http://www.ck12.org Chapter 18. Thermal Energy
FIGURE 18.13
The warm-air vent is placed near the floor
of the room. Warm air is less dense than
cold air so it rises. If the warm-air vent
were placed near the ceiling instead, how
would this affect the transfer of thermal
energy throughout the room?Cooling Systems
It’s easy to see how thermal energy can be used to keep things warm. But did you know that thermal energy can also
be used to keep things cool? Cooling systems such as air conditioners and refrigerators transfer thermal energy in
order to keep homes and cars cool or to keep food cold. In a refrigerator, for example, thermal energy is transferred
from the cool air inside the refrigerator to the warmer air in the kitchen. You read in this chapter’s "Transfer of
Thermal Energy" lesson that thermal energy always moves from a warmer area to a cooler area, so how can it move
from the cooler refrigerator to the warmer room? The answer is work. The refrigerator does work to transfer thermal
energy in this way. Doing this work takes energy, which is usually provided by electricity.Figure18.14 explains
how a refrigerator does its work.
For an animated demonstration of how a refrigerator works, go to this URL: http://www.chemistry.wustl.edu/~cour
ses/genchem/LabTutorials/Thermochem/fridge_movie.html.
The key to how a refrigerator or other cooling system works is the refrigerant. Arefrigerantis a substance, such as
FreonTM, that has a low boiling point and changes between liquid and gaseous states as it passes through the cooling
system. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a
gas. As a gas, it releases thermal energy to the warm air outside the refrigerator and changes back to a liquid.
Combustion Engines
Acombustion engineis a complex machine that burns fuel to produce thermal energy and then uses the energy to
do work. Two basic types of combustion engines are external and internal combustion engines.