School Home | Physics Home
By Chris McGinlay, ©Sandwick Junior High School 2000, 2001,2002.
This is a glossary of the terms used in Standard Grade Physics. Let the entire
document load or get 'Transfer Interrupted' errors! Most diagrams or figures
will be kept in separate linked documents. There are approximately 275
definitions at present.
. Not all terms are inter-related. Still
to be error checked.Webmaster. Please notify me of
any errors or suggestions.
- Acceleration TR, SP
- Acceleration is the rate of increase of velocity. Acceleration tells you
how much faster or slower a moving object gets every second. A negative
acceleration is called a deceleration. The SI Unit of
acceleration is the metre per second
per second (ms-2)
- Acceleration due to
Gravity TR, SP
- This is the acceleration caused by the
gravitational pull of a planet. On Earth, the strength of the acceleration due to gravity is roughly 10
ms-2. The acceleration due to gravity is
often given the symbol 'g'. See also weight and mass
- Accommodation HP
- Accommodation relates to the ability of the eye to
change its focal length and so focus on objects at a
range of distances. To focus on a close object, the eye lens will be fat
(having a short focal length). To focus on a far object, the lens will be
pulled into a thin shape having a long focal length. Failure of accommodation
gives rise to long-sightedness and short-sightedness.
- Activity HP, EM
- The activity of a sample of radioactive material
is the number of disintegrations per second taking place in that particular
sample. The SI unit of activity is the becquerel
- Aerodynamic TR
- Aerodynamic objects have minimal air resistance. This means that they can
move at greater speed through the air. Remember that as objects go faster the
aerodynamic drag increases. Therefore, an aerodynamic object will be able to
travel faster than a non-aerodynamic one!
- Alpha (a) Radiation
- a (alpha) radiation
consists of slow moving helium nuclei. These nuclei are released during the
disintegration of an unstable nucleus. a radiation
will be stopped by a few centimetres of air, or a sheet of paper. It is a
strongly ionising radiation, due in part to its relatively
large mass - it is about 8000 times as massive as the electrons in b radiation! The
helium nuclei are positively charged.
- Alternating Current (a.c.) UE, EM
- Electrical current continually changes direction in
an a.c. power supply (as opposed to direct current (d.c.))
In the UK, mains electricity is a.c., with a frequency of 50 Hz and an r.m.s
voltage of 230 V.
- Alternator EM
- This is a device for generating a.c. electricity. It
consists of rotor coils (with a d.c.
supply) and stator coils which produce the a.c. electricity.
- Ampere UE, EL
- The ampere is the SI unit of electrical current. At a simple level it can be defined as the number
of coulombs of electrical charge
flowing per second. The full definition is beyond the scope of the S-grade
- Amplifier TC, UE, EL
- An amplifier is a component of an electronic system, such as a radio or TV which makes all the input signals louder.
Ideally, an amplifier will increase the amplitude of
all input signals by the same factor. Amplifiers require a power supply which
can be battery or mains. The voltage gain or the power
gain can be used to describe how much amplification is taking place.
- Amplitude (Diagram) TC, UE, HP, EL
- This is the height of a wave, measured vertically from
the centre line to a crest or a trough. The SI unit is the metre, although in some contexts amplitudes may be given in volts.
- Amplitude Modulation (Diagram) TC
- Amplitude modulation refers to the changing of the amplitude of a radio-frequency or microwave frequency
carrier wave. The other method of modulation is called frequency modulation.
- Analogue EL
- Analogue signals should be compared with digital
signals. Analogue signals are continuous. As an example, a microphone is an
analogue input device. This is because it can
produce a range of voltage levels.
- AND Gate EL
- An AND gate is a logical device. It usually takes two inputs (although
more are possible) and produces only one output. The logic state of the output depends on the logic state of
the inputs. This is shown in the truth table below.
A and B are the inputs and Z is the output state.
From this it is easy to see that the output is on only if both inputs are
- Angle of Incidence (Diagram 1) (Diagram 2
- The angle between the normal line and a ray or
wave incident to a surface.
- Angle of Reflection (Diagram) TC
- The angle between the normal line and a ray or
wave reflected from a surface
- Armature UE
- A multicoil rotating assembly, used in commercial motors instead of the
single rotating coil of the model motor built in class. A segmented commutator
is used to connect each of the armature windings to the power supply in turn,
thus providing smoother rotation of the motor.
- Asteroid SP
- An asteriod is a lump of rock which orbits around the sun. (The biggest
asteroids are about 100km across, although many are 1km or less across.
Asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and
- Atom HP, EL, EM, SP
- An atom is the smallest possible particle of any chemical element. For
example, an atom of carbon is the smallest possible unit of carbon. All of the
atoms of an element are identical to each other, but different from the atoms of
any other element. For example, all carbon atoms are essentially identical to
each other, but carbon atoms and oxygen atoms are different from each other.
The size of an atom is approximately 10-10m.
The reason why atoms of oxygen are different from carbon (or any other
element) is down to the structure within the atom itself. Atoms are made up
from protons, neutrons and electrons. Atoms of different elements have different
numbers of protons. Atoms of the same element have the same number of protons.
For example, atoms of carbon always have six protons. Atoms of oxygen always
have eight protons.
The protons and neutrons are found in the nucleus of the atom. The tiny
electrons 'orbit' the nucleus. The nucleus is very small, approximate diameter
See also: Ion.
This link has more
information on atoms
- Angle of Refraction (Diagram) TC
- The angle between the normal line and a ray or
wave refracted at a surface
- Average Speed TC, HP, TR, SP
- The average speed of a vehicle is usually calculated over the whole
distance and time of the journey. For example a train may cover a distance of
240 km in a time of 3 hours. The average speed can be worked out as follows:
In kilometres per hour: speed = 240 km / 3 hr = 80 km/hr
In metres per second: speed = 240 000 m / 10 800 s = 22.2 m/s
Remember that average speed is different from instantaneous speed.
- Audio Wave TC
- A sound wave, but usually in the context of modulation and radio
- Background Radiation HP
- Background radiation is the radiation which occurs
naturally in the rocks, air and water around us.
- A battery is a number (or battery) of cells, connected
in series with the positive terminal of one cell connected to the negative of
the next. Batteries provide a d.c.(direct current) source of
- b (beta) radiation
- b (beta) radiation
consists of fast moving electrons. These electrons are
produced as a result of transitions that take place in unstable nuclei. b radiation can be stopped by a few millimetres of
aluminium. It is an ionising radiation, not as strongly
ionising as a (alpha)
- Big-bang Theory SP
- The 'big-bang theory' of cosmology tries to explain the origin of the
universe as an explosive event, where space, time and matter came into being.
Evidence for the big-bang includes redshift measurements
(and microwave background radiation studies of the universe - look up CMBR on the web if you are
- Binary EL
- A system of counting in twos instead of in tens (counting in tens is called
decimal). Here is a comparison of the decimal and binary counting system:
You should notice that each column (or place) in decimal is ten times the
value of the previous one, whereas in binary, each column is only twice the
value of the previous one. Also in decimal, you can have any digit from 0 up to
9 in any of the columns, whereas in binary, you can have either a zero or a one
in any of the columns. In the example given, you can see that 12 in decimal is
the same as 1100 in binary.
- Black Hole SP
- A collapsed star from which no light can escape, due to
the intense gravitational field.
- Block Diagram EL
- In electronics, a block diagram provides a simple way of analysing how a system works. Block diagrams don't show details
of how components work, but simply show how information or signals should travel
through the system. They are usually drawn using rectangles and arrows.
- Cancer HP
- Cancer is a growth of out-of-control cells.
Localised cancer growths are called tumours. a, b, g or X-ray radiation can be used to diagnose and treat cancer.
- Capacitor (Diagram) TC, UE, EL
- A capacitor is a device which can store electrical charge. Capacitors are often used to introduce a time delay
in electronic circuits. The SI unit of capacitance is
- Carbon Dating HP
- See radiometric dating.
- Carrier Wave TC
- Carrier waves are used to carry information from
transmitter to receiver. The carrier wave will be radio or
microwave frequency. There
are two methods of using the carrier wave to carry information - amplitude modulation and frequency modulation. In radio transmissions
for example, the carrier is of a much higher frequency
than the audio wave which is to be transmitted. The
higher the frequency of the carrier, the higher the quality of the transmitted
- Cell (Diagram)
- In electricity, a cell is a source of d.c. electricity.
A cell can be made using two different metals inserted into a salt solution, for
example. The metals react with the salt solution at different rates and in so
doing, generate a small electrical current. Many cells
can be combined to produce a battery of cells.
In biology, cells are the basic building blocks of all living things. Plant
and animal cells alike have a tiny nucleus which controls the behaviour of the
cell, a cell membrane which controls the entry and exit of substances. The
inside of the cell is filled with a liquid called cytoplasm. These cells can be
damaged or destroyed by ionising radiation such as a, b, or g. The amount of cell damage caused by radiation
- the type of radiation
- the type of tissue or cells
- the total amount of energy absorbed
See also: Dose Equivalent.
- Charge UE
- There are two types of electrical charge. One type is called 'positive',
the other is called 'negative'. Like charges repel each other, whilst unlike
charges attract each other. The amount of charge affects how strong this
attraction or repulsion actually is.
The SI unit of charge is the coulomb (C)
See also the references for the electron, the proton and the neutron.
In electrical circuits, charge is related to current
by the equation Q = I t
- Chemical Potential Energy
- Chemical energy is a form of potential
energy because it is stored energy. Energy stored in food, torch batteries
and explosives like dynamite is all chemical energy. Chemical energy is
released in a chemical reaction (like combustion of explosives or respiration in
plant and animal cells).
- Clock Pulse Generator EL
- A simple circuit using a capacitor, resistor and transistor. The circuit
generates 'clock pulses' - simply a regular sequence of logic LO and logic HI.
The capacitor charges and discharges through the resistor. As the capacitor
charges to a certain level, the inverter's input goes HI and its output goes LO.
This this causes the capacitor to start discharging. When its voltage gets low
enough, the inverter output goes back to HI and the cycle starts again.
By increasing the resistance or increasing the capacitance, the pulses can
be generated more slowly.
