Andrew1234
- 18
- 1
- 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.
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.
Last edited: