Modes of heat transfer
- Heat can be conducted between two bodies which are in contact with each other; heat "flows" from one to the other.
- Materials which conduct heat well are called conductors of heat. Electrical conductors (such as metals) are good conductors of heat.
- Materials which do not conduct heat well are called insulators.
Electrical insulators (for example, wood or glass) are usually good
insulators of heat. Materials with low density, such as air or foamed
plastic, are normally also good insulators unless they happen to be
electrical conductors. To prevent heat from moving from one place to
another, we usually place an insulator between.
Once a good insulator becomes hot, however, it stays that way for a long time, because it is difficult for the material to lose heat by conduction. Think of a hot ceramic pan and a hot metal pan: which cools faster?
This is a different kind of heat transfer than conduction. In
conduction, heat itself is moving; in convection, hot portions of a
fluid move through the body of the fluid. The hot fluid mixes with the cold fluid, and heat is transferred more quickly than by conduction.
What we commonly call a "rolling boil" results from convection. Hot
fluids rise through surrounding, cooler fluid because they are less
dense; cooler fluids sink through warmer fluids because they are more
dense. This causes circular motion of the fluid away from a source of
heat. Convection in water drives ocean currents; convection in air
drives weather patterns; and convection of molten rock inside the earth
is thought to drive plate tectonics.
- Radiation is the simplest means of heat transfer. Heat radiation is carried not by moving atoms (as in conduction or convection) but by electromagnetic waves. Radiation is the only way that heat can move through a vacuum, and is the reason that even a closed thermos bottle (which has a vacuum between the inner and outer parts) will eventually come to the same temperature as its surroundings.