Test: Second Law of Thermodynamics - NEET MCQ

We know that the efficiency of refrigeration for a refrigerator is T₂ / T₁ + T₂
Where T₁ is source temperature and T₂ is sink temperature
For refrigerator X we have T₁ = 298K and T₂ = 268K
Hence the efficiency of refrigeration = 268 / 298 - 268
= 268 / 30
= 8.93
For refrigerator Y we have T₁ = 293K and T₂ = 253K
Hence the efficiency of refrigeration = 253 / 293 - 253
= 253 / 40
= 6.35

It works on the principle that no machine is 100% efficient. For instance if taking an ideal machine into consideration, work can be completely converted to heat. Now if the conditions are reversed, all heat cannot be converted to equal amount of work until and unless backed up by an external force.

A heat pump is an electrical device that heats a building by pumping heat in from the cold outside. In other words, it’s the same as a refrigerator, but its purpose is to warm the hot reservoir rather than to cool the cold reservoir (even though it does both).

If first law efficiency is close to unity, the all the energy carried in by heat transfer is used and no heat is lost to the surroundings.

The second law of thermodynamics gives a fundamental limitation to the efficiency of a heat engine and the coefficient of performance of a refrigerator. It says that the efficiency of a heat engine can never be unity or 100%, this implies that the heat released to the cold reservoir can never be made zero.
For a refrigerator the second law says that the coefficient through performance can never be infinite, this implies that the external work can never be zero.

If you leave the door open, heat is merely recycled from the room into the refrigerator, then back into the room. A net room temperature increase would result from the heat of the motor that would be constantly running to move energy around in a circle.

Refrigerator follows the principle of Clausius statement of second law of thermodynamics. It does not violate second law of thermodynamics because it takes energy to transfer heat from low temperature body to high temperature body. Electrical work is given to refrigerator to extract heat from low temperature body and to transfer it to higher temperature body. If any refrigerator is transferring heat from low temperature body to higher temperature body without any external energy then we can say that it violates second law of thermodynamics. But in actual it takes energy to do.

Entropy is defined as a thermodynamic property which serves as a measure of how close a system is to equilibrium, as well as a measure of the disorder in the system.

None

A is the correct answer.