# Thermodynamics piston-cylinder closed system

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1. Feb 12, 2017

### Andrew Pierce

1. The problem statement, all variables and given/known data
An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 175 kPa. Water is stirred by a paddle wheel while a current of 8 A flows for 45 min through a resistor placed in the water, If one-half of the liquid is evaporated during this constant pressure process and the paddle -wheel work amounts to 400 kJ, determine the voltage of the source. Also, show the process on a P-v diagram with respect to saturation lines.

2. Relevant equations
ΔE = Ein - Eout
Welectric = VIΔt
if ΔP = 0, ΔH = m(h2 - h1)
ΔH = ΔU + Wboundary

3. The attempt at a solution
V = 5 L
m = 5 kg (by conversion)
P = 175 kPa
I = 8 A
Δt = 45*60 sec = 2700

Assuming that KE, and PE = 0,
ΔE = ΔU

Also since it is insulated we are assuming that the heat lost is 0

We + Wsh - Wboundary = ΔU
We + Wsh = ΔU + Wboundary
We + Wsh = ΔH
We + Wsh = m(h2 - h1)

Here is where I get lost. I know that we can find h1 from the saturated water tables
h1 = 487.01 kj/kg (at Psat = 175 kPa)
I'm confused as to where the second enthalpy value comes from since our state has constant P.

Also the P-v should look something like this I believe:

#### Attached Files:

• ###### Drawing.jpeg
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2. Feb 13, 2017

### Staff: Mentor

If you are using the saturation tables, you can get $h_2$ using the lever rule, or equivalently, as $h_2=h_1+\Delta h_{vap}/2$.

Regarding the P-V diagram, your result is correct.