Heat engines & refrigerators worksheet with answers

A refrigerator removes heat from its cold space at a rate of 4.1 kW while its compressor uses 1 kW of power. Find the rate Q̇_H at which it rejects heat to the kitchen.

A heat engine takes in 1400 kJ of heat from a hot reservoir and rejects 1050 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.

A heat engine receives heat from a source at 750 °C and rejects heat to the surroundings at 20 °C. Its actual thermal efficiency is 46%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

A reversible (Carnot) heat engine takes in 2900 kJ of heat from a reservoir at 525 °C and rejects 1210 kJ per cycle. Find the temperature T_L of the cold reservoir in °C.

A refrigerator keeps its cold space at -25 °C in a kitchen at 30 °C, removing heat from the cold space at 2.5 kW. Find the minimum power input Ẇ_min.

Two Carnot engines work in series. Engine A receives heat from a reservoir at 1200 °C and rejects heat to an intermediate reservoir at temperature T_m. Engine B takes all that heat and rejects heat to a reservoir at 35 °C. Both engines produce the same work. Find T_m in °C.