Adiabatic compressed air and energy calculations

AI Thread Summary
The discussion revolves around solving a thermodynamics problem involving adiabatic compression of air in a pump. Key calculations include determining the volume and temperature of the compressed air, as well as the increase in internal energy during compression. Participants share their attempts at calculations, correcting each other's mistakes, particularly in applying the correct formulas and understanding the relationships between pressure, volume, and temperature. The conversation also touches on the heat transfer between the compressed gas and the steel cylinder, emphasizing the assumptions needed for accurate calculations. Ultimately, the group aims to clarify the thermodynamic principles at play while addressing specific homework questions.
  • #51
I was thinking since we can assume that all of the energy of the hot gas is transferred to the steel. Wouldn't what you said before still work?
 
Physics news on Phys.org
  • #52
Akaramos45 said:
I was thinking since we can assume that all of the energy of the hot gas is transferred to the steel. Wouldn't what you said before still work?
Will all of the gained internal energy will be transferred? Even when the gas cools back to 18C it will be at a higher pressure than it started. That represents a gain in energy, no?
 
  • #53
That's true. but this question allows me to state assumptions
 
  • #54
is there anything i can assume in order to make this true?
 
  • #55
Could i say that the increase in energy due to an increase in pressure is neglible?
 
  • #56
Akaramos45 said:
Could i say that the increase in energy due to an increase in pressure is neglible?
I would not have thought so, but I could be quite wrong about this. Maybe the internal energy is just PV, so it all does end up in the steel.
@Chestermiller , you'll be able to settle this immediately, I'm sure.
 
  • #57
haruspex said:
I would not have thought so, but I could be quite wrong about this. Maybe the internal energy is just PV, so it all does end up in the steel.
@Chestermiller , you'll be able to settle this immediately, I'm sure.
In my judgment, the way he did it in the first place is correct. The assumption is that the gas adiabatically heats up first, and then equilibrates thermally with the cylinder. So, the way you originally had it doped out was correct.
 
  • #58
Chestermiller said:
the gas adiabatically heats up first, and then equilibrates thermally with the cylinder.
Yes, I understand that, but I had this nagging thought that surely the compressed gas, even after equilibration, is storing energy. But I now think that its potential to do work comes from its low entropy, in the same way that an uneven temperature distribution has the capacity to do work.
Thanks for clearing it up.
 
  • #59
haruspex said:
Yes, I understand that, but I had this nagging thought that surely the compressed gas, even after equilibration, is storing energy. But I now think that its potential to do work comes from its low entropy, in the same way that an uneven temperature distribution has the capacity to do work.
Thanks for clearing it up.
Rather than assuming the the gas heats up first and then equilibrates with the cylinder, it is also possible to assume that the two heat up simultaneously. In this case $$dU=-PdV$$or $$MCdT=-\frac{nRT}{V}dV$$or equivalently $$MC\frac{dT}{T}=\frac{nR}{V}dV$$where M is the mass of the cylinder and C is the heat capacity of the steel. I think that this is more likely what they had in mind. In this case, the expansion would be nearly isothermal (because of the large thermal inertia of the cylinder).
 
  • #60
Chestermiller said:
it is also possible to assume that the two heat up simultaneously... I think that this is more likely what they had in mind.
The question statement says the air is compressed adiabatically.
 
  • #61
haruspex said:
The question statement says the air is compressed adiabatically.
Hmmm. Well, if it is truly adiabatic, then the gas can't exchange heat with the cylinder. So there is some ambiguity here. However, the version I just presented assumes that the combination of gas and cylinder is adiabatic.
 
Back
Top