In a closed system where volume is held constant, there is a direct relationship between Pressure and Temperature. In a direct relationship, one variable follows the same change when it comes to increasing and decreasing. For example, when the pressure increases then the temperature also increases. When the pressure decreases, then the temperature decreases.

There are two examples that demonstrate this principle. The first is that of a spray can. The volume of the can is constant. When contents are removed from the can, then the mass of the contents will be reduced in the can. Because there is less mass in the can with a constant volume, the pressure will decrease. This pressure decrease in the can results in a temperature decrease. The contents coming out of the can also cool in temperature but for another reason. Outside of the can the volume is not held constant. Thus the contents are free to expand when they move from the high pressure in the can to the low pressure outside of the can. Expansional cooling causes the contents coming out of the can to cool. This effect can be noticed in spray-on deodorants.

In the second example, a compressor is shown. This is used in appliances such as refrigerators. The refrigerant air is pressurized into the compressor meaning the pressure increases as more air is squeezed into the same volume. This causes a temperature increase. With time, this heat is lost to the environment through coils. Additional heat is given off as the refrigerant releases latent heat by turning from a gas to a liquid as it cools. The compressor will be location is a position such as the back of the refrigerator. Thus, the outside portion of the refrigerator can give off heat to the air due to this process. The high pressure liquid in the compressor is then transferred to the inside of the refrigerator and gradually released. This results in a process similar to the first example with the addition of latent heat cooling that takes places as the refrigerant changes phase from a liquid to a gas. The rapidly expanding air cools and this causes the temperature to decrease inside the refrigerator and freezer due to the very cold coils being in contact with the surrounding air inside the refrigerator.