Engine question

[xyz]

Hi,there...

 

[xyz]

question?

Consider a cyclic engine.Which one of the following variables might NOT equal zero over one complete cycle? That is,some of these can be zero, but one will not be, over an entire cycle. Which one? Explain.
a) Work-Heat in
b) the amount of change pressure
c) Q_in
d) the amount of change volume
e) the amount of change temperature

Thanks in advance!

 

[bogdan]

a) work-heat in = - variation of internal energy = 0 (cycle)
b) variation of pressure = 0 (cycle)
c) Q_in = ?
d) variation of volume = 0 (cycle)
e) variation of temperature = 0 (cycle)

So, I think that the correct answer is c) Q_in <>0 ...

 

[Will]

Originally posted by bogdan
a) work-heat in = - variation of internal energy = 0 (cycle)
b) variation of pressure = 0 (cycle)
c) Q_in = ?
d) variation of volume = 0 (cycle)
e) variation of temperature = 0 (cycle)

So, I think that the correct answer is c) Q_in <>0 ...

For part a, I think that you are confusing the fact that Internal E = W- total Q. Q means the same as heat, and total Q is the sum of Q in and Q out. If Qin was equal to W, that would violate the Kelvin-Planck statement of the 2nd law of TD, which says it is impossible to construct a heat engine that can absorb energy as heat and do an equal amount of work. So a) can never be zero. But W-Q is always zero for cyclic process.
c) can also never be zero. If there is only supposed to be one answer then by Qin they mean Q total, in which case the answer is c).

 

[bogdan]

Well...if you say so...termodynamics is quite strange for me...
I didn't pay too much attention on part a) because I saw c), which was <>0...so...my mistake...sorry...