SAT Physics Subject Test

Chapter 15 Thermal Physics

HEAT TRANSFER

There are three principal modes by which energy can be transferred:

Conduction

An iron skillet is sitting on a hot stove, and you accidentally touch the handle. You notice right away that there”s been a transfer of thermal energy to your hand. The process by which this happens is known as conduction. The highly agitated atoms in the handle of the hot skillet bump into the atoms of your hand, making them vibrate more rapidly, thus heating up your hand.

Convection

As the air around a candle flame warms, it expands, becomes less dense than the surrounding cooler air, and thus rises due to buoyancy. (We”ll study buoyancy in the next chapter.) As a result, heat is transferred away from the flame by the large-scale (from the atoms” point of view anyway) motion of a fluid (in this case, air). This is convection.

Radiation

Sunlight on your face warms your skin. Radiant energy from the sun”s fusion reactions is transferred across millions of kilometers of essentially empty space via electromagnetic waves. Absorption of the energy carried by these light waves defines heat transfer by radiation.

When a substance absorbs or gives off heat, one of the following can happen:

(1) the temperature of the substance can change

or

(2) the substance can undergo a phase change.

There are three phases of matter: solid, liquid, and vapor (or gas). When a solid melts (or liquefies), it becomes a liquid; the reverse process occurs when a liquid freezes (or solidifies) to become a solid. A liquid can evaporate (to become vapor), and vapor can condense to become liquid. These are the most common phase changes, but others exist: A solid can sublimate, going directly to vapor form, and vapor can experience deposition, going directly to solid.

Since either the first or second change—but not both—takes place upon heat transfer, let”s study these changes separately.