Consider an electric refrigerator located in a room. Determine the direction of the work and heat interactions (in or out) when the following are taken as the system: (a) the contents of the refrigerator, (b) all parts of the refrigerator including the contents, and (c) everything contained within the room during a winter day

(a) Heat flows out

Work flows in

(b) Heat flow out

work flows in

(c) Heat flow out

work flows out

Explanation:

(a) According to the first law of thermodynamics, the change in internal energy of the system, ΔU is given as follows;

ΔU = Q - W

According to the second law of thermodynamics, heat will flow from a hot body to a cold body however through work input, a refrigerator causes heat to flow from a cold body to a hot body

Therefore;

Heat flows out of the system (the contents of the refrigerator)

Work is inputted, flows in to the system

(b) Given that the refrigerator pumps heat from the content out, the body of the refrigerator becomes hot relative to the room, and according to the second law of thermodynamics

Heat flow out from the hot system, the refrigerator and its content, to the room

h₁ > h₂, h₂ - h₁ = -ve = -w

Therefore, work flows out the system as the refrigerator operates

(c) During a winter day, the temperature of the room relative to the external environment where it is snowing is higher

Therefore, heat will flow out from the system

Work done, -w = h₂ - h₁, where

h₂ = The heat of the environment

h₁ = The heat in the room

Given that h₁ > h₂, h₂ - h₁ = -ve = -w, therefore, work is done by the system which is the room

Work will flow out of the system