Before cooking up those delicious potatoes tonight, how about using them to learn a bit about the awesomeness of chemical reactions? You can actually build a clock that runs purely on potatoes. Give it a go.
1Gather the items required. These are listed below under "Things You'll Need".
2Separate the two potatoes. Each potato works as a "galvanic cell", releasing electrical energy through chemical reactions. Call one potato A (or 1) and the other potato B (or 2).
- The bulk of the potato serves as a case for the cell, holding everything in place. The potato juice serves as the "electrolyte," in which charged atoms and molecules called "ions" dissolve and can flow over time. The dilute phosphoric acid in it also provides hydrogen ions for the reaction.
- Some atoms (or molecules) strongly attract extra electrons and become negatively charged ions, called anions ("an-ions"); others are easily stripped of some electrons and become positively charged "cations" ("cat-ions"). Each element attracts electrons with different force, due to the differing charge of protons with which their nucleus attracts electrons and the manner in which the marginal "valence" electrons arrange themselves around the nucleus and other electrons.. The potato juice has dilute phosphoric acid, some of which dissolves dissociated into as hydrogen cations (basically) and phosphate anions.
3Place one galvanized nail in each potato.
4Place one copper nail or wire piece into each potato ensuring that the nails don't touch. At this stage it is really important to understand that this experiment will work only if the galvanized nails and the copper nails/wire are as far apart as possible on the potato because the distance between them is what generates the power.
5Remove the battery compartment lid from the clock. If there is already a battery in it, remove it. Take notice of the battery's polarity in relation to how it connects to the clock.
6Connect the first jumper wire from copper nail or wire of potato A to the positive terminal of the clock. To do this, take one jumper lead and attach the alligator clip to the copper nail or wire and then the other end's alligator clip to the positive terminal (denoted by the symbol + ).
7Connect the second jumper wire from the galvanized nail of potato B to the negative terminal of the clock, denoted by the symbol "-".
- When you stick the zinc electrode (the plating on the galvanized nail) and the copper electrode (the copper nail or wire) in the potato, each metal, having less electron affinity than the hydrogen ions, would tend to have its atoms stripped of their electrons by the hydrogen ions. The neutral hydrogen atoms would join together into hydrogen gas molecules and escape; the metal cations would tend to disperse away and more hydrogen diffuse in to continue the reaction slowly bubbling away the acid's hydrogen and consuming the metals.
- Zinc has even less electron affinity than copper. So, if you connect the electrodes with a conductor, the wire, some electrons will tend to flow in it from the zinc to the copper.
- The removal of some electrons from the zinc electrode then that will make a few of its atoms fall off as cations which are positive and be dissolved away. The excess of electrons at the copper will make hydrogen preferentially take them up there, but also tend to make the copper keep its electrons and not be corroded (or, more precisely, shift the reaction equilibrium so that the copper ions that do fall off tend to regain electrons and reattach for little or no net corrosion). The removal of electrons from the copper and the addition of electrons to the zinc would tend to build up areas of electric charge that would hinder the reaction. But more electrons keep conducting through the wire, and more hydrogen and zinc ions keep dispersing through the potato to maintain fresh solution at each electrode, to keep the reaction going. The force with which the copper thus draws the electrons can be used to do work, like run a clock..
9Check the clock. It should now be running on potato power.
- The reaction slows as the zinc metal and the hydrogen ions are depleted. The force with which the hydrogen pulled the electrons off the zinc, in a very roundabout way, is what the cell makes available to you for work--with a little being "wasted", of course.
- Wiring the potato cells end-to-end makes a "series circuit", pulling the stream of electrons through the clock with twice as much force as one potato would apply. It also makes them properly called a "battery", because you're using more than one electrochemical "cell".
- Once it's working, why not set the time on the clock to the current time!
- If the clock doesn't work, check that all connections are secure and in the right order, and double-check the polarity of the clock. If it still doesn't work, your clock might require more power than the very little a vegetable battery can provide. You can satisfy yourself that the battery is producing some electricity with a multimeter and measuring its voltage (electrical pressure) and current (electrical flow, amps).
How long with the potato clock work?
- How long a potato battery will last depends on a lot of things. First, it will only be "used up" if it's actually hooked up to a wire. If it's not hooked up to a wire, it will theoretically last until the potato goes bad. Also, it depends on how much zinc you have, because the zinc gets used up in the reaction that powers the battery. If you're using a piece of metal that only has a little bit of zinc on it, then it won't last very long at all. But if you use a big, solid piece of zinc, your battery could go for a really long time! It also depends on the potato - some work better than others. If you want to know exactly how long a specific potato will last, the only way to find out for sure.
