EML 3100 Homework No. 2
1.
A piston cylinder device is to be analyzed.
At the initial state, the cylinder holds 25
grams of saturated water vapor at 300 kPa.
A resistance heater is installed in the walls of
the cylinder and is turned on for 5 minutes.
During this time 0.2 amps of current flows
through the heater from a 120V power source.
The cylinder is fairly well insulated but
loses during this time period a total amount of heat equal to 3.7kJ.
Assume that during
the heating that the piston is allowed to move so that pressure is constant.
a.
show that for this closed system that the boundary work Wb and the change in
internal energy deltaU, in the first law relation can be combined into one term,
deltaH for the constant pressure process.
b.
Determine the final temperature of the steam
State the assumptions that you should make for this analysis.
Soln.
Tank is stationary so KE and PE changes are zero
Therefore deltaE = deltaU
Neglect the energy changes in the wires and cylinder
Q Wother –Wb = U2 – U1
Wb = P (V2V1) for a constant pressure process
Therefore
Q – Wother –P(V2V1) = U2 – U1
P1 = P2 = P
Therefore
Q – Wother = (U2+P2V2) – (U1+ P1V1)
Or
Q – Wother = H2 – H1
Wother = V*I*deltat = 120 V * 0.2A * 300 seconds *(1kJ/s/1000VA) = 7.2