MediumPhysicsHigh School (11–12)Heat & Thermodynamics › Heat Engines, the Second Law and EntropyConceptualBoard Exam
Say whether each process is nearly reversible or irreversible, and give a reason…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Say whether each process is nearly reversible or irreversible, and give a reason.
- A gas expands very slowly against a piston, touching a large reservoir at its own temperature.
- A gas rushes into an empty vessel when a tap is opened.
- Heat flows along a metal rod from a hot body to a cold one.
- A block slides on a rough floor and stops.
- An ideal spring is stretched very slowly and let back slowly.
Need a reminder? The key ideaShow
The second law. Kelvin-Planck: no engine working in a cycle turns all the heat it takes in into work. Clausius: heat does not flow from a colder body to a hotter one by itself. A reversible process is very slow, with no friction and no heat flow across a gap in temperature; every real process is irreversible and raises the total entropy.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Nearly reversible: slow, in balance, no gap in temperature.
Step 2. (b) Irreversible: a sudden, unbalanced change. The gas never gathers back by itself.
Step 3. (c) Irreversible: heat crosses a finite gap in temperature.
Step 4. (d) Irreversible: friction turns kinetic energy into heat.
Step 5. (e) Nearly reversible: slow, and an ideal spring returns all its energy.
Answer: (a) and (e) are nearly reversible; (b), (c) and (d) are irreversible.
Check. Each irreversible one raises the total entropy.
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