- Combined Heat and Power EM
- Most power stations end up trying to turn heat energy into electrical
energy. This can never be done with 100% efficiency (due to the second law of
thermodynamics). Most of the energy produced (by combustion for example) will
be lost as waste heat. A combined heat and power plant uses the waste heat to
heat nearby homes and factories. This is why half of Lerwick was dug up around
2000-2001 - for the installation of pipes for the CHP plant.
- Comet (Image 1)
A comet is a small body orbiting our Sun, usually at great distance from
Earth and with a highly elliptical orbit. They are composed of 'ices' and dust.
As a comet approaches the inner solar system, the heat of the Sun vapourises
volatile material from the surface and pushes the material directly away
from the Sun. This gives rise to the 'ion tail', usually a ghostly
blue colour and quite straight, it always points directly away from the Sun and
has nothing to do with the direction of motion of the comet. A second tail
exists, usually white and curved. This is dust which falls off the comet and is
left in track of its orbit. Here is a photo of comet Hale-Bopp, taken from
Hoswick, Shetland in 1998. Scientists recently deliberately crashed a space
probe onto the surface of a comet in order to learn more.
Comets and their dust tails give rise to regular meteor showers.
- Conductor UE, EL
- A material that allows electricity to flow easily through it. Copper, for
example is a good conductor. Good conductors have a very low resistance. See also insulators
- Conduction (Heat) EM
- This refers to the ability of heat energy to travel through a material.
For example, copper is a good conductor of heat as it will heat up quickly if
left on a radiator. Wood, for example does not conduct heat so well.
Conduction is not possible in a vacuum. See also convection and radiation.
- Consumer Unit UE
- Commonly known as the fusebox, the consumer unit is the part of the meter
board which separates the incoming electrical supply into the various lighting
and ring main circuits. Each of these circuits is
protected by either a fuse or miniature circuit breaker (MCB)
- Continuous Spectrum SP
- The light from a tungsten filament lamp has a
range of colours present within it. Each colour corresponds to a wavelength of
visible (EM) radiation. In a continuous spectrum like this, there are no
missing wavelengths. However, the light from the Sun (or from any star) has
certain wavelengths missing. The missing wavelengths can be used to identify
the 'fingerprint' of chemical elements in stars' atmospheres, or in the
intervening dust and gas clouds between us and a particular star. In class, we
use a direct vision spectroscope to observe the missing colours of
- Control Rods EM
- These are used in nuclear reactors. They control the rate of nuclear
reactions by absorbing the neutrons which cause the reactions to happen. By
lowering the rods into the reactor, the rate of reaction slows down.
- Convection EM, SP
- This is a way of moving heat energy around. In a fluid (i.e. a liquid or
gas) heat can travel by convection. For example, hot air is less dense than
cold air, so the hot air rises up, carrying its heat energy with it. Convection
is not possible in a vacuum. See also conduction and radiation.
- Core (Soft Iron) UE, EM
- The presence of an iron core in a solenoid will produce a stronger magnetic
field than the solenoid would manage on its own. The iron core thus magnifies
the effect of the solenoid's magnetic field.
- Coulomb UE, EL
- The coulomb is the SI Unit of electrical charge. It can be defined as the amount of charge which
flows in 1 second due to a current
of 1 ampere.
- Crest (Diagram)
- This refers to waves. The crest of a wave (also called
the peak) is simply the part of the wave with the greatest amplitude.
- Critical Angle TC, HP
- The critical angle is associated with refraction of light. This angle is the smallest angle of incidence at which total
internal reflection (TIR) occurs.
- Current UE, EL, EM
- Electrical current is the rate of flow of electrical charge round a circuit. It is measured in amperes and should be measured in series with an ammeter.
See also: Ohm's Law
- Curved Reflector (Diagram) TC, SP
- Curved reflectors, with a parabolic shape, are used to produce parallel beams, or to collect light, radio waves or similar, over the area of the reflector and
focus this energy onto a detector. This makes the
received signal at the detector much greater than if no reflector was used.
- Cycle TC
- A complete cycle of a wave is equivalent to one
complete wavelength of that wave. The time taken to
produce one complete cycle is called the period of the
wave. The number of cycles per second is called the frequency of the wave
- Direct Current (d.c.) UE, EL
- Electrical current only flows in one direction from
a d.c. power supply (as opposed to alternating current (a.c.))
- Deceleration TR, SP
- Deceleration basically means slowing down - reducing speed. A negative answer for an acceleration would actually be a deceleration. Like acceleration, deceleration is also measured in
- Decibel (dB) Scale HP
- The decibel scale is used to measure the volume (intensity or loudness) of
sound. The abbreviation for this unit is dB. For the S-grade physics course, a
sound level of 80dB is considered to be the danger level - i.e. the level at
which hearing damage starts to occur.
- Decoder TC
- The decoder section of a radio essentially removes the
carrier wave from the modulated signal, leaving only the audio wave which is then amplified. In TVs there are two separate decoders - one
for the picture and one for the sound.
- Deflection Plates TC
- Deflection plates are used to direct the electron
beam to all parts of the cathode ray tube of an oscilloscope. There are two
sets of plates, one set for controlling horizontal movement of the beam and the
other set for vertical movement. Each set of plates controls the beam by having
a certain voltage applied across the plates.
- Diffraction TC, HP, SP
- Diffraction is a wave phenomenon - i.e. all types of
wave can diffract. Diffraction means the bending of wave (change in direction)
and it occurs whenever a wave passes near to an obstacle. Longer wavelength waves diffract through a greater angle than
shorter wavelength waves. Do not confuse diffraction with refraction.
- Digital EL
- Digital can refer to electronic signals or to components of an electronic system. Digital signals have only two
voltage levels, described as 'HI' and 'LO', or 'OFF' and 'ON'. In a real
component, 'HI' might be represented by +5V d.c. and 'LO' by 0V d.c. A digital
component is one which works with only digital signals. Here are some examples
of digital components: LED (output), photodiode (input), switch (input).
Contrast digital signals with analogue signals.
- Diode (Diagram) UE,
- A diode is a semi-conductor device. Diodes will allow current to pass through them in only one direction.
- Discharge Lamps UE
- See under Gas Discharge Lamps
- Double Insulation (Diagram) UE
- An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way
that the electrical parts can never come into contact with the outer casing of
the device. Common double insulated appliances are hair dryers, radios and
A wet double insulated appliance is exceptionally dangerous - water is a good
conductor of electricity and will easily reach the live
electrical components within the case. Any human user touching the casing will
then receive an electric shock. It is important to note that the human
conductivity is increased when hands are wet, allowing a greater current to flow through the casualty.
For this reason, do not operate a mains radio, hairdryer or double insulated
appliance in any wet area - especially a bathroom
Do not touch any person who has been electrocuted until you are sure that the
electricity supply has been shut off.
- Dose Equivalent HP
- This is the biological risk due to exposure to radiation. The SI unit of dose
equivalent is the sievert (Sv). The damage caused to
cells depends on three major factors (see under cells).
- Drag TR, SP
- Drag is a type of friction force usually associated with movement through a fluid like air or water. Drag forces generally increase at
- Dynamo UE, EM
- A device (a tranducer) which converts kinetic energy into electrical energy. Dynamos are
often used to produce a.c. electricity on a push-bike. Unlike
a full-blown generator, the rotor
coil is replaced with rotating permanent magnets.
These magnets rotate near to the stator coil, (wrapped around a soft iron
core) and cause a small current to flow (via electromagnetic induction).
- Earth (planet) SP
- Planet Earth has the third most distant orbit from our star the Sun. To the best of our knowledge it is the only place in the
entire universe where we know that life exists (well, some people have made this
claim at least) - although this may change as we explore Jupiter's moon Europa.
It is the only place in the Solar System where large quantities of liquid water
- Earth (wire) UE
- If fitted, the earth core of a flex connects the casing
of the electrical appliance to ground via the earth terminal of the plug. Its insulation is coloured
green and yellow. In the event of an appliance malfunction resulting in the
casing becoming live, the earth wire provides a route to ground for the current. This will help prevent electric shock. If the
earth wire is not present and the casing is live, any person touching the casing
will receive a shock as the current flows to earth through them. If the earth
wire is intact, its low resistance will result in a
very high current flowing through live. This should cause the fuse in the plug to blow and remove any fire hazard.
Note that some devices are double insulated
and have no earth. These devices must not be used near water or steam
Do not touch any person who has been electrocuted until you are sure that
the electricity supply has been shut off.
- Efficiency EM
- No machine can be 100% efficient. Efficiency is defined as 'Useful Energy
Out' / 'Total Energy In'. It is a fraction, sometimes written as a percentage.
For example, if an electric motor in a winch consumes 2000J of energy per second, and does 420J of useful work per second, then the efficiency of the winch is, eff =
420/2000 = 0.21 = 21%. Note that efficiency has no units. Note that since
2000J per second is the same as 2000W power, you can do the efficiency
calculations with power too.
- Electromagnet (Diagram) UE, EL, EM
- An electromagnet is a solenoid with an iron core
inserted into it. If a current flows in the coil, a magnetic field is
generated. All the randomly oriented domains of the iron core then align in the
presence of the field of the solenoid. Thus, the core greatly enhances the
strength of the electromagnet.
- Electromagnetic Induction
- If a conductor is moved within a magnetic field,
then a current will be induced (caused) in the conductor.
If the magnetic field is reversed or if the direction of movement is reversed,
then the direction of the current flow will also reverse.
This is a very similar effect to the movement of a current carrying wire in a
magnetic field: a non-magnetisable copper wire placed in a magnetic field can be
made to move if a current is passed through it. If either the direction of the
magnetic field or the direction of the current is reversed, then the movement
will be in the opposite direction.
- Electromagnetic Spectrum
TC, UE, HP, EL, SP
- See also under spectrum. This includes includes
Gamma, X-rays, Ultraviolet, Visible, Infra-red, Microwave and Radio - in order
of increasing wavelength.
- Electron TC, UE, EL, EM
- Electrons are sub-atomic particles. They carry a
negative electrical charge (qe=-1.602 x
They have a mass of me = 9.110 x 10-31 kg.
Beams of electrons are used in TV sets where they are called cathode rays.
