Practice question · Match the pairs
Match each device to what it does with heat and work.
- Heat engine
- Refrigerator
- Heat pump
- Perpetual motion of the second kind
- Takes heat from hot, delivers work, dumps heat to cold
- Would convert heat wholly to work, and is impossible
- Uses work to deliver heat into the warm space
- Uses work to move heat from cold to hot
Hints
- Three are real devices; one is what the second law forbids.
- A fridge and a heat pump are the same cycle judged by different outputs.
Show the answer
- Heat engine → Takes heat from hot, delivers work, dumps heat to cold
- Refrigerator → Uses work to move heat from cold to hot
- Heat pump → Uses work to deliver heat into the warm space
- Perpetual motion of the second kind → Would convert heat wholly to work, and is impossible
Why
The engine and the reversed cycle are the same four steps; what differs is which flow you count as the useful output. The forbidden device is the one that takes heat from a single reservoir and turns it entirely into work.
Practise Cyclic Processes and the Carnot Cycle
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