Why must a cooling system do work to keep things cool?

The most prominent use of thermal energy for direct heat is solar energy. 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. Thermal energy naturally 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... Show more

The most prominent use of thermal energy for direct heat is solar energy.
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. Thermal energy naturally 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.    
The key to how a refrigerator (or other cooling system) works is the refrigerant. A refrigerant is a substance, such as FreonTM, that has a low boiling point and changes back and forth between liquid and gaseous states as it cycles through the refrigerator. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator.    
The thermal energy causes the refrigerant to change to a gas. As a gas, the refrigerant releases thermal energy to the warm air outside the refrigerator. This causes the refrigerant to change back to a liquid, and the cycle repeats.   

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