Difference between qv and qp


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Alex Chen 2L
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Joined: Wed Sep 18, 2019 12:21 am

Difference between qv and qp

Postby Alex Chen 2L » Sun Feb 02, 2020 2:04 pm

In systems of constant pressure, qp is equal to the change in enthalpy, delta H. However, I'm a bit confused on how it differs from qv, or the energy transferred by heating in a system of constant volume. Does it still equal delta H or does it have to be calculated through q = mCdelta(T)?

Sahil Jog 1F
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Joined: Thu Jul 11, 2019 12:16 am

Re: Difference between qv and qp

Postby Sahil Jog 1F » Sun Feb 02, 2020 2:35 pm

Under conditions of constant pressure, the piston has to be freely movable, thus accommodating for the constant pressure when heated. This would mean that under constant pressure the q would be the enthalpy change or equivalent to the amount of heat added. However, under conditions of constant volume, the piston is not allowed to move, thus the q would be calculated through mCv(delta)T, not just the enthalpy change.

KBELTRAMI_1E
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Joined: Sat Jul 20, 2019 12:17 am

Re: Difference between qv and qp

Postby KBELTRAMI_1E » Sun Feb 02, 2020 2:54 pm

Sahil Jog 1F wrote:Under conditions of constant pressure, the piston has to be freely movable, thus accommodating for the constant pressure when heated. This would mean that under constant pressure the q would be the enthalpy change or equivalent to the amount of heat added. However, under conditions of constant volume, the piston is not allowed to move, thus the q would be calculated through mCv(delta)T, not just the enthalpy change.


So is this just a difference in notation or would the calculations differ significantly?

Sahil Jog 1F
Posts: 126
Joined: Thu Jul 11, 2019 12:16 am

Re: Difference between qv and qp

Postby Sahil Jog 1F » Sun Feb 09, 2020 7:43 pm

It is just a difference in notation. I do not really know why he makes it this confusing.

AKhanna_3H
Posts: 104
Joined: Wed Sep 18, 2019 12:19 am

Re: Difference between qv and qp

Postby AKhanna_3H » Sun Feb 09, 2020 7:44 pm

In general the subscripts are just indicating volume and pressure. This is similar to saying Cv and Cp as heat capacities at constant volume and constant pressure.

Nare Nazaryan 1F
Posts: 101
Joined: Fri Aug 09, 2019 12:17 am

Re: Difference between qv and qp

Postby Nare Nazaryan 1F » Sun Feb 09, 2020 9:06 pm

When you have q[v], it means that volume is constant (v=0), so if you plug that into the deltaU equation, your work will also be 0 since it's based on volume (w=-PdeltaV) and therefore, you get deltaU=q. On the other hand, when you are doing q[p], that means your pressure is constant (p=0) and therefore you have deltaH (which is enthalpy/heat under constant pressure) -pdeltaV for work.


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