Engines must dump heat

Science · Thermal & Statistical Physics · ages 19-20

Name ______________________ Date ____________

Every heat engine must reject some heat to a cold store.
Circle one: True False
How is the efficiency of a heat engine found?
- Qc over Qh
- W over Qh
- Qh over W
What does the engine form of the second law ban?
- All use of cold stores
- Any engine below full efficiency
- One store of heat fully to work
You push 100 J of work into a perfect refrigerator. Where can heat go?
- From cold to hot, moved by your work
- From hot to cold for free
- Nowhere, since work destroys heat
An engine takes 500 J and rejects 200 J. Give the work output in J.
Answer: ______________
Why are the engine and refrigerator forms equivalent?
- Both mention Carnot by name
- A violator of one could run a violator of the other
- Both use the same fuel
An engine takes 1000 J at efficiency 0.4. Give the rejected heat in J.
Answer: ______________
A catalogue advertises an engine with 120 percent efficiency.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Skipping the dump would convert heat fully into work.
W over Qh · Efficiency is useful work per unit of heat paid in.
One store of heat fully to work · One store to pure work with no dump is impossible.
From cold to hot, moved by your work · Cold to hot flow is allowed when work pays for it.
300 · Work is 500 minus 200, which is 300.
A violator of one could run a violator of the other · Linking the cheats shows one impossible machine builds the other.
600 · Work is 400, so 1000 minus 400 leaves 600.
False · Efficiency above one means work from nothing, which the law bans.