See also b (beta)
- Electronic System EL
- An electronic system can simply be considered as consisting of three parts:
input, process and output. Input devices include microphones and LDRs,
thermistors and switches. Process sections can be made from one or more
transistors, perhaps built into logic gates or computer chips. The output
section will consist of one or more output devices, for example, a buzzer, LED,
- Energy TC, UE, HP, EL, TR, EM, SP
- It is difficult to define in a precise way what energy actually is. For
the Standard grade, it is enough to think of energy as the ability to do
mechanical work - to lift up weights for example. There are many different
types of energy including:
The SI unit of energy is the joule
(J). Energy can not be created or destroyed, but can be changed from one
form to another. It is an abstract concept, but makes many problems in science
easier to solve.
- kinetic energy,
- chemical potential energy,
- elastic potential energy,
- gravitational potential energy,
- heat energy,
- electrical energy,
- light energy,
- magnetic energy,
- nuclear energy,
- sound energy.
- Eye HP
- The various parts of the eye are shown in a diagram (not included yet).
Light entering the eye passes through (in order) the cornea, aqueous humour,
pupil, lens and vitreous humour before striking the retina. Electrical signals
from the retina are sent to the brain via the optic nerve. The pupil is
actually a hole in the iris. The lens is held in place by the ciliary muscles
and suspensory ligaments.
Common eye conditions are long(presbyopia)-
and short(myopia)- sightedness.
- Eyepiece Lens SP
- The lens on a telescope (or microscope) which the observer places his or
her eye next to. The eyepiece's sole funciton is to magnify
the image. image
- Farad TC, UE, EL
- The farad (F) is the SI Unit of capacitance. Since the farad is a massive
unit, most capicitances are given in micro-farads. The definition of the farad
is beyond the scope of the standard grade course, but it is based on
measurements of voltage and charge.
- Far-sighted HP
- See under long-sighted.
- Fibrescope HP
- Fibrescope are sometimes called endoscopes. Fibrescopes use optical fibres to allow doctors to view inside a
patient without cutting them open. There are two separate fibre bundles - one
for taking light down into the patient, called the lightguide. The other is for
forming an image and is called the image guide. The image guide must be a
coherent fibre bundle.
- Filament Lamp UE
- These lamps rely on an incandescent metal filament to
produce light. The interior of the lamp is filled with an inert gas in order to stop the filament from corroding. An
electrical current, passed through the filament makes it
extremely hot. When sufficiently hot, the filament itself will emit light
radiation. Tungsten metal is
chosen for the filament due to its high melting point of about 3956 celsius.
Note that filament lamps produce both light and unwanted heat. As such they
are less efficient than fluorescent tubes
- Fission Process HP, EM
- This usually refers to nuclear fission, where an atomic
nucleus splits apart into smaller pieces. Fission should not be confused with
fusion. When a heavy, unstable nucleus undergoes fission, energy is released as
heat. In nuclear power stations, uranium is often used as the fissile material,
where a nuclear chain reaction is controlled to release energy.
- Flex UE
- The flex allows electrical current to flow into
domestic appliances from the mains socket. The flex should be correctly wired
to a plug.
- Fluorescent Lamps UE
- These lamps produce light by passing electrical current through a mercury vapour at low
pressure. The electrical current excites ("energises") the electrons of the mercury atoms. These electrons quickly
give up their energy as light. The light which is
produced is invisible ultra-violet and would be useless. To get round this
problem, the inside of the tube is coated with fluorescent paint, which converts
U.V. light to visible light.
Note that fluorescent lamps are more efficient than filament lamps as they waste less energy as heat
- Focal Length HP, SP
- The focal length of lens is measured in metres. It is the distance from
the centre of the lens to the point at which rays passing through the lens meet
(i.e. focus). It is related to lens power.
- Force TR, SP
- Forces can cause an object to speed up, slow down, change direction or
change shape (if they are unbalanced). Forces
are either push forces or pull forces. The SI unit of
force is the newton.
- Fossil Fuel EM
- Fossil fuels are coal, oil and gas. They are non-renewable resources.
Fossil fuels come from ancient remains of plants and animals, compressed and
heated over many millenia. The combustion of fossil fuels is now (nearly)
universally accepted as causing global warming. They are well known to produce
acid rain. Most of the world's energy is still produced from fossil fuel. Here
in Shetland, we paid the price in the form of the Braer tanker disaster.
- Frequency TC, UE, HP, EM, SP
- The frequency of a regular event is the number of times the event occurs
in a given time. e.g. The Sun rises with a frequency of once per day.
The frequency of a wave is the number of complete
wavelengths which pass any point in one second. The SI unit of frequency is the
hertz (Hz) (equivalent to 'per second' or s-1).
The concept of wave period is closely related to
See also: cycle.
- Frequency Modulation TC
- Frequency modulation refers to the changing of the frequency of a radio-frequency or microwave frequency carrier
wave. The other method of modulation is called amplitude modulation.
- Friction TR
- Friction is a force which tends to make moving objects
slow down. Friction also prevents objects from slipping over one another - so
for example, friction helps keep your backside on a seat and it allows people to
stand upright without their feet slipping out from under them. Friction due to
fluid flow is called drag.
- Fridge EM
- A fridge is an example of a heat pump. It extracts heat energy from inside
the fridge cabinet and releases it into the room. The basic principle relies on
the latent heat of vapourisation of a coolant liqid. The coolent is evaporated
within the pipes inside the cabinet - the heat required for this is taken from
the cabinet itself, with the result that the cabinet becomes cooler.
- Fuse UE
- A fuse, where fitted, is designed to protect the flex (and not the appliance) from current overload and the associated risk of fire. The fuse
rating should always be less than that of the flex which it is protecting. This
will make sure that it blows before the flex melts. If the fuse in the
plug blows, it is likely to be because of a fault in the
appliance which is drawing too much current.
The fuse (and switch) should always be connected to the live wire in a plug.
- Galaxy SP
- A grouping of about approximately 100 billion stars, held together by their
own gravity. We live in the Milky Way galaxy, visible on any clear, moonless
night away from Lerwick street-lights. Within a galaxy, there are many
different types of star, some older, younger, brighter, dimmer, or more/less
massive than the Sun. Galaxies do not all look the same - astronomer's classify
them according to their appearance. Ask your teacher to show you the Andromeda
galaxy - easily visible on a clear, dark night as a large misty patch.
Andromeda is one of the nearest galaxies to our galaxy (the Milky Way), at a
distance of two million light-years.
- Gamma (g) Radiation
HP, EM, SP
- g (gamma) radiation is
high frequency electromagnetic radiation. It is a part
of the electromagnetic spectrum. Gamma radiation
carries no mass, no charge, travels at the speed of light.
(It can be thought of as being made of particles called photons or as being a wave). It can be stopped by several centimetres of lead or a
few metres of concrete. Gamma radiation is an ionising
radiation, but is not as strongly ionising as a radiation. It is released during the disintegration
of an unstable nucleus.
For the purposes of treating cancer, medical gamma rays
from cobalt-60 are used.
- Gamma Camera HP
- This is a device used to detect the gamma radiation
from a patient during an examination using a radioactive tracer. Since gamma radiation passes straight through most
materials, it is not possible to build a camera with glass lenses or mirrors.
Instead an array of holes drilled in a thick layer of lead is placed between the
actual gamma detectors and the patient. This allows an image to be built up.
Scintillation counters are used to detect
the radiation after it has passed through the holes in the commutator.
- Gas Discharge Lamps UE
- These lamps produce light by passing electrical current through a gas at low pressure. The electrical
current excites ("energises") the electrons of the atoms in the gas. These electrons quickly give up their energy as light. Sodium is often used
(in street lights) or an inert gas for colourful
Note that discharge lamps are more efficient than filament lamps as they waste less energy as heat
- Geostationary EM
- 'Geostationary' refers to a satellite of planet Earth. Such a satellite
will appear to hover above the surface of the Earth, since it is placed in an
orbit with a period of 24 hours.
- Geothermal Energy EM
- This is a renewable energy resource, unlike fossil fuels. In some parts of
the world, hot subsurface rock heats water which reaches the surface and can be
used for heating or electrical power generation. Geothermal Resources
- Generator EM
- A machine which uses motion to produce electrical energy. Sometimes called
an alternator. A generator is like a dynamo, but
with the permanent rotor magnets replaced with (d.c) rotor coils
(electromagnets). The stator coils have an a.c. induced
- Gravitational Field TR, SP
- The Earth generates a gravitational field which attracts all other masses.
It is a called a force field - the word field means that the force changes as
you move away from the surface of the Earth (in fact the force becomes steadily
less as you move away from the surface of the Earth). It is the combined mass
of all particles on Earth which cause the Earth's gravitational field. The
strength of the field is measured in Nkg-1.
- Gravitational Potential
Energy TR, SP
If a mass is lifted up then its potential energy is given as follows:
Ep = mgh
Ep is the potential energy
m is the mass in kilograms
g is the strength of the gravitational field (10ms-2 on
h is the height through which the object is lifted
- Geiger-Muller tube HP
- This is a device used to detect the presence of radiation. It relies on
ionisation to produce tiny bursts of current which can be counted by a
ratemeter. A popular exam question, and worth checking your class notes.
- Gold Grain HP
- Radioactive gold grains are used in the
treatment of cancer. Gold (which is unreactive) doped with radioactive (gamma
emitting) nuclei is implanted in a patient, near or in a tumour. The radiation will destroy or damage cells near the grain. The radioactive material chosen will
have a short half-life so that the grain's activity level will drop to background levels after several
- Half-life HP
- The half-life of a radioactive material is the
time taken for its activity level to drop by a half.
Half-life is measured using any unit of time (seconds, hours etc). For example,
if the activity of a sample is 20MBq at 12:00 pm,
falling to 10 MBq at 2:00 pm, then the half-life is 2 hours. At 4:00 pm the activity will be 5 MBq, etc. There is a dramatic range in
the half-life from element to element. For example the half-life of
113Cd (cadmium) is 9 x 1015 years (nine million billion
years), whilst the half-life of 213At (astatine) is only 0.11
millionths of a second!
- Heat Energy EM
- Heat is a form of energy and is not to be confused with temperature. There
is no such thing as 'cold energy'. At the molecular level, heat energy is
transferred from particle to particle by vibration (this is called conduction).
A particle vibrating violently can be said to have a lot of heat energy. As
well as by conduction, heat can also be transferred by convection or
- Heater UE
- The operation of a bar heater is very similar indeed to a filament lamp. The difference is in the thickness of
the 'filament' or element of the heater. Curiously, heaters operate at a lower
temperature than lamps, but produce far more heat - as infra-red radiation.
