- #1
Peter G.
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Hi,
So I am learning about how refrigerators work. First I have to show the Carnot cycle and explain why it does not work in reverse - from my understanding, it does not.
And then show how it actually works in real life, with the phase transitions, which I am guessing don't exist for ideal gases because the intermolecular forces are ignored - is this correct?
I find understanding the "real life" freezer O.K.. The problem is when it comes to the reverse Carnot cycle...
This is why I believe it does not work, and I would appreciate any help/corrections on what I said above and will write now:
If we look at the regular Carnot Cycle, the first part, that is: Isothermal contraction - the following occurs:
Δ U = 0 Because it is isothermal
Δ W = negative because work is being done on the gas, hence:
ΔQ = 0 - ΔW which means that the ΔQ is negative: Heat is transferred from the gas - heat is lost, and, in order for that to happen, the gas must be contact with a cold source.
Now if we revert the cycle and still keep the cold source where it is, this is what happens:
ΔU = 0
ΔW = positive because the gas is doing work
Therefore ΔQ = positive: The gas absorbs heat from a cold (I am assuming colder) surroundings, which is impossible
Is that it?
Thanks,
Peter G
P.S: I haven't done the Second Law of Thermodynamics yet, so I don't know about entropy. Furthermore, if any of you could explain how there is no temperature change in an isothermal situation even though heat energy is moving in and out? Maybe it is because it loses the heat to the surroundings as shown by:ΔQ = ΔW?
So I am learning about how refrigerators work. First I have to show the Carnot cycle and explain why it does not work in reverse - from my understanding, it does not.
And then show how it actually works in real life, with the phase transitions, which I am guessing don't exist for ideal gases because the intermolecular forces are ignored - is this correct?
I find understanding the "real life" freezer O.K.. The problem is when it comes to the reverse Carnot cycle...
This is why I believe it does not work, and I would appreciate any help/corrections on what I said above and will write now:
If we look at the regular Carnot Cycle, the first part, that is: Isothermal contraction - the following occurs:
Δ U = 0 Because it is isothermal
Δ W = negative because work is being done on the gas, hence:
ΔQ = 0 - ΔW which means that the ΔQ is negative: Heat is transferred from the gas - heat is lost, and, in order for that to happen, the gas must be contact with a cold source.
Now if we revert the cycle and still keep the cold source where it is, this is what happens:
ΔU = 0
ΔW = positive because the gas is doing work
Therefore ΔQ = positive: The gas absorbs heat from a cold (I am assuming colder) surroundings, which is impossible
Is that it?
Thanks,
Peter G
P.S: I haven't done the Second Law of Thermodynamics yet, so I don't know about entropy. Furthermore, if any of you could explain how there is no temperature change in an isothermal situation even though heat energy is moving in and out? Maybe it is because it loses the heat to the surroundings as shown by:ΔQ = ΔW?
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