The second law of thermodynamics can be expressed in several equivalent ways. This lesson focuses on the Kelvin statement, the Clausius statement, and why neither allows a perfect cyclic heat engine or refrigerator.
The Kelvin statement says that it is impossible to convert heat from a single source entirely into work without any other effect. The phrase without any other effect is crucial. It rules out a device that absorbs heat from one reservoir, produces an equal amount of work, and returns to its original state.
All flashcards
Kelvin statement
No cyclic device can convert heat from a single source entirely into work without any other effect.
Clausius statement
Heat cannot flow from cold to hot as the sole effect without external energy input.
Why does the Kelvin statement specify a cycle?
A cycle returns the device to its initial state, so no lasting change can serve as another effect.
Reversible-engine efficiency
A reversible engine is more efficient than an irreversible engine operating between the same reservoirs.
Efficiency of reversible engines
All reversible engines operating between the same two reservoirs have equal efficiency, determined by the reservoir temperatures.
Quiz questions
What does the Kelvin statement of the second law forbid?Answer: A cyclic device converting single-source heat fully into work with no other effect.
According to the Clausius statement, when can heat transfer from cold to hot occur?Answer: When external energy input drives the transfer.
A hypothetical refrigerator moves heat Q from cold to hot without work. A real engine takes Q+ΔQ from hot and rejects Q to cold. What is the combined net result?Answer: It removes ΔQ from the hot reservoir and produces work W=ΔQ.
How do the efficiencies of reversible and irreversible engines compare when they operate between the same two reservoirs?Answer: The reversible engine is more efficient than the irreversible engine.
What is true of two reversible engines operating between the same hot and cold reservoirs?Answer: They have equal efficiencies, regardless of their construction.
Based on University Physics Volume 2 by Samuel J. Ling, William Moebs, Jeff Sanny (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.