Carnot

[MedPR]

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If I showed you this graph and asked if it was a heat engine or a heat pump, how would you choose? Assume you don't have them memorized as clockwise/counterclockwise PV diagrams.

 

[milski]

More work is being done by the system (area under the curve left to right) than on the system (area under the curve right to left). As a whole that means that the system is doing work and taking in heat to return to the same initial state. That would be an engine.

 

[MedPR]

More work is being done by the system (area under the curve left to right) than on the system (area under the curve right to left). As a whole that means that the system is doing work and taking in heat to return to the same initial state. That would be an engine.

What do you mean by area under the curve left to right and right to left?

 

[milski]

What do you mean by area under the curve left to right and right to left?

The area under the 1-2-3 curve is the work done by the system. The area under 3-4-1 is work done on the system. You can easily tell that what's under 1-2-3 is larger than what's under 3-4-1.

 

[MedPR]

The area under the 1-2-3 curve is the work done by the system. The area under 3-4-1 is work done on the system. You can easily tell that what's under 1-2-3 is larger than what's under 3-4-1.

I see...

So 1-2-3 is work done by the system because it is expanding (volume increasing) and 3-4-1 is work done on the system because it is compressing (volume decreasing). And for an engine, work out > work in (while for a pump work in > work out).

What does the area between 1-2-3 and 3-4-1 indicate? Geometrically, I understand what you are saying about the area under 1-2-3 being greater because it includes everything under 3-4-1 as well as what is in between 1-2-3 and 3-4-1, but I don't understand it conceptually.

 

[milski]

I see...

So 1-2-3 is work done by the system because it is expanding (volume increasing) and 3-4-1 is work done on the system because it is compressing (volume decreasing). And for an engine, work out > work in (while for a pump work in > work out).

What does the area between 1-2-3 and 3-4-1 indicate? Geometrically, I understand what you are saying about the area under 1-2-3 being greater because it includes everything under 3-4-1 as well as what is in between 1-2-3 and 3-4-1, but I don't understand it conceptually.

It's the difference between the work done by the system and the work done on the system. In other works, that's how much work you managed to get out of the system for the whole cycle. Since it is in the same initial state, ΔU=0 and that area also will be the magnitude of heat absorbed.

 

[MedPR]

It's the difference between the work done by the system and the work done on the system. In other works, that's how much work you managed to get out of the system for the whole cycle. Since it is in the same initial state, ΔU=0 and that area also will be the magnitude of heat absorbed.

So wouldn't doing less work on the system result in a greater area between 1-2-3 and 3-4-1?

 

[milski]

So wouldn't doing less work on the system result in a greater area between 1-2-3 and 3-4-1?

Yes, it would.

 

[MedPR]

Ok thanks!