(Remember that temperature and heat are very different concepts). Note that
some energy is wasted as (red) light.
- Heat Pump EM
- This is a device which can move heat energy from a colder object to a
hotter one, but requiring an external energy source to do it. A fridge is an
example of a heat pump. Due to the second law of thermodynamics, a heat pump
can never be 100% efficient.
- Hyperopia or hypermetropia (hyperopic or
hypermetropic eye) HP
- See under long-sighted.
- Inert UE
- A chemical element is inert if it does not react with anything. Examples
are helium, neon, argon, krypton, xenon and radon. Inert gases can
be used in filament lamps to protect the filament
from destruction due corrosion.
- Inertia TR, SP
- Inertia is a property of matter. It describes how difficult it is to
either get something moving or to stop it when it is moving. So an elephant has
a lot more inertia than a paper clip. The SI unit for
inertia is the kilogram. It's important to realise
that, e.g. when in a spacecraft in orbit, the effects of gravity can be ignored,
but inertia cannot - an astronaut can still be crushed by a stray satellite!
See also: Mass.
- Input Device EL
- The first part of an electronic system, an input device typically processes
requests from a human user. For example in a CD player, the 'ON' button and
volume controls (potentiometer) are all input devices. Other input devices
include switches, thermistors, LDRs, photodiodes.
- Infrared TR, UE, HP, SP
- Infrared radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just longer than that of visible red light.
Infrared light is use in telecommunications (via optical fibres). It is used in medicine to diagnose
illness (thermograms) and accelerate healing in physiotherapy. Loosely
speaking, infrared radiation can be thought of as 'heat rays'.
- Instantaneous TR
- This means an event which occurs over a very short (infinitesimal) period
of time. It usually refers to speed, i.e. instantaneous speed as opposed to average speed.
- Instantaneous Speed TC, TR
- The speed at any particular stage or time during a
journey. Instantaneous speed can be calculated by measuring short distances and
time intervals. For example a train coming into a station may move forward by
0.2 m in a time of 0.5 s, giving an instantaneous speed of 0.4 m/s. Compare
this with the average speed of the train over a
- Insulator UE
- A material which does not usually conduct electricity. Air, for example is
an insulator. Insulators have a very high (ideally infinite) resistance. Given a suitably high voltage, insulators can be forced to conduct - take for
example lightning during a thunderstorm. See also conductors and semi-conductors
- Insulation (Heat) EM
- An insulator in this sense is a material which does not conduct heat well.
Ceramics, stone and plastics are good insulators against heat conduction.
Metals are poor insulators as they will quickly heat up or cool down.
- Inverter (NOT gate) EL
- A processing device within an electronic system, the NOT gate is a single
input logic gate which 'inverts' its input. A logic 'HI' input will become a
logic 'LO' output and visa-versa. Here is the truth table:
- Ion HP
- An ion is an electrically charged atom. In a neutral atom, the number of protons in the nucleus is balanced by the number of electrons around the nucleus. If an electron is knocked
away from the nucleus (perhaps as a result of radiation), then the atom will be left
with a net positive charge. The process of creating ions is called
- Joule UE, TR, EM, SP
- The joule (J) is the SI Unit of energy and of
(mechanical) work. One joule is the same as one
newton-metre (1 J = 1 Nm).
- Jupiter SP
- Planet Jupiter has the fifth most distant orbit from our star the Sun. It is the largest planet of our solar system, with a
diameter which is 11 times that of the Earth. A so-called 'gas giant', most of
the planet is made of swirling clouds of toxic gas, with wind speeds up to 450
mph. (For comparison, typical wild winter storms in Shetland which reach well
over 100mph, would be very tame).
- Kelvin HP, EM, SP
- The kelvin is the SI Unit of absolute temperature.
273.15 K is the same as 0 Celsius and 373.15 K is the same as 100 Celsius. Its
formal definition is beyond the scope of the S-grade course.
- Kilogram TR, EM, SP
- The kilogram is the SI unit of mass. It is
defined as the mass of a particular cylinder of platinum which is kept at the
International Bureau of Weights and Measures at Sevres, near Paris.
- Kilowatt-hour (kWh) UE, EM
- This is a unit of energy, usually used in electrical
situations. Electricity companies bill their customers on the number of kWh
used. One kWh means that energy has been used at a rate of 1000 W
(Js-1) for one hour. Using this, the kWh can be related to the joule as follows:
E = P t
1 kWh = 1 000 Js-1 x 3 600 s
1 kWh = 3 600 000 J = 3.6 MJ
kWh is a unit of electrical energy and NOT power!
- Kinetic Energy TR, EM, SP
Kinetic energy is the energy of motion. It is given by the formula:
Ek=0.5 * mv2
where, Ek is the kinetic energy in joules
m is the mass of the object in kilograms
v is the velocity of the object in metres per second.
- Lamp UE
- A device intended to produce light energy. Modern
lamps convert electrical energy to light energy via one of two main mechanisms -
the filament lamp or the gas discharge lamp. Note that filament lamps are
less efficient than discharge lamps (i.e. fluorescent lamps) as they produce more waste heat
- Laser TC, HP
- The word laser was originally an abbreviation for Light Amplification by
the Stimulated Emission of Radiation. Lasers have countless applications, many
in medicine and communications. Lasers produce light which is monochromatic,
coherent and tightly focussed. Lasers are used in the technique of
photocoagulation (used to treat retinae).
- Latent Heat EM
- Latent heat is heat energy used or released during change of phase.
When matter changes phase, energy must be given out or taken in. Change of
phase takes place at constant temperature. For example, if a bunsen is used to
bring 100ml of water to the boil, it is easy to understand that the heat energy
from the bunsen increases the heat energy of the water - because the water gets
hotter. During boiling however, the temperature of the water does not increase.
The energy from the bunsen no longer causes an increase in temperature, but is
now carried away in the steam. The energy is used to break the bonds between
the water molecules instead of increasing the temperature. (Remember: heat and
temperature are different concepts!)
- Liquid Crystal Display (LCD) EL
- This is an output device. They run on almost no power since they don't
produce their own light, but work by reflecting room or daylight.
- Light Dependent Resistor (LDR) (Diagram) EL
- LDRs are designed so that their resistance
depends on the intensity(brightness) of light falling on them.
When in low intensity illumination (dark!), the resistance of an LDR is
When in bright illumination, the resistance of an LDR is low.
- Light Emitting Diode (Diagram) EL
- LEDs are simply diodes which emit light when connected
correctly to a power supply.
- Light-year SP
- The light-year (ly) is a unit of distance which is useful in astronomy.
It is the distance which light (or any other form of electromagnetic radiation)
travels in one year. Note that real astronomers do not use light-years,
but use a unit called the parsec instead.
It is easy to work out how far a light-year is as follows:
distance = speed x time
1 ly = 3 x 10 8 x 365x24x60x60 metres
1 ly = 9.46 x 10 15 metres
To appreciate the scale of this unit, recall that the distance from the Earth to
the Sun is 1.5 x 10 8 metres. This is about 8 light-minutes. The
distance to the next-nearest star (Proxima Centauri) is about 4 ly!
- Line Spectrum SP
- Plural: Line Spectra. A spectrum of light with only certain wavelenghts or
colours present. (See also continuous spectrum). If you use a direct vision
spectroscope to view light from a (yellow) sodium vapour street-lamp, you will
see that only a few wavelengths are present. This is an emission line spectrum
and is unique to sodium. Sodium can also absorb these wavelengths too, so if a
continuous spectrum of white light were to be shone through sodium vapour and
then observed through a spectroscope, certain colours or wavelengths would be
missing. This is called an absorption spectrum.
These facts can be used to identify which elements are present in stars'
atmospheres, by matching lines in their spectra with the spectra of elements
tested on Earth.
- Logic Diagram EL
- A simplified diagram of an electronic system, using rectangles to represent
input and output devices along with the usual logic gate symbols for the AND, OR and NOT gate etc. Inputs and outputs are given
letters (A,B,C,.. for inputs; Z for output) to help layout a truth table.
- Logic Gate EL
- For standard grade, we study AND, OR and NOT gates.
Other common gates are the NAND, NOR and XOR (or EOR). Logic gates allow an
electronic system to make decisions based on a number of inputs. For example an
OR gate can be used to illuminate a warning light if engine temperature is too
high or if it is over-revved.
- Logic State EL
- In a digital system, there are two logical states. One state is 'ON', 'HI'
or 'logic 1' (all mean the same). The other is 'OFF', 'LO', 'logic 0' (again
all mean the same).
- Long-sighted HP
- This refers to a problem with the accommodation ability of an eye.
Basically, the eye can focus on far away objects with no problem, but can't
properly focus on nearby objects. There can be two causes of this. The first
occurs in older people, where the aging eye lens becomes less flexible and so
cannot adopt the highly curved shape needed to focus at short range. This
condition is called presbyopia. The second cause is that retina is too near to
the lens - so that the focal point falls behind the retina. This is called
hypermetropia or hyperopia. Long-sightedness can be treated by wearing
spectacles which are convex in shape and therefore help to focus the image
properly by reducing the overall focal length of the
glasses and eye.
- Lubricant TR
- A lubricant is a substance which reduces friction
between two surfaces. Examples are oil, water, air, bananas, seaweed.
- Magnet UE, EL, EM
- A simple permanent magnet like a bar magnet has two poles, called 'North'
and 'South'. Like poles repel and unlike poles attract. A magnet sets up a
magnetic field in three dimensions around itself. Field lines can be drawn to
try to represent this magnetic field. The arrows on a field line point from
North to South and show the direction a North pole would move. Field lines
never cross. Where they are closest together, the field is strongest.
- Mars SP
- Planet Mars has the fourth most distant orbit from our star the Sun. It is slightly smaller than Earth and has a thinner
atmosphere, made of carbon dioxide. Mars has carbon dioxide ice caps at its
north and south poles.
- Mass TR, SP
- Mass refers to the quantity of matter present in an object. The SI unit of mass is the kilogram. In
actual fact there are two kinds of mass - inertial mass
and gravitational mass. Gravitational mass describes how an object responds to
(and creates) a gravitational field.
