As mentioned earlier, heat is always transferred from hotter substances to cooler ones. This heat transfer takes place in three ways: conduction, convection and radiation.
|Conduction is the form of heat transfer that takes place within a solid. Conduction causes the handle of a cast iron frying pan to be nearly as hot as the bottom. Conduction occurs within a solid or between two solids in contact with each other. It is generally believed that conduction occurs on the atomic level, much as the conduction of electricity takes place. Free electrons transport the thermal energy from molecule to molecule away from the heat source.|
|Convection is the form of heat transfer that takes place within gases or liquids, or between solids and liquids or gases. Convection causes the air from your heating system to be distributed throughout the room. Convection also requires physical contact. It takes place because of the difference in density in fluids as they are heated and cooled. Liquids or gases that are heated become less dense and rise. As they cool, they become more dense and fall. In a system, this cycle of movement is called a convection current.|
|Radiation is the form of heat transfer that takes place between two bodies that aren't in physical contact. Radiation is responsible for those yearly sunburns many of us must endure. Radiation is the only form of heat transfer that requires no physical contact. While we do know that it occurs because of electromagnetic radiation (usually in the infrared band), much of our understanding of it is still theoretical. We do know that it can take place between two bodies separated by a vacuum and is partially explained by wave theory and quantum mechanics.|
Often, heat transfer occurs as a combination of the methods we've discussed. For example, fire heats a frying pan by convection. The heat is then transferred from one side of the frying pan to the other by conduction. The egg in the pan is cooked by conduction heating. Bon appétit!