The Carnot refrigerator

Problem statement

A refrigerator that works on a Carnot cycle absorbs an amount of heat of $[[Qf]]$ kJ in one hour. The temperature inside the refrigerator must be $-[[tf_abs]]\ ^\circ\text{C}$, while the ambient temperature outside is $[[tc]]\ ^\circ\text{C}$. Determine:

a) (1 mark) The power the motor must have to achieve the temperature set for the interior.

b) (1 mark) If the efficiency of the refrigerator is now $[[frac]]\ \%$ of that of the Carnot cycle, what should the power of the motor be?

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · Technology and Engineering · Question 3.2. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.