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Thermodynamics Cycle Question

  • Thread starter Boom100
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  • #1
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Homework Statement


Determine Whether or not this cycle violates the first law and if it is reversible, irreversible, or not possible.

2000kW or power are produced by taking 3000 kW of heat transfer from a 750C reservior and rejecting 1000kW of heat to a 20C reservoir.


Homework Equations



dU=Q-W

The Attempt at a Solution



My attempt was to simply sum the heat transfer (3000-1000)kW = Q and set dU=2000kW (The energy produced, or the internal energy it gathered). I set W=0 because this is just heat transfer and there is no work being done.

So 2,000 does = 2,0000 so I'm thinking this is true. It seems a bit easy though... so im thinking something is off and I might have to incorporate the temperatures.

I said this is irreversible because it is heat transfer from a reservoir. So, once the heat is transferred, both heat sources change temperatures (The cold gets hotter and the hot gets cooler) so they are tending towards an equilibrium temperature. This can not be reversed and heat can not be sent back.
 

Answers and Replies

  • #2
Redbelly98
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Welcome to Physics Forums.

Homework Statement


Determine Whether or not this cycle violates the first law and if it is reversible, irreversible, or not possible.

2000kW or power are produced by taking 3000 kW of heat transfer from a 750C reservior and rejecting 1000kW of heat to a 20C reservoir.


Homework Equations



dU=Q-W

The Attempt at a Solution



My attempt was to simply sum the heat transfer (3000-1000)kW = Q and set dU=2000kW (The energy produced, or the internal energy it gathered). I set W=0 because this is just heat transfer and there is no work being done.

So 2,000 does = 2,0000 so I'm thinking this is true. It seems a bit easy though... so im thinking something is off and I might have to incorporate the temperatures.

I said this is irreversible because it is heat transfer from a reservoir. So, once the heat is transferred, both heat sources change temperatures (The cold gets hotter and the hot gets cooler) so they are tending towards an equilibrium temperature. This can not be reversed and heat can not be sent back.
The "power produced" mentioned in the problem statement actually refers to the work done by this heat engine. That contradicts your thinking that W=0.
 
  • #3
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Welcome to Physics Forums.


The "power produced" mentioned in the problem statement actually refers to the work done by this heat engine. That contradicts your thinking that W=0.
Ahhh Ok great, thanks Red. So with that thinking then- Since the engine is doing the work, that means that W=-2000, So dU=2000-2000=0. So 2000 still = 2000 So am i correct in saying that this means that this statement is valid? Is my reason correct for why it is irreversible as well?

Once again, thank you!
 
  • #4
Redbelly98
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I have asked around if any of the other Homework Helpers can help out on the question of reversible/irreversible/impossible. Meanwhile, I'll respond as I can.

I said this is irreversible because it is heat transfer from a reservoir. So, once the heat is transferred, both heat sources change temperatures (The cold gets hotter and the hot gets cooler) so they are tending towards an equilibrium temperature. This can not be reversed and heat can not be sent back.
But an ideal Carnot cycle also involves heat transfer from a reservoir, yet it is reversible.

Ahhh Ok great, thanks Red. So with that thinking then- Since the engine is doing the work, that means that W=-2000, So dU=2000-2000=0. So 2000 still = 2000 So am i correct in saying that this means that this statement is valid?
What statement are you referring to?
Is my reason correct for why it is irreversible as well?
I think not, for the reason I stated above on Carnot cycles.
 
  • #5
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I have asked around if any of the other Homework Helpers can help out on the question of reversible/irreversible/impossible. Meanwhile, I'll respond as I can.


But an ideal Carnot cycle also involves heat transfer from a reservoir, yet it is reversible.


What statement are you referring to?

I think not, for the reason I stated above on Carnot cycles.
The statement I was referring to was the first law. So I was just seeing if that was proof enough that it doesnt violate the first law.

As far as the reversibility thing, I see what you mean. Im still pretty confused on this one.
 
  • #6
vela
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You want to compare the efficiency of the given engine to that of the Carnot engine and use Carnot's theorem to answer the question about whether the given engine is possible, and if so, if it's reversible or irreversible.
 

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