A refrigerator is a cooling device. An ideal refrigerator can be regarded as Carrot’s heat engine working in the reverse direction. Therefore, it is also call a heat pump. In a refrigerator the working substance would absorb certain quantity of heat from the sink at lower temperature and reject a large amount of heat to the source at a higher temperature with the help of an external agency like an electric motor (Figure).
In an actual refrigerator vapours of freon (dichloro difluoro methane CCI2F2) act as the working substance. Things kept inside the refrigerator act as a sink at a lower temperature T2. A certain amount of work W is performed by the compressor (operated by an electric motor) on the working substance.
Therefore, it absorbs heat energy Q2 from the sink and rejects Q1 amount of heat energy to the source (atmosphere) at a temperature T1. Since this is a reversible cyclic process, the change in the internal energy of the working substance is zero, dU = 0.
According to the first law of thermodynamics,
dQ = dU + dW
but, dQ = Q2 – Q1
dW = – W and dU = 0
So, dQ = Q2 – Q1 = W
Negative sign with W represents work is done on the system, W = Q1 – Q2