I understand how we heat them, by putting energy into the system. but how do we use the energy to cool them down?
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“Heat” is the **quantity** of thermal energy in a substance. “Temperature” is the **concentration** of thermal energy in that same substance.
Heat moves from regions of high concentration to regions of lower concentration. From high temperature to lower temperature.
If you keep the same amount of energy in a substance, but reduce the volume that substance occupies, the concentration of thermal energy increases. The temperature rises, even though no thermal energy was added or removed.
If you keep the same amount of thermal energy in the substance, but increase the volume the substance occupies, you decrease the temperature.
To cool something, you first increase the pressure of a gas. Temperature rises. Heat flows from high temperature gas to ambient temperature environment, until the compressed gas temperature equalizes with ambient. Now you put the compressed gas next to the object you want to cool, and you expand it. Temperature falls. The heat from the object flows into the gas, lowering the temperature of the object, and raising the temperature of the gas.
Now take the gas away from the object, and compress the gas again, repeating the cycle.
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