Thermodynamics energy balance for control volume

[Andrew1234]

Homework Statement

Explain why the energy balance for a control volume includes an enthalpy term instead of an internal energy term.

Relevant Equations

Energy balance for a control volume:

dE/dt = dQ/dt-dW/dt-dm/dt(ΔH+ΔKE+ΔPE)

0 = dQ/dt-dW/dt-dm/dt(ΔH+ΔKE+ΔPE)

Energy balance for other systems:

ΔE =Q-W-(ΔU+ΔKE+ΔPE)

0 = Q-W-(ΔU+ΔKE+ΔPE)

Why is energy balance for a control volume

dE/dt = dQ/dt-dW/dt-dm/dt(ΔH+ΔKE+ΔPE)

0 = dQ/dt-dW/dt-dm/dt(ΔH+ΔKE+ΔPE)

whereas for other systems it is

ΔE =Q-W-(ΔU+ΔKE+ΔPE)

0 = Q-W-(ΔU+ΔKE+ΔPE)

with enthalpy, h = u +pv, replaced by only the internal energy? How is the pv term accounted for?

What I have tried

In a control volume, the dW/dt term fails to account for the pV work which therefore needs to be included in the separate enthalpy term. This is not true of systems that are not control volumes.

 

[Chestermiller]

You have to make an effort to answer the question before we can help you. So...