Heat engines & refrigerators worksheet with answers

A refrigerator removes heat from its cold space at a rate of 0.42 kW while its compressor uses 0.2 kW of power. Find the rate Q̇_H at which it rejects heat to the kitchen.

A heat engine takes in 800 kJ of heat from a hot reservoir and rejects 640 kJ to a cold reservoir in each cycle. Find the net work W it produces per cycle.

A heat engine works between a hot reservoir at 300 °C and a cold reservoir at 15 °C. It takes in 5000 kJ of heat. Find the most work W_max it could possibly produce.

A heat engine receives heat from a source at 350 °C and rejects heat to the surroundings at 35 °C. Its actual thermal efficiency is 20%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

A Carnot heat engine receives 70 kW of heat from a source at 700 °C and rejects heat to the surroundings at 25 °C. All of its work drives a Carnot refrigerator that removes heat from a freezer at -5 °C and also rejects heat to the surroundings at 25 °C. Find the rate Q̇_L at which heat is removed from the freezer.

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 30 °C. Both engines produce the same work. Find T_m in °C.