- Mercury (metal) HP
- Mercury is a metal which is liquid at room temperature. It is often used
in liquid in glass thermometers. It is a toxic substance. Chemicool web-page for
- Mercury (planet) SP
- Planet Mercury has closest orbit to our star the Sun.
It is much smaller than Earth and has no atmosphere. It is very hard to see
Mercury from Earth, due to it being very close to the Sun.
- Meteor SP
- Commonly called 'shooting stars', meteors are tiny specks of dust which the
Earth sweeps into in its orbit. These dust grains enter the Earth's atmosphere
at up to 70km per second - very fast! They cause the atoms
in the atmosphere which they hit to glow with the heat produced as the meteor
slows down. The meteor is destroyed by this process. Many meteors are
particles which are left behind in the orbit of comets as they pass through the
inner solar system.
- Meteorite SP
- This is a lump of material which manages to reach Earth's surface, having
only partially burned up on the way down.
- Meteroid SP
- These are small lumps of material found in space. They range from less
than 1mm across, up to tens of metres. Only if a meteroid enters Earth's
atmosphere it is called meteor.
- Metre TC, HP, TR, EM, SP
- The metre is the SI unit of distance. It is
defined as distance light travels in the fraction 1 / 299 792 458 of a
- Microphone TC, EL
- A microphone is an input device in an electronic system. It converts sound
energy into electrical energy. It is therefore a transducer.
- Microwaves TC, SP
- Microwaves, along with light, UV, x-rays and gamma radiation form part
of the electromagnetic spectrum. The wavelength of
microwaves is longer than that of infra-red radiation, but shorter than that of
radio waves. Microwaves are used commonly in mobile phones
and in microwave ovens.
- Milky Way SP
- This is the name of the galaxy we live in. There are about 100 billion
stars in the Milky Way. The Milky Way is a spiral galaxy, made up of several
arms. Our arm is imaginatively called the 'local arm' (by us at least). The
Milky Way actually has a few smaller satellite galaxies, two of them are the
Large and Small Magellanic Clouds, visible from the southern hemisphere.
- Miniature Circuit Breaker (MCB) UE
- Instead of using a fuse to protect an electrical flex,
a miniature circuit breaker (MCB) can be used instead. MCBs are sometimes known
as Earth Leakage Circuit Breakers (ELCBs) or Residual Current Devices (RCDs).
The MCB has several advantages over fuses as follows:
- MCBs can be reset once the fault is eliminated, whereas a fuse must be
- MCBs cut-off the current very quickly in comparison
- Some designs of MCB are tamper proof in that the cut-off current can not
be altered. With a fuse, it is always possible to use a too-highly-rated
Disadvantages of the MCB are as follows:
- MCBs cost more than fuses
- MCBs require regular testing to ensure they will work
- The cut-off current of the MCB can be affected by temperature
The Australian government's website has more information on RCDs, useful for background
- Moderator EM
- In a nuclear fission reactor, the moderator slows fast neutrons down.
Slower moving neutrons are more effective at keeping the chain reaction going by
causing more uranium nuclei to break apart. Each fission event releases fast
neutrons - the moderator is therefore required to slow them down.
- Modulation TC
- Modulation means change. In physics, this is usually used to
describe changes made to the properties of a radio-wave or microwave
carrier wave in order to carry information. There
are two types of modulation - amplitude
modulation and frequency
- Morse Code TC
- Morse Code is a versatile and simple method of transmitting information.
It uses any method of telecommunication (such
as flashing lights, tap keys and wires or radio transmissions) to send
information coded as sequences of dots and dashes (e.g. short and long pulses of
voltage or flashes of light). This is its great advantage over any other means
of information transmission: morse can be sent with extremely simple equipment.
Its disadvantage is that it has a very slow rate of transmission and takes
considerable practice to use properly. The codes are shown in Telecommunications Activity 3
- Motor (Diagram) UE,
- Motors convert electrical energy into kinetic energy,
along with some unwanted sound and heat energy.
A simple motor, which can run on d.c. electricity, has a
rotor coil and permanent field magnets. As the rotor coil spins, the direction
of the current flowing in the coil is reversed by means
of a split-ring commutator. Carbon brushes allow the commutator to slide whilst
maintaining an electrical connection. Carbon brushes are used as carbon is a
good conductor and is hard-wearing.
In commercial motors, the permanent field magnets are replaced with
electromagnets called field coils. This allows a more powerful magnetic field
to be created than would be possible using permanent magnets. Also, the single
rotor coil of the simple motor is replaced by a number of rotor coils (called a
multi-coil or armature) and a segmented commutator
replaces the split ring commutator. Each coil of the armature is connected to a pair of contacts in the
segmented commutator. This allows for a smoother rotation of the motor. Note
that in a commercial motor, because the magnetic field is produced by
electromagnets, the motor will work equally well with a.c. or
d.c. electricity. This is because a reverse current in the field coil will
coincide with a reversal in the armature current.
Therefore the armature will be forced round in the same
To read more about how motors work, try "How Stuff Works"
- Myopia (Myopic eye) HP
- See under short-sighted.
- National Grid EM
- This is a network of power lines which connect together all the mainland UK
power stations. Grid controllers can monitor the demand for electrical energy
across the UK and allow more power to be diverted to areas with high
- Near-sighted HP
- See under short-sighted.
- Neptune SP
- Planet Neptune usually has the eighth most distant orbit from our star the
Sun. It is gas giant, although smaller than Jupiter or
Saturn. It was discovered after observing abnormalities in Uranus' orbit.
Calculations were made and Neptune was observed where the figures said it had to
be. Neptune has three very faint rings (not visible from Earth). Sometimes
Pluto is closer to the Sun than Neptune.
- Neutron HP, EM, SP
- Neutrons are sub-atomic particles. They are uncharged
They have a mass of mn = 1.675 x 10-27 kg.
- Neutron Star SP
- The remnant of a dead star after it has exploded in a supernova. Neutron
stars are extremely dense and have intense graviational fields, so strong that
all atoms are crushed, forcing their nuclei together.
Electrons and protons from the smashed atoms are crushed
together to form neutrons. Neutron stars are a few kilometres across and have
masses comparable to that of the Sun! (100 million tonnes per teaspoonful)
- Newton TR, SP
- Sir Isaac Newton, 1642-1727. Newton tried farming before going on to
develop his three Laws of Motion (see below). Newton also contributed to other
branches of science and maths.
- Newton TR, SP
- The newton (N) is the SI unit of force. An unbalanced force of 1 newton will cause a mass of 1 kg to accelerate at 1ms-2
- Newton Balance TR
- This is a spring balance (called a pundler in Shetland). It can be used to
measure force, including weight
forces. Remember all forces are measured in newtons!
- Newton's First Law TR, SP
- If the forces acting on an object are all balanced,
then the object will not change in its motion. It will not speed up, slow down,
change direction or shape. IF it is not moving, then it will stay
- Newton's Second Law TR, SP
If forces are not balanced on an object then it will
either speed up, slow down or change direction - this means it will have to accelerate. Remember that a change of direction is also
an acceleration, even if the speed does not change. The formula for this law is:
F = ma
F is the force in newtons, m is the mass in kilograms and a is the acceleration in ms-2
- Newton's Third Law TR, SP
- For any force in a situation, there will be an equal
and opposite force. For example if you stand on some grass, then your weight
force pulls you down and the grass pushes you up with the same force, but in the
opposite direction. This means that the forces are balanced - so by Newton's first law - you don't move.
- Non-Renewable Energy Sources
- Fossil fuels are termed non-renewable sources because they will run out one
day. See also renewable energy sources.
- Normal Line (Diagram) HP
- In optics and wave applications, the normal line is
an imaginary line drawn at right angles to a surface. If the surface is curved,
then the tangent to the surface must be drawn first, with the normal being drawn
at right angles to the tangent.
- NOT gate (inverter) EL
- See under inverter
- Nuclear Chain Reaction
- In a nuclear fission power station (or a nuclear fission bomb) an unstable
nucleus of uranium is hit by a neutron, causing it to break up, releasing a
small quantity of energy plus several more neutrons. These new neutrons go on
to hit more nuclei, causing them to break up, releasing even more energy and
neutrons. This quickly builds up to a nuclear chain reaction. In a power
station, control rods are used to regulate the rate of energy release. In a
fission bomb, the chain reaction is allowed to proceed out of control with
- Nuclear Energy EM
- Nuclear energy is the energy contained within the nucleus of an atom. It can be released in nuclear fission or nuclear fusion.
Much of the world's electricity is produced using nuclear fisson power
stations. Fusion power remains elusive, although progress is slowly being
- Objective Lens SP
- In a refracting telescope the objective lens is the large lens which
collects the dim light from objects in the night sky. The purpose of the
objective is to collect as much light as possible. It does not magnify the
image (see eyepiece). With a larger objective, the image will be brighter and
fainter objects will become visible.
- Optical Fibres TC, HP
- Optical fibres rely on the principle of total internal
reflection for their operation. Optical fibres are made from extremely
thin, flexible hairlike strands of glass. They allow light to be sent over
great distances and round corners. This makes possible devices such as the
medical endoscope (or fibrescope) and various high
capacity data communications. In communications applications, fibre optics have
several advantages over copper wires - they are lighter in weight, more
difficult to eavesdrop and require far fewer booster stations.
- Open Circuit UE, EL
- This is one of two types of circuit fault (the other being a short circuit). In an open circuit fault, there is a
break in the circuit. This break will prevent any current from flowing. Open circuit faults register on an
ohmmeter with an extremely high (ideally infinite) resistance.
- Ohm (W) UE, EL
- The ohm is the SI unit of resistance. The resistance of an electrical component is
one ohm (1 W) if, when a voltage of 1 V applied across it causes a current of 1 A to flow through it.
- Ohm's Law UE, EL
- Georg S. Ohm discovered that for a conductor at
constant temperature, the current which flows through the
conductor is proportional to the voltage across it. The
constant of proportionality is called the resistance
of the conductor. The law is usually written as:
V = I R
- OR Gate EL
- An OR gate is a logical device. It usually takes two inputs (although more
are possible) and produces only one output. The logic
state of the output depends on the logic state of the inputs. This is shown
in the truth table below. A and B are the inputs and
Z is the output state.
From this it is easy to see that the output is on if at least one of the
inputs is on.
- Oscilloscope TC, UE, EL
- The oscilloscope provides a visual way of investigating electrical signals.
