Heat engines & refrigerators worksheet with answers

Free printable Heat engines & refrigerators worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

A refrigerator removes heat from its cold space at a rate of 8.17 kW while its compressor uses 1.9 kW of power. Find its coefficient of performance COP_R.

Q̇_L = 8.17 kWẆ = 1.9 kW

COP_R =

2.Easy

A heat engine takes in 700 kJ of heat from a hot reservoir and rejects 385 kJ to a cold reservoir in each cycle. Find the net work W it produces per cycle.

Q_H = 700 kJQ_L = 385 kJ

W = kJ

3.Medium

A heat engine receives heat from a source at 800 °C and rejects heat to the surroundings at 25 °C. Its actual thermal efficiency is 40%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

T_H = 800 °CT_L = 25 °Cη = 40%

η_II = %

4.Medium

A heat engine works between a hot reservoir at 325 °C and a cold reservoir at 35 °C. Find the highest possible thermal efficiency η_max.

T_H = 325 °CT_L = 35 °C

η_max = %

5.Hard

A heat engine takes in 4500 kJ of heat from a reservoir at 350 °C, produces 1800 kJ of work and rejects the rest to a reservoir at 35 °C, per cycle. Find the total entropy generated S_gen per cycle.

T_H = 350 °CT_L = 35 °CQ_H = 4500 kJW = 1800 kJ

S_gen = kJ/K

6.Hard

A refrigerator keeps its cold space at -18 °C in a kitchen at 35 °C, removing heat from the cold space at 1.5 kW. Find the highest possible coefficient of performance COP_max.

T_H = 35 °CT_L = -18 °CQ̇_L = 1.5 kW

COP_max =

Answer key

1. COP_R = 4.3
2. W = 315 kJ
3. η_II = 55.39 %
4. η_max = 48.48 %
5. S_gen = 1.541 kJ/K
6. COP_max = 4.814

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