Question

A house wife, on a warm summer day, decides to beat the heat by closing the windows and doors in the kitchen and opening the refrigerator door. At first she feels cool and refreshed, but after a while the effect begins to wear off. 

Evaluate the situation as it relates to First Law of Thermodynamics, considering the room including the refrigerator as the system.

Answer 1

Initially, the temperature of air in the room falls when it communicates with the cool refrigerator with its door open. This makes the house wife feel cool. 

Considering the room and its contents as the system, and assuming the walls, windows and doors non-conducting, we find, Q = 0. 

To operate the refrigerator, electricity is supplied from outside and hence external work W is done on the system. 

Applying the first law to the system,

Q = ∆ U + W 

0 = ∆ U + (– W) 

∴ ∆ U = W

The right hand side is a positive figure indicating the increase in energy of the system with time. As the energy is increasing the temperature of air increases and hence the effect of coolness gradually begins to wear off. 

It may be pointed out here that in this case the energy rise manifests itself in a rise in temperature.