Question

How can I differentiate between QH and QL in the refrigerator case in the second law of thermodynamics?

Answer 1

2nd law of thermodynamics tells us which processes in nature may or may not occur. For example, in 2 objects in thermal contact, it is spontaneous for heat to flow from the warmer to cooler object, but it is non-spontaneous to occur in the opposite direction.

Firstly, it is commonly referred to as and not QL, as you mentioned in the question.

Let's now study the refrigerator case:

Refrigerator is a device that converts work into thermal energy ()

During the cycle,

Heat () is absorbed from a source at low temperature (e.g. outside air or food) by a circulating fluid.

Work is done on the engine by a compressor.

Heat () is expelled to a source at higher T (e.g. room).

Since it is a cyclic process, ∆U = 0. Thus,

coefficient of performance = ratio of heat transferred to work required to transfer the heat.

can be much larger than 100% (denominator < 1).

A perfect refrigerator would transfer heat from a colder body to a hotter body without doing any work. From 2nd law of thermodynamics, this is impossible.

Hope this answers the question. Let me know if any doubts.