The second law of thermodynamics is considered to be one of the most fundamental laws across all sciences, it alone can explain a variety of different things including The Big Bang, Expansion of the Universe as well as why refrigerators need to be supplied with electricity.

The second law states that it is impossible for heat to travel from a cold reservoir to a hot reservoir without work being provided, this is simply because energy will not travel from a cold region to hot region but in fact the opposite, when energy travels from a hot region to a cold region work is produced. [1,2]

Therefore we can say by looking at Fig 2.1 that

Qc = Qh W [4] (because energy cannot be created or destroyed)

This is in fact a heat engine also named the Carnot Engine. The equation above is what allows heat engines to work, heat is transferred from a hot region to a cold region and work is produced which can then be used in various ways. A refrigerator is therefore the reverse of a heat engine, which is why it is also named a reversed Carnot theorem (see Fig2.2) [1,3,2]

In the reversed Carnot theorem external work has to be supplied to the system to transfer energy/heat from the cold reservoir to the hot reservoir lowering the temperature of the cold reservoir further.

Therefore in this system by looking at Fig 2.2 we can say that

Qc + W = Qh [4

So in our refrigerator the freezer and contents of the refrigerator are maintained at a lower temperature than the surroundings by taking heat away from the inside and dumping that heat in to the surrounding environment i.e. the kitchen, all as a result of providing electrical energy to the compressor in the refrigerator, but where does the refrigerant fluid come in to this?

Heat will not move freely from the cool inside to the hot outside so its made to by the intermediate fluid. The intermediate fluid absorbs heat on the inside and takes it to the air surrounding the refrigerator. The fluid circulates through condensing and evaporating pipes that run in and out through the back of the refrigerator, the fluid is kept moving by the compressor driven by electricity. This is the work thats provided to the system to produce the reversed Carnot theorem. [4]

This very same principle with a few adjustments can be applied to explain how air conditionings and heat pumps work all three engines follow the reversed Carnot theorem. Air conditionings also use a refrigerant to keep a room cool when the outside temperature is very high, by removing heat from the room and dumping it in to the outside of the house. Heat pumps are used to keep a room hot when the outside temperature is significantly lower the inside temperature, it takes heat from the surrounding outside environment and dumps it in to the room, increasing the temperature of the room. [3,2]

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The second law of thermodynamics as a fundamental law across all sciences

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