EasyPhysicsHigh School (11–12)Heat & Thermodynamics › Heat Engines, the Second Law and EntropyConceptualBoard ExamIB
An inventor claims a machine that takes 3200 J of heat from the water of a lake in each cycle and…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
An inventor claims a machine that takes of heat from the water of a lake in each cycle and turns all of it into of work, with no other effect. Does it break the first law? Does it break the second law?
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. First law. Energy in, ; energy out, . Energy is kept, so the first law is not broken.
Step 2. Second law. The machine works in a cycle and turns all the heat from one reservoir into work, giving none to a colder body. That is exactly what Kelvin-Planck forbids.
Answer: It obeys the first law but breaks the second, so it cannot be built.
Check. The lake's entropy would fall by with nothing gaining any: the total would fall.
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