Difference between qv and qp

$\Delta U=q+w$

Alex Chen 2L
Posts: 86
Joined: Wed Sep 18, 2019 12:21 am

Difference between qv and qp

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
Posts: 126
Joined: Thu Jul 11, 2019 12:16 am

Re: Difference between qv and qp

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
Posts: 108
Joined: Sat Jul 20, 2019 12:17 am

Re: Difference between qv and qp

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

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

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

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.