Heat Engines And Efficiency
During each cycle a heat engine draws 800 J from its hot reservoir and discharges 600 J into the cold surroundings. What is the thermal efficiency of this engine?
Select the correct option:
🔒 Solution Hidden from View
Submit your answer to unlock the detailed step-by-step solution.
More thermodynamics Practice Questions
For a diatomic ideal gas the molar specific heat at constant volume is 5R/2, so what is the value of...
For a diatomic ideal gas the molar specific heat at constant volume is 5R/2, so what is the value of...
A gas is taken around a complete thermodynamic cycle and returns to its original state after several...
A gas is taken around a complete thermodynamic cycle and returns to its original state after several...
Two moles of an ideal gas expand isothermally at 300 K from a volume V to a volume 2V in a slow reve...
Two moles of an ideal gas expand isothermally at 300 K from a volume V to a volume 2V in a slow reve...
A balloon suddenly bursts and the compressed gas inside rushes out into the open atmosphere very rap...
A balloon suddenly bursts and the compressed gas inside rushes out into the open atmosphere very rap...
When 2 moles of a monatomic ideal gas are heated at constant pressure causing a temperature rise of ...
When 2 moles of a monatomic ideal gas are heated at constant pressure causing a temperature rise of ...
On a pressure-volume diagram an isothermal curve and an adiabatic curve pass through the same point ...
On a pressure-volume diagram an isothermal curve and an adiabatic curve pass through the same point ...
A heat pump uses 200 J of work to deliver a total of 800 J of heat into a warm room during each oper...
A heat pump uses 200 J of work to deliver a total of 800 J of heat into a warm room during each oper...
Two identical samples of the same ideal gas are kept at the same temperature but stored at very diff...
Two identical samples of the same ideal gas are kept at the same temperature but stored at very diff...
One mole of an ideal gas is compressed isothermally at temperature 300 K, so what happens to its int...
One mole of an ideal gas is compressed isothermally at temperature 300 K, so what happens to its int...
Heat of 400 J is supplied to a gas sealed inside a rigid steel container whose volume cannot change,...
Heat of 400 J is supplied to a gas sealed inside a rigid steel container whose volume cannot change,...
An ideal gas expands at a constant pressure of 2 × 10^5 Pa, increasing its volume from 0.01 m^3 to 0...
An ideal gas expands at a constant pressure of 2 × 10^5 Pa, increasing its volume from 0.01 m^3 to 0...
A gas confined in a perfectly insulated cylinder is suddenly compressed by a fast-moving piston, so ...
A gas confined in a perfectly insulated cylinder is suddenly compressed by a fast-moving piston, so ...
About This Question
- Subject
- physics
- Chapter
- thermodynamics
- Topic
- heat engines and efficiency
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
25%
The thermal efficiency of a heat engine measures the fraction of absorbed heat converted into useful work, defined as η=QHW=1−QHQC. The net work per cycle is W=QH−QC=800−600=200 J. Dividing by the input heat gives η=200/800=0.25, or 25 percent. The value 75 percent mistakenly reports the rejected fraction QC/QH=0.75 rather than the useful fraction. The value 33 percent comes from dividing work by the rejected heat, 200/600, which is not the definition of efficiency. The value 20 percent results from dividing the net work by the sum of the heats instead of the input heat. The efficiency can never reach 100 percent for a cyclic engine because the Second Law forbids converting all absorbed heat into work without rejecting some to a colder reservoir. Notice also that only the difference QH−QC appears as work, so reducing the rejected heat is the only way to raise efficiency for a fixed intake. As a plausibility check, real engines always reject most of their input heat, so an efficiency well below fifty percent is expected, and 25 percent is consistent with rejecting 600 of the 800 joules supplied.
This medium difficulty physics question is from the chapter thermodynamics, covering the topic of heat engines and efficiency. It appeared in the 2025 exam.
Looking for more practice? Explore all physics questions or browse thermodynamics questions on RankGuru.