#1


For point A in the figure, the pressure is 3 atm, the volume is .08 m3 and the internal energy is 750 J. At point C the values are 1.2 atm, .03 m3, and 420 Joules.
(a) For the cyclic process abcda, how much heat is exchanged with the environment?
(b) For the process abc, what is the change in internal energy?


I'm totally lost on this problem, our teacher is foreign and difficult to understand . Could UG give me some legit help on this lol?
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#2
Hin`t B and A have equal pressure and a and d have equal temp. also p1/t1=p2/t2
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#3
Quote by enceps
Hin`t B and A have equal pressure and a and d have equal temp. also p1/t1=p2/t2


C and D also have equal Pressure and A/D have equal Volume. It doesn't really help you much in this problem...
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#4
Quote by Flux'D
C and D also have equal Pressure and A/D have equal Volume. It doesn't really help you much in this problem...



should help, also if you state the former, the latter is usually inferred.
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#5
^ True. The problem asks about Internal Energy and I'm not sure how to relate the Pressure and Volume with it
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#6
well your lowering pressure and volume, so internal energy should be going down.\

B) change in internal energy from abc should be 750-420
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Last edited by enceps at Dec 1, 2009,
#7
Personally I don't think you're given enough info to complete the problem. I mean, what kind of fluid/substance are we talking about? If it's just water, then you should probably use steam tables to find the thermodynamic data you need. If it's another substance, you'll have to look up wither thermodynamic tables for it, or use something like an equation of state (EOS) to find what you need. The pressures are pretty low, so I guess you can assume ideal behaviour, and this should give you a relatively good answer, but like I said, the question isn't very well defined and is pretty open ended.

The basic relations for the volume dependence of internal energy are:

(dU/dV)=T(dP/dT)-P (all partial derivatives). You may have to use an EOS to find the P and dP/dT as functions and then integrate with respect to volumes, but since the pressures are low to moderate, I would personally assume ideal behaviour.

As for finding heat exchanged with the environment, use the first law energy balance, dU=dQ+dW

I'll admit, I've never been asked any questions regarding internal energy in Thermo, but I did Chemical Engineering Thermodynamics, which may be different to what you're doing. If you are confused with thermo, it's completely normal-- everyone is, even the tutors, but seriously, make an appointment to see your lecturer for any difficulty you have, and do outside reading for the subject. I would recommend "Introduction To Chemical Engineering Thermodynamics" (J Smith, H. Van Ness, M. Abbott).

Hope the above equations can be of some use to you, and sorry if they aren't!
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#8
Quote by whosthat?
Personally I don't think you're given enough info to complete the problem. I mean, what kind of fluid/substance are we talking about? If it's just water, then you should probably use steam tables to find the thermodynamic data you need. If it's another substance, you'll have to look up wither thermodynamic tables for it, or use something like an equation of state (EOS) to find what you need. The pressures are pretty low, so I guess you can assume ideal behaviour, and this should give you a relatively good answer, but like I said, the question isn't very well defined and is pretty open ended.

The basic relations for the volume dependence of internal energy are:

(dU/dV)=T(dP/dT)-P (all partial derivatives). You may have to use an EOS to find the P and dP/dT as functions and then integrate with respect to volumes, but since the pressures are low to moderate, I would personally assume ideal behaviour.

As for finding heat exchanged with the environment, use the first law energy balance, dU=dQ+dW

I'll admit, I've never been asked any questions regarding internal energy in Thermo, but I did Chemical Engineering Thermodynamics, which may be different to what you're doing. If you are confused with thermo, it's completely normal-- everyone is, even the tutors, but seriously, make an appointment to see your lecturer for any difficulty you have, and do outside reading for the subject. I would recommend "Introduction To Chemical Engineering Thermodynamics" (J Smith, H. Van Ness, M. Abbott).

Hope the above equations can be of some use to you, and sorry if they aren't!


Agreed there isnt enough info, which leads me to believe it's one of those trick questions, where you don't need all the info to do the actual problem.
sigh...