q, qv, qp

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Elizabeth Bowen 1J
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q, qv, qp

Postby Elizabeth Bowen 1J » Mon Feb 10, 2020 2:38 am

Why does q (heat) need different values/symbols for qv and qp? I know that it's for constant pressure and volume, but why do these make the values of q different (why isn't it all just q)?

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Re: q, qv, qp

Postby lilymayek_1E » Mon Feb 10, 2020 10:25 am

you can use qv and qp to denote which constant the system is in, but they also serve an important purpose because depending on whether a system is in a constant volume or pressure state changes the capacity to do work regardless of having the same internal energy.
For example, because deltaU = q+w, and w=PdeltaV, you can use this equation to differentiate between constant pressure or volume systems.
In a constant volume system (so you'll be using qv), there is no change in volume, which means deltaV from the work equation=0. Thus, in a constant volume system, your method to find internal energy would be deltaU=qv. In a constant pressure system, it should be noted that qp is also equal to deltaH (change in enthalpy). Because a constant pressure is present & volume can change, the equation would be deltaU=deltaH-PdeltaV.
The distinction between qv and qp will be that work will either be performed or not (in the case of constant volume), and that. changes energy levels within the system. A system could have the same internal energy at both constant pressure and volume, but the work and energy levels could vary.

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