1.Easy
A heat engine takes in 4400 kJ of heat from a hot reservoir and rejects 3080 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 4400 kJQL = 3080 kJ
η = %
Heat engines & refrigerators worksheet with answers
2.Easy
A refrigerator removes heat from its cold space at a rate of 5.72 kW while its compressor uses 1.3 kW of power. Find the rate Q̇H at which it rejects heat to the kitchen.
Q̇L = 5.72 kWẆ = 1.3 kW
Q̇H = kW
3.Medium
A heat engine works between a hot reservoir at 775 °C and a cold reservoir at 5 °C. Find the highest possible thermal efficiency ηmax.
TH = 775 °CTL = 5 °C
ηmax = %
4.Medium
A reversible refrigerator keeps a cold room at -5 °C by removing 7.5 kW of heat, and rejects heat to the surroundings at 20 °C. Find the rate Q̇H at which it rejects heat.
TL = -5 °CTH = 20 °CQ̇L = 7.5 kW
Q̇H = kW
5.Hard
A refrigerator keeps its cold space at 4 °C in a kitchen at 32 °C, removing heat from the cold space at 1 kW. Find the minimum power input Ẇmin.
TH = 32 °CTL = 4 °CQ̇L = 1 kW
Ẇmin = kW
6.Hard
Two Carnot engines work in series. Engine A receives heat from a reservoir at 650 °C and rejects heat to an intermediate reservoir at temperature Tm. Engine B takes all that heat and rejects heat to a reservoir at 30 °C. Both engines produce the same work. Find Tm in °C.
TH = 650 °CTL = 30 °C
Tm = °C
Answer key
- 1. η = 30 %
- 2. Q̇H = 7.02 kW
- 3. ηmax = 73.46 %
- 4. Q̇H = 8.199 kW
- 5. Ẇmin = 0.101 kW
- 6. Tm = 340 °C
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