Past exam of the physics B course of the University of Cambridge / 2010 / Paper 1 / Section A / 3

Let $Q_c$ be heat extracted from the cold reservoir, $Q_h$ the heat delivered to the surroundings and $W$ the supplied work. The entropy inequality for a cyclic refrigerator isEquality is the reversible Carnot refrigerator limit. Thereforeenergy conservation gives $W_{\min }=Q_h-Q_c=74\mathrm{J}$. The requested rejected heat is 75 J, while 74 J is the minimum work; both have to be distinguished. Any entropy production increases the required rejection at these fixed reservoir temperatures.