In standard grade, you need to understand about the voltage gain and timebase
controls of the oscilloscope. The vertical scale shows the voltage of the
signal - its scale can be modified by changing the Y-gain setting (in V/div).
The voltage can then be deduced by calculating V = Y-gain x div, where 'div' is
the number of divisions on the oscilloscope screen. The horizontal scale shows
time and its control is called the timebase (or X-gain) (in ms/div). The period
T of a wave-like signal can be calculated from T = timebase x div, where 'div'
is the number of divisions taken to draw one full cycle.
- Parallel Beam (Diagram) TC, SP
- A parallel beam is one which does not diverge or converge - its width
stays the same as the wave (which could be light or microwaves for example) travels out from its source.
Parallel beams are produced using curved (parabolic)
reflector dishes (e.g. in car headlamps, satellite transmitter dishes,
mobile phone mast relay dishes). Parallel light beams can also be produced
using convex lenses (e.g. in binoculars or telescopes). Parallel beams can be
used to send waves over much greater distances than would otherwise be possible.
This is because the wave energy stays much closer together when reaching its
destination. However, this would be no use for sending a signal out which was
intended to cover a wide area.
In astronomy, the light from stars has come from such a great distance that
the starlight does effectively form a parallel beam travelling down the tube of
- Parallel Circuit UE, EL
- In a parallel circuit, each component is connected directly across the
terminals of the power supply. Therefore, the voltage across any component is
the same as the voltage across the power supply terminals. The rules for current, voltage and resistance in a parallel circuit are as follows:
Vsupply = V1 = V2 = V3 = ...
Isupply = I1 + I2 + I3 + ...
1/Rtotal = 1/R1 + 1/R2 + 1/R3 +
Note that the calculation for total resistance in parallel circuits requires
an extra step to get the final answer. Here is an example calculation for a 30
ohm and a 40 ohm resistor connected in parallel:
1/Rtotal = 1/R1 + 1/R2
1/Rtotal = 1/40 + 1/30
1/Rtotal = 3/120 + 4/120 (change to common denominator)
1/Rtotal = 7/120
Rtotal/1 = 120/7 (extra step)
Rtotal = 17.14 W (remember the
Calculators with a reciprocal button can make these calculations easier.
See also: Series circuits
- Period TC, SP
- Satellite period is the time taken for a satellite to make one complete
orbit around its parent body (e.g the time taken for the Moon to orbit the Earth
is called the Moon's period). Satellite period is usually measured in hours or
Wave period is the time taken for one complete wave to pass
a point. The SI unit of wave period is the second (s).
A closely related concept is that of frequency. See
- Peak (Diagram) TC,
- This refers to waves. The peak of a wave (also called
the crest) is simply the part of the wave with the greatest amplitude.
- Peak Voltage UE
- This the maximum voltage reached by an a.c. source in each cycle. It is very similar to the peak or crest of a wave. Compare with the r.m.s voltage of an a.c. source.
- Persistence of Vision TC
- This is the effect which allows the human brain to be fooled into thinking
that the 25 frames per second of TV is actually continuous motion. The retina
retains each image for a fraction of a second and so
provided that the changes between the frames are small enough, smooth motion
will be seen. (The phosphor dots which make up the screen also continue to emit
light for a short time after the electron beam has
passed - this effect also helps to give the impression of a smooth motion)
- Photographic_film HP
- Photographich film can be used to detect visible light, x-rays, or alpha, beta and gamma radiation. These radiations will blacken film due to a
chemical reaction. Photographic film badges are used by medical and nuclear
workers to monitor their level of exposure to
- Planet SP
- Our solar system has nine planets, including the Earth. Pluto's status as
a planet is a subject of current debate, with many scientists pointing out that
it is really just a big rock. The table below shows some data for the planets.
A planet does not produce its own light but shines by reflected star-light
(sunlight!). The word 'planet' literally means 'wanderer' due to the way the
planets move across the night sky. From Earth, we can easily observe Mars,
Venus, Jupiter and Saturn using just binoculars or the naked eye. With 20x60
binoculars, firmly mounted it is also easy to observe Jupiter's four Galilean
moons - a treat well worth the effort- they will be visible as tiny pin-points
of light all in a row (but you may not see all four at once). These moons move
around pretty rapidly and if you sketch their positions, you should easily see
them shift position after several hours.
|Name||Distance from Sun
(approx)/AU||Period||Day/hr||Mass/ mass of
Earth||Radius / radius of Earth|
Mercury, Venus, Earth and Mars are called 'inner planets'. Jupiter, Saturn,
Uranus, Neptune and Pluto are 'outer planets'.
- Plug (Diagram)
- A plug is correctly wired as follows:
Green/Yellow insulated core to the earth terminal
Brown insulated core to the live terminal. The live terminal can be
identified by the fuse. The fuse and the switch should
always be connected to live as it is the live which supplies electrical energy to the device. If the fuse and switch were connected
to neutral, the appliance could appear to be off, yet still be connected to
Blue insulated core to the neutral terminal.
- Pluto SP
- Planet Pluto usually has the furthest orbit from our star the Sun. It is the smallest planet, really just a large lump of
rock. Its status as a planet has been challenged by some, due to its similarity
to other similar sized objects. Currently the IAF still list Pluto has a
planet. Due to its highly elliptical orbit, Pluto is nearer to the Sun than
Neptune for part of its orbit.
- Potential Divider (potentiometer)
- A variable resistor, when used with all
three of its terminals, it is called a potentiometer or potential divider. This
arrangement is for controlling voltage. See also: Rheostat.
- Potential Energy TR, EM, SP
- Potential energy means stored energy. Some examples
are chemical, elastic, nuclear and gravitational - all forms of potential
energy. For Standard grade, you should know the formula for gravitational potential
- Power UE, EL, TR, EM, SP
- Power is the rate of transfer of energy. The SI unit of power is the watt (W). One
watt is the same as one joule per second
The formula which relates power (P), energy (E) and time (t) is:
E = P x t
In electrical applications, there are other equations for power, which
involve current, voltage and resistance. They are:
P = I V
P = I2R
P = V2 / R
- Power Gain UE, EL
- Power gain has no units of its own. It expresses how many times more power
the output signal from a process has compared to its input power. A closely
related concept is that of voltage gain. Power Gain = Output Power / Input
- Power (of a lens) HP
- Lens power describes the curvature of a lens. High power lenses have a
short focal length and are highly curved. A positive lens power indicates a
convex lens and negative power indicates a concave lens. Lens power, P, is
measured in Dioptres (D) and is related to the focal length, f, in metres
through the formula:
P = 1 / f
- Presbyopia (presbyopic eye) HP
- See under long-sighted.
- Primary Colour (Diagram) TC
- There are three primary colours (of light): red, green and blue. Any two
of these colours can be mixed to give a secondary colour as follows:
Red + Green = Yellow
Red + Blue = Magenta (pink)
Green + Blue = Cyan (Turquoise)
If all three colours are mixed together, white light can be obtained.
Note that these are the primary and secondary colours of light. The primary
and secondary colours of pigment (paint) are not the same.
- Primary Mirror SP
- In a reflecting telescope, the primary mirror collects the light from the
objects under study in the night sky. Reflectors can be made much bigger than
the objective lenses of a refracting telescope. They are also easier to make,
since their is only one optical surface to prepare. The primary mirror itself
is usually made from glass, with an aluminium coating on its forward face
(unlike the mirror in your bedroom). The mirror is made as large as possible
for the same reasons as for the objective lens of a refractor. See also
- Principle of
Reversibility TC, SP
- This applies to light rays passing through any system of mirrors or lenses.
Basically, it means if a ray of light follows a path when going, say, from left
to right, then another ray of light going from right to left will follow the
same path in reverse.
- Proton HP, EM
- Protons are sub-atomic particles. They carry a
positive electrical charge (qp = +1.602 x
10-19 C). They have a mass of mp = 1.673 x 10-27 kg.
- Pumped Storage EM
- At times of low electricity demand, typically at night, the National Grid
has overcapacity. Since it is difficult and expensive to shut down power
stations, this extra energy would have to be wasted. However, by using it to
pump water back uphill into reservoirs, a small percentage of this energy can be
stored as gravitational potential energy of the water. The process is
inefficient, but dumping the excess power would be 0% efficient.
- Quasar SP
- Quasar means 'quasi-stellar radar source'. These are objects which look
like stars but are not. (They are only visible in big telescopes, because of
their great distance from us. A typical quasar differs most noticeably from a
star because it gives off the same intensity of light as would one million
million suns. This is quite impressive, but even more impressive is the fact
that quasars spew out much more radiation at radio wavelengths. Quasars are
amongst the most distant objects observed from Earth.
- Radar Gun TR
- A hand-held device used by police to measure the speed
of passing cars.
- Radiation HP, EM
- The word 'radiation' can refer to electromagnetic radiation (including gamma radiation) or to alpha and beta particles released from radioactive decay. Radiation can
cause ionisation of neutral atoms, by
knocking away electrons. This can damage or kill cells. In this way, radiation can cause cancer and be used to treat cancer.
- Radiation (heat) EM
- Heat radiation is a part of the electromagnetic spectrum, conventionally
called 'infra-red'. Heat can be transferred from a hot object to a cold one by
radiation, convection or conduction. Transfer by radiation is possible in a
vacuum (or in presence of matter).
- Radio TC
- A radio is an electronic system which
allows a listener to hear programmes sent from a transmitter. The main components of a radio (in
order) are: the aerial, tuner, decoder, amplifier and loudspeaker.
- Radioactive HP
- A material is said to be radioactive if it emits radiation. Radioactive materials have unstable nuclei which have a tendency to decay. It is when a decay
occurs that radiation is released from the nucleus. Important related concepts
are the activity and half-life
of a source.
- Radioactive Decay HP
- This refers to the disintegration of an unstable nucleus. The number of disintegrations per second taking place
in the sample is called the activity of the sample. Radiation is given out when a disintegration takes place.
Considering each nucleus on its own, radioactive decay is a random process.
