How does a refrigerator obey the second law of thermodynamics?

Answer 1

The cooling of the interior of a refrigerator does not violate the laws of thermodynamics because work has to be input to the system in order to run the compressor that helps pump the heat out of the refrigerator. The typical refrigeration cycle runs like this:

1. The refrigeration fluid is compressed by the compressor. Energy must be supplied to the compressor to make this happen. As the fluid is compressed, it gets hotter because the compression is nearly adiabatic.
2. The hot compressed fluid is warmer than the surroundings so it can be cooled down by passing it through coils where it gives off heat to the surroundings.
3. The cooled, compressed fluid is passed through a valve where it expands nearly adiabatically and at least partially vaporizes. As it does so, the temperature drops significantly to a point below the temperature of the interior of the refrigerator.
4. The lower pressure fluid (gas and possibly some liquid) is passed through coils in the interior walls of the refrigerator where it absorbs heat from the somewhat warmer interior.

Note that in each instance where there is heat transfer, the heat moves from the warmer to the colder part of the system - in keeping with the second law. Overall the heat taken out of the fridge and the heat given off by the compressor creates a greater increase in entropy of the surroundings than the decrease in entropy of the interior of the fridge - again in keeping with the second law. Overall the energy input to the compressor exactly balances the heat released by the compressor and given off by the coils minus the energy removed from the interior of the fridge - thus complying with the first law.

Answer 2

The refrigerant is compressed by an input of work (in the form of electricity) to the pump motor. The 2nd law dictates that the refrigerant would not spontaneously compress. The compressed refrigerant also gets warmer - a consequence of the 1st law. Calculations of the work required and the outlet temperature of the compressor require application of thermodynamic equations.

The compressed refrigerant is passed through cooling coils, usually on the back of the fridge, where it gives off heat to the air in accordance with the 2nd law of thermodynamics.

The cooled, compressed liquid is expanded across a valve where, due to the expected effects of isenthalpic expansion, it gets quite cold. This is predicted by application of both the 1st and 2nd laws. The laws of thermodynamics predict how cold the refrigerant will get.

The cold, decompressed refrigerant then absorbs heat from the contents of the fridge as predicted by the 2nd law. The amount of cooling power provided is governed by both the 1st law (heat absorbed equals heat removed from the contents of the fridge) and the 2nd law (heat will only be absorbed so long as the refrigerant is colder than the contents of the fridge).

The whole cycle roughly follows the ideal thermodynamic refrigeration cycle. Thermodynamics also describes the efficiency of heat removal, the maximum cooling possible, the dependency of cooling on the temperature of the ambient air, the dependency of cooling on the specific heat capacity and mass of the contents, and other refrigeration related factors

Answer 3

A refrigerator characteristically reduces the temperature of the contents below that of the surroundings. Such a change does not occur spontaneously; it requires energy input to pump the heat out of the interior. If you don't plug in the refrigerator, it gradually comes into thermal equilibrium with the surroundings. Running the refrigerator heats up the area around it both from the energy pumped out of the refrigerator and the heat coming from operating the compressor motor. The second law has several consequences that are illustrated here:

1) heat only moves spontaneously from higher temperature to lower temperature

2) to move energy from a region of lower temperature to higher temperature energy must be input to the system

3) more heat is released to the surroundings than is removed from the refrigerated compartment

Answer 4

At first glance a refrigerator seems to violate the 2nd law since heat moves from a region of lower temperature to a region of higher temperature. A better understanding of the refrigeration cycle can dispel that notion.

1. The refrigerant coming from the interior of the refrigerator enters a compressor.
2. The compressor - as the name implies - compresses the refrigerant and in the process also raises the temperature of the refrigerant above that of the surroundings.
3. The hot refrigerant is sent through cooling coils - usually on the back of the refrigerator - where it dumps some of that heat into the comparatively cooler air around it.
4. The somewhat cooled refrigerant is then expanded almost adiabatically through a valve, resulting in it dropping to a temperature below the interior of the refrigerator.
5. The cold refrigerant circulates through coils in the interior walls of the refrigerator where it absorbs heat from the interior.
6. At this point, the cycle returns to step 1 of the process.

Note that the movement of heat in steps 3 and 5 is still from higher temperature to lower temperature - a natural process that follows the 2nd law. Step 2 where work is input to the system to make it function is necessary for it to succeed in the overall effect of pumping heat from the cold interior of the refrigerator to the warmer surroundings. If you actually measure it you will find that the heat sent to the surroundings in step 3 is greater than the heat absorbed in step 5. The difference is the extra energy added to the system in step 2 as work done by the compressor.