If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Carnot efficiency 2: Reversing the cycle

Seeing how we can scale and or reverse a Carnot Engine (to make a refrigerator). Created by Sal Khan.

## Want to join the conversation?

• - I've never understood (i took thermodynamics in 1986 and wondered ever since) why the adiabatic expansion is needed in the Carnot cycle. Isn't the isothermal expansion enough?
• Im not sure I aswer you correctly, but I think about it this way: In order to work in cycle, you need to expand and compress to get to the same point (imagine a piston). If you want to compress something at high temperature, you have to do more work, because the pressure is higher...so thus the adiabatic expansion - you lose some energy (temperature)and now you can compress it easily - you have to do less work to the system...
Now if you look at the graph, the work done by the system is higher (it expands the same volume, but at higher temperature/pressure) than the work done to the system) - both can be expresed as the area under the curve)
IF you compress the system at the same temperature, you also have to do the same work as the engine and that would be useless
So I think the key idea is, that this is a cycle - you cant just expand forever...
I hope that helps, also, pardon my english
• I'm still confused, why do we not consider the work done adiabatically when calculating efficiencies, proving relationships and so on?
• Hi. This is because the work done adiabatically during expansion and compression cancel each other out exactly in the carnot cycle.
• In the Carnot refrigerator the heat is obtained from the colder reservoir T2?
• Yep, that's why we keep our milk there.

Jokes aside, this is an important takeaway lesson from Carnot cycles; with reversible processes, if you can increase heat to do work, then you can use work to remove heat.
• Correct me if I'm wrong, doesn't that mean that in case the hot reservoir has higher temperature than usual (like a hot day), then your compressor has to do more work to heat up your coolant which then transfers heat to this reservoir?
• Theoretically you're right, the compressor will need to do more work because our isothermal compression at the top isotherm is occuring at a higher temperature. However, it's also possible that our compressor simply cannot compress the coolant enough to reach the original volume at a higher pressure (and temperature). So, we will have a shortened cycle. Then, when the coolant expands it will not be able to absorb the same quantity of heat, Q2, from the inside of the refrigirator, because our adiabatic expansion may not take us all the way to the low Temperature setpoint.
• at , you say adiabatic contraction, dont you mean adiabatic compression?
or is contraction the same as compression?
• If something is pushing on the system, it's a compression.
If the system is doing the pulling, it's a contraction.
• This is not a THERMODYNAMICS question
Is a compressor a machine used to "compress" a gas? If this is right then from ideal gas equation can we say that it helps in cooling a gas, right? Isn't this how an A/C works?
• There are three variables(pressure, temperature and volume) in ideal gas law. To say anything about one you need to fix other two. Also considering a real A/C you can't really use ideal gas law. But you can find an approximate approach(not a very good one) with ideal gas law...
• At , he said when we isothermally contract work is done on system so it's negative,but when work is done on a system work done is positive and work done by the system is negative.so the area under the reversible process is negative or positive?"