There is no way of knowing when a particular nucleus will decay - it may decay
within the next millionth of a second, or it may not decay for another billion
years! This isn't a measurement problem - the information just doesn't
However, when you take a very large number of nuclei, the number of nuclei
decaying (i.e. the activity) becomes predictable. Once
a nucleus has decayed and given out radiation, the remains of the nucleus will
still be in whatever material it started in. However that nucleus will not be
able to decay in the same way again. (You can only smash a cup once, but you
can go back over and stamp on all the bits again!) This means that the level of
activity will gradually drop away over time as the
nuclei all decay. For some elements, the activity will
drop away to background very rapidly, for
other elements activity drops more slowly. Half-life is a measure of how quickly the activity level drops.
- Radioactivity HP
- See Activity.
- Radiometric Dating HP
- This is the name given to the general technique of using half-life to find out the age of samples of rock, fossil
or material which is organic in nature. Carbon dating is used to date organic
material up to about 30 000 years.
- Radio Telescope SP
- Astronomers use radio telescopes to help build up a picture of the
Universe. Since radio waves can penetrate Earth's atmosphere, we can detect
them by building large antennae or dishes. The radio part of the spectrum
provides an alternative and complimentary view of the Universe - importantly
radio waves convey information about much lower energy events than visible light
does, much of what a radio telescope 'sees' will be not appear in visible light
- Radio Transmitter TC
- A source of radio waves. Radio transmitters are
usually large vertical conducting masts which emit
radio waves. Moving electrons in the metal of the mast
radiate the radio waves.
- Redshift SP
- This refers to the fact that the spectrum of distant galaxies is shifted
towards the long wavelength 'red' end of the electromagnetic spectrum.
(Redshift is caused by Doppler shift, beyond scope of standard grade physics).
Bigger redshifts mean that the object is moving away from us faster. An
astronomer by name of Edwin Hubble discovered that the more distant galaxies are
moving away from us the most quickly (he measured the greatest redshifts for the
farthest galaxies). This means that the Universe is expanding and leads
directly to the currently popular Big-Bang theory of the origin of the
- Reed Relay UE, EL
- A reed switch, controlled by an electromagnet. Relays can be used for remote control
of electrical circuits in dangerous places. For example, a circuit within a
nuclear reactor can be switched on or off with a reed relay. In general, relays
can be used to switch high current circuits using low current circuits. In this
way, control panels can be made safer for their operators because the more
dangerous high current circuits can be separated from the control panel
- Reed Switch UE, EL
- A simple switch, controlled by a magnet. The reed switch can be designed
either to make or break contact when a magnetic field is present. Reed switches
can be used (with permanent magnets) as door switches, for example to switch on
an oven light when the oven door is opened, or to switch off a microwave oven
when the door is opened. In a reed relay, an electromagnet is used to control switch.
- Reflection (Diagram) TC, HP, SP
- All waves can be reflected from an obstacle - for example, water waves can
reflect off a harbour wall and light waves can reflect off a mirror. The law of
reflection is very simple:
The angle of incidence is equal to the angle of reflection.
- Refraction (Diagram) TC, HP, SP
- All waves can be refracted. Refraction means bending. Light rays refract
when they pass through a medium of different density (e.g. when light
travels from air into glass). When light passes into a region of increased
density (e.g. air to glass) it bends towards the normal. When light passes into a region of reduced
density, it bends away from the normal.
- Renewable Energy Sources EM
- Solar, wind, geothermal, tidal and wave power plants are examples of
renewable energy resources. They will not run out, unlike fossil fuels.
Currently political will exists to develop these sources and good progress has
been made. Shetland already uses solar and wind plants to generate non-trivial
power. There are plans to develop an off-shore wave power station too. For
solar power, the following link is noteworthy, although a quick search on google
will yield more. Solar power
- Resistance UE, EL
- The electrical resistance of a component (e.g. a lamp) is the
component's opposition to the flow of current. The SI unit of resistance is the ohm (W). See also: Ohm's Law
- Resistor UE, EL
- A resistor is an electronic component which opposes the flow of current.
See resistance and variable resistors.
- Rheostat UE, EL
- A variable resistor, when used with just
two of its terminals, it is called a rheostat. This arrangement is for
controlling current. See also: potential divider.
- Ring Circuit UE
- This is a special type of parallel circuit,
used in household wiring. The advantage of the ring circuit is that thinner,
cheaper cable can be used. This is because there are two separate paths to any
socket. Therefore, if each cable is capable of carrying up to 18 A, then the
maximum current the ring can draw from the consumer unit is 36 A.
- R.M.S. Voltage TC, UE, EL, EM
- This is a kind of average voltage of an a.c. source (averaged over a complete cycle). It is the effective voltage delivered by the source
and can be used in power calculations (P=IV etc). Compare
rms voltage with the peak voltage of an a.c. source.
In the UK, the rms voltage of a.c. mains is quoted at 230
- Rocket Engine TR, SP
- Rockets are propelled upwards by ejecting exhaust gases. Because of
Newton's third law, as the rocket pushes the exhaust gases down, the exhaust
exerts an equal and opposite upwards force on the rocket. It is much easier to
understand rocket motion using the concept of momentum, which you will meet in
- Rotor Coil UE, EM
- In an a.c. generator, the rotor coil is a rotating
electromagnet, with a d.c. power supply. It replaces the permanent rotating
magnets of a dynamo.
- In astronomy and space science, a satellite is simply any object which is
in orbit about a larger parent orbit. The force required to keep the satellite
in orbit is the graviational attraction between the satellite and parent object.
The moon and the ISS are
satellites of Earth. Earth is a satellite of the Sun.
- Planet Saturn has the sixth most distant orbit from our star the Sun. It is slightly smaller than Jupiter and is also a gas
giant planet. Saturn's ring system is easily seen in a small telescope from
- The second is the SI unit of time. It is
defined as the time taken to produce 9 192 631 770 complete wavelengths of electromagnetic radiation from a
particular event in cesium atoms.
- Sievert (Sv)
- The sievert is the SI unit of dose equivalent. Since 1 Sv is a massive dose of
radiation, units of microsieverts are often used instead.
- Scintillation Counter
- These devices detect gamma rays and form part of the
gamma cameras used in radioactive tracer examinations. Scintillations are flashes of light
given out by fluorescent materials (e.g zinc sulphide. When radiation is absorbed by zinc sulphide, it causes the zinc
sulphide to produce a small burst of light. These can be electronically
- Series Circuit
- In a series circuit, each component is connected to the other to form a
'chain' between the terminals of the power supply. The rules for current, voltage and resistance in a series circuit are as follows:
Vsupply = V1 + V2 + V3 + ...
Isupply = I1 = I2 = I3 = ...
Rtotal = R1 + R2 + R3 + ...
The mnemonic VISA may help to remember the rules for voltage and current in
series and parallel circuits:
VISA: Voltages In Series Add
- Short Circuit
- This is one of two types of circuit fault (the other being an open circuit). A short circuit fault is often caused
by wires touching, allowing current to bypass a component
in the circuit. Short circuit faults register on an ohmmeter with almost zero
- This refers to a problem with the accommodation ability of an eye.
Basically, the eye can focus on near-by objects with no problem, but can't
properly focus on far-away objects. This is called myopia. This is because the
retina is too far from the lens - so that the focal point falls short of the
retina. Short-sightedness can be treated by wearing spectacles which are
concave in shape and therefore help to focus the image properly by extending the
overall focal length of the glasses and eye.
- SI Units
- SI stands for Systeme International. This is the standard system of units
used worldwide and includes the metre, the second, the kilogram, the ampere, the kelvin, and the coulomb.
More information is available from the University of
Alberta, CA or the University of Exeter,
- A solenoid is a coil of wire with an electrical current flowing through it. An iron core, inserted into the
coil, has the effect of increasing the strength of the magnetic field produced.
An increased current or a greater number of turns on the coil will also increase
the strength of the magnetic field. See also: Electromagnet
- Solar Cell
- A solar cell is a photovoltaic semiconductor material which converts light
energy into electrical energy. Solar cells are transducers.
- A longitudinal wave which travels through a medium,
usually air. Sound also travels through liquids (e.g. water) and solids (e.g.
steel). The volume (loudness) of sound is measured using the decibel (dB)
- Specific Heat Capacity
- The energy required to increase the temperature of 1kg of a material by 1
degree celsius. This is also the energy released during cooling. Water has a
high specific heat capacity at 4200 Jkg-1 oC-1.
Metals have much lower heat capacities. Physically, this is why metals heat up
and cool down more rapidly than water - less energy is required to bring about a
given change in temperature.
- Specific Latent Heat of
- This the latent heat per kilogram of material required to change its phase
from solid to liquid (i.e. heat energy required to melt 1 kg of material). It
is also the heat energy released (in joules) if 1 kg of material 'freezes' from
liquid to solid.
- Specific Latent Heat of
- This the latent heat per kilogram of material required to change the
material's phase from liquid to gas (i.e. heat energy required to boil or
vapourise 1 kg of material). It is also the heat energy released (in joules) if
1 kg of material condenses from solid to liquid.
- A device using the diffraction of light to split the visible spectrum up
into the rainbow of colours. The spectroscope is used in astronomy to study the
spectra of light from the stars.
- (Plural: spectra) Usually refers to the 'rainbow' of colours seen by the
human eye, Violet, Indigo, Blue, Green, Yellow, Orange, Red
(in order of increasing wavelength). It is important
to remember that this visible spectrum is only a tiny part of the full electromagnetic spectrum.
- Speed is the rate at which distance is covered. The SI unit of speed is
the metre per second. Other units include feet per second, kilometres per hour
and miles per hour.
The simple formula for movement at constant speed is: distance = speed x
For waves, the formula v = f l can be used.
- Speed Time Graph
- This is a graph with time plotted along the horizontal axis, and with speed
up the vertical axis. You need to be able to interpret these graphs for
Standard grade. The area under the graph is the total distance travelled.
- The electromagnetic spectrum includes all types of electromagnetic
radiation(EMR). All types of EMR travel at the same speed: the speed of light,
which is 3 x 108 ms-1. Starting with the lowest frequency
and longest wavelength radiation, the spectrum consists of
The visible spectrum includes red, orange, yellow, green , blue, indigo,
violet. Red light has a wavelength of approximately 700 nm and violet light's
wavelength is roughly 400 nm. All other colours have a wavelength between these
- A star is a hot ball of gas which shines by producing its own light. The
closest star to Earth is called the Sun. The next closest is Proxima Centauri
at around four light-years distant.Other types of stars include red giants,
white dwarfs and neutron stars.