- You can buy a potato clock kit. Cheaper ones just have a clock that plugs into the potatoes with wires and electrodes; more expensive ones hold the potatoes in an integrated assembly with cups.
- A lemon battery works about the same way, but with citric acid rather than phosphoric providing the hydrogen ions. Other kinds of batteries similarly allow you to harness the energy of electrons' flow from an area where they are more easily given up to where they are more easily accepted but differ in many details.
- It's not just potatoes you can do this with. Try other fruits and vegetables, such as citrus, avocados, bananas, etc. It will even work in soda.
- The phenomenon of the extra electrons keeping the copper from corroding can be useful, too. If that's your objective, you're said to be using "cathodic protection" rather than making a battery. A cheap chunk of reactive metal, like zinc or magnesium, can corrode itself away and replenish electrons to protect a valuable structure of less-reactive metal, like a bridge or a ship. (Water isn't nearly as reactive as battery acid, so the reactive metal lasts a while, even with no resistance to the current in the conductor.) For a huge structure, one could use direct-current "impressed current" from an on-site generator or line power (with a professionally-designed machine--electricity and water can be dangerous).
- The clock is probably a quartz clock, which applies electricity to a tiny quartz-crystal tuning fork to physically "ring" it at its resonant frequency. The fork gives back an electric signal with each vibration. Other parts electronically count the vibrations, compute the corresponding time, and arrange the crystals in the display to show it off.
- Israeli researchers have found that they can produce up to 10 times the power when the potato is boiled.
- Do not eat the potatoes afterwards.
- Young children should be supervised when performing this experiment - nails and wires are sharp and can cause injury if handled incorrectly. Removal of batteries should also be supervised.
Things You'll Need
- 2 large clean potatoes, or 2 halves of a large potato.
- 2 galvanized nails (zinc)
- 2 copper nails or wires
- 3 jumper wires (with alligator clips on each end)
- Battery operated LCD clock (gray display with black numbers, like a cheap watch, normally taking just one 1.5V battery)
Sources and Citations
- Show Me How, 500 Things You Should Know, pp. 27-28, (2008), ISBN 978-1-74196-327-4 – research source
- ↑ http://en.wikipedia.org/wiki/Galvanic_cell
- ↑ http://en.wikipedia.org/wiki/Solvation
- ↑ http://en.wikipedia.org/wiki/Electron_affinity
- ↑ http://en.wikipedia.org/wiki/Phosphoric_acid
- ↑ http://en.wikipedia.org/wiki/Hydronium
- ↑ Show Me How, 500 Things You Should Know, p. 27, (2008), ISBN 978-1-74196-327-4
- ↑ http://en.wikipedia.org/wiki/Electrical_conduction#Metals
- ↑ http://www.teachengineering.com/view_activity.php?url=http://www.teachengineering.com/collection/cub_/activities/cub_energy2/cub_energy2_lesson04_activity2.xml#procedure
- ↑ http://en.wikipedia.org/wiki/Chemical_equilibrium
- ↑ http://www.carrotmuseum.co.uk/battery.html
- ↑ http://en.wikipedia.org/wiki/Irreversible_process_%28thermodynamics%29
- ↑ http://en.wikipedia.org/wiki/Series_and_parallel_circuits
- ↑ http://en.wikipedia.org/wiki/Battery_%28electricity%29#History
- ↑ http://www.physlink.com/education/askexperts/ae516.cfm
- ↑ http://en.wikipedia.org/wiki/Lemon_battery
- ↑ Show Me How, 500 Things You Should Know, p. 27, (2008), ISBN 978-1-74196-327-4
- ↑ http://en.wikipedia.org/wiki/Cathodic_protection
- ↑ http://www.corrosionsource.com/FreeContent/1/Cathodic+Protection
- ↑ http://www.cathodicprotection101.com/
- ↑ http://www.explainthatstuff.com/quartzclockwatch.html
- ↑ http://www.huffingtonpost.com/2010/07/30/potato-battery-could-prov_n_665578.html
In other languages:
Español: hacer un reloj que funcione con papas, Italiano: Costruire un Orologio a Patate, Português: Fazer um Relógio com Batatas, 中文: 做土豆钟, Deutsch: Eine Kartoffeluhr herstellen, Français: fabriquer une batterie en pommes de terre pour une horloge, Русский: сделать часы из картофеля
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