- Stator Coil
- In a dynamo, the stator coil is the the fixed coil which has an electric
current (a.c.) induced in it by the motion of the nearby rotor
- The stethoscope is a device which allows a doctor or nurse to listen to
sounds coming from within a patient's body. The sounds can be used to diagnose
medical conditions. The two bells of a stethoscope are connected to the
earpieces via a flexible hollow tube. The bells can be selected by turning a
valve which allows only sound from one of the bells to enter the tubing at a
time. The open bell is used to listen to lower frequency (e.g. heart) sounds and the closed bell is used
to listen to higher frequency (e.g. breathing sounds).
- Streamlined objects are designed to reduce fluid drag forces. All rough or
angular edges are removed, leaving only smooth curved surfaces. Fish, dolphins,
falcons, air interceptors and racing cars are good examples of streamlined
- The Sun is our nearest star. It is a dim yellow dwarf star, not
particularly big or bright by astronomical standards. It has a mass of
2x1030kg and diameter 110 times that of the Earth. As an example for
comparison, Betelgeuse in Orion has around 20 times the mass of the Sun, and a
diameter 290 times that of the Sun. Also in Orion, the bright star Rigel is
intrinsically 60 000 times brighter than the Sun.
- Communication (by any means) over a distance
- A telescope is a device used in astronomy, including radio and optical
telescopes. In a refracting telescope, an objective lens is used to collect
light. The biggest optical telescopes are of the reflecting type, using a
primary mirror to collect light. The aperture of the telescope determines how
good it will be for observing faint objects. For use by a human, all optical
telescopes also require an eyepiece lens.
- Total Internal Reflection (TIR) (Diagram)
- Total internal reflection occurs when light, instead of being refracted at a boundary between media of different
densities, the light is totally reflected off the
inside surface. The usual case is inside glass, where the light ray bounces off
the inside of the glass instead of escaping out. This effect is the basis of fibre optics - used in communication and medicine.
Total internal reflection occurs only when the angle of incidence is above a certain value
called the critical angle.
- Thermistor (Diagram)
- Thermistors are designed so that their resistance
depends on temperature.
When subjected to low temperatures, the resistance of a thermistor is
When subjected to a high temperature, the resistance of a thermistor is
- When two different metals are joined together, they will convert heat
energy into electrical energy. This is the basis of the thermocouple.
Thermomcouples are transducers.
- Radioactive tracers are injected into a patient
to examine the function of organs and blood supplies within the body.
Technetium is often used as it emits gamma radiation which
can be detected outside the patient's body. A gamma
camera is used to detect the radiation as it travels around the parts of the
body of interest.
- Any device which converts one form of energy into another. For example, a
microphone, a TV, a car or a solar cell.
- A semiconductor device which can be used as an electronic switch (popular
exam question). In standard grade physics you need to understand the operation
of the NPN type transistor only.
- A device which consists of two separate coils. Transformers only work with
a.c. and can be used to step voltages up or down. A popular
exam topic, transformers are well worth some study time.
- Trough (Diagram)
- This refers to waves. The trough of a wave is simply
the part of the wave with the least (most negative) amplitude.
- Truth Table
- A table showing the output logic states of a logic gate or logic system for
all given combinations of input logic states. Input states should be listed
systematically to avoid confusion. As examples, have a look at the truth tables
for the AND, OR and NOT gates. As a further example,
here is the truth table for a 3 input XOR gate (exclusive OR), showing how to
list the inputs in the correct order.
- Tuner (Radio or TV)
- The tuner in a radio or TV allows the user to select
one channel from the many channels which arrive at the aerial. The output
signal from the tuner section is then passed to the decoder.
- Two-stage Rocket
- Essentially, two rockets one on top of the other. The top rocket will not
ignite until the first rocket is finished and jettisoned. The advantage over a
big single stage rocket is that all the dead weight of the empty first stage can
be cut away and left to fall back to Earth. This can greatly reduce the total
mass of fuel required to reach orbit.
- Ultrasound is sound which has a frequency (or pitch) greater than 20 kHz. This is above
the maximum frequency which a human can hear. Many other animals however can
hear sounds with a frequency above 20 kHz. Ultrasound can be so high pitched
that no animal can hear it. Here are several application areas of ultrasound:
All these applications rely on the fact that ultrasound is reflected when the density of the material through which
they pass changes (in other words, they echo back when passing from skin to
- Animal navigation
- Submarine navigation
- Fishing - detecting shoals
- Medical - looking inside the body without need of surgery. Often used
- Ultraviolet (UV) radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just shorter
than that of visible violet light. UV radiation can be used to treat skin
conditions like acne. Humans use UV light from the Sun to produce vitamin D3
(used in the kidneys and liver). UV light causes sun-tans, but too much can
cause skin cancer.
- The Universe in which we live consists of several hundred billion galaxies.
These galaxies are grouped as superclusters and clusters. Astronomers
currently favour the Big-Bang theory of the origin of the Universe, with recent
results (of cosmic microwave background radiation) tending to support this
- Planet Uranus has the seventh most distant orbit from our star the Sun. It is slightly smaller than Jupiter and Saturn and is also
a gas giant planet. Uranus has very faint rings around it (not visible from
- Variable Resistor
- Variable resistors are resistors whose resistance can be altered, usually by means of a rotating
or sliding contact. They can be used as potential
dividers or rheostats.
- The volt is the SI unit of voltage. In fact 1 V = 1 JC-1 (i.e. a charge of
one coulomb will gain or lose one joule of potential energy when moved through a
potential difference of one volt).
- Planet Venus has the second most distant orbit from our star the Sun. It is slightly smaller than Earth. In many ways it is our
sister planet - it receives similar heat energy from the sun (twice what Earth
receives), with a similar internal structure to Earth. Its atmosphere is quite
different, with 90 times the pressure of ours, and consisting of very corrosive
gases. Its surface temperature can be well over 400 degrees celsius, due to a
runaway greenhouse effect.
- Voltage is an electrical concept. In circuits, voltage is a measure of
the electrical energy available between two points.
Voltage must be measured between two points in a circuit - e.g. "The voltage
between point X and point Y is 3 volts". Voltages are measured using a device
called a voltmeter. It is not necessary to break into a circuit in order to use
It is wrong to say: "The voltage flows into the lamp". Voltage does not
flow. It is current that flows. A difference in voltage
between the terminals of a lamp causes current to flow through the lamp. (An
analogy for voltage is the difference in heights between two points on a
hillside. This difference in heights (voltage) causes water to flow downhill
(electrical current). One would not speak of height flowing into a
Voltage, current and resistance are related through Ohm's
- Voltage Divider
- A simple circuit branch consisting of two resistors connected in series.
The voltage accross the terminals of the branch is shared between the two
resistors, with the bigger resistor getting the biggest share. Because the
resistors are connected in series, the current through each is the same. The
easiest way to calculate the voltage division for given resitances is to use
division in ratios as follows:
Problem: Given resistors of 100R and 200R, connected in series, calculate
the voltage division when connected to a 12V power supply.
Solution: 100:200 is equivalent to 1:2, so split the 12V supply voltage into
1+2=3 parts. Hence 1 part = 12/3 = 4 V and then 2 parts = 8 V. Do a quick
check to see that 4+8=12V and then the answer can be given: the 100R resistor
has a p.d. across it of 4V, the 200R p.d. is 8V.
- Voltage Gain
- Voltage gain has no units of its own. It expresses how many times greater
the output signal voltage is (from some component) compared to the input
voltage. A closely related concept is that of power gain. Voltage Gain =
Output Voltage / Input Voltage.
- Watt UE, EL, TR, EM
- The watt (W) is the SI Unit of power. It is equivalent to one joule per
second (Js-1). Another common unit for power
is the horsepower (hp). 1 hp = 746 W.
- Wave (Diagram)TC, UE, HP, SP
- A travelling disturbance, carrying energy from
place to place. Examples are water waves and sound waves
where it is the movement of particles which constitutes the wave motion. In the
example of light waves and other electromagnetic waves it is changing electric
and magnetic fields which carry energy from place to place.
- Wavelength, l (lambda)
(Diagram) TC, UE, HP, SP
- This is the length of a wave between any two equivalent points. It is
easiest to measure from crest to crest or from trough to trough. The SI unit of wavelength is the metre,
although kilometres, millimetres, micrometres and nanometres are also used.
See also: cycle.
- Weight TR, EM, SP
- Weight is a force. The SI
unit of force is the newton - not the kilogram!
- Wind Turbine EM
- Work TR, EM
- When a force acts on a moving object, that force does mechanical work. The
work done is defined by the equation:
Work Done = Force x Distance
The SI unit of work is the same as that of energy - the joule
The force only does work when it is at least partially aligned with the
direction of movement. If the force acts at right angles to the direction of
movement, then that force does no work. Work done can be used to calculate
energy transferred. For example, if a box is pushed for 20 m with a force of 30
N, the pushing force does 600 J of work on the box (Work = 30 x 20 = 600 J).
This work will either make the box accelerate or heat up due to friction (or
both) - i.e. it will cause an increase in the kinetic energy of the box, or the
heat energy within the box.
Note from the equation W = F d, the unit of one joule is equivalent to one
newton.metre (1 J = 1 Nm)
- X-rays TC, HP
- X-rays were discovered accidentally by Wilhelm Röntgen in 1895. X-ray
radiation is a part of the electromagnetic spectrum.
With wavelengths ranging from about 0.001nm to about
100 nm, X-rays are much shorter than ultraviolet.
They travel at the speed of light.
X-rays are produced in an X-ray tube (soft X-rays for diagnosis) or in a
linear accelerator (hard X-rays for treatment).
X-rays can be detected using photographic
film or scintillation counters.
In medicine, 'soft' X-rays can be used to take photographs of broken bones.
Bone absorbs X-rays more than soft flesh and therefore shows up white on a
photographic plate. A break in the bone will show up dark. (X-rays blacken the
photographic chemicals on the film).
Alternatively, scintillation counters can be used to detect x-rays and
display an image on a TV type screen. This method is used with barium meals for
investigating the gut.
For a three dimensional view, computed tomography can be used where the
X-ray source and detector rotate around the patient to build up a series of
- This is a phrase often uttered by those people who finish reading the zenith entry below, having read the whole of this file.
- Zenith SP
- The zenith is the point in the sky directly above an observer's head. This
term is often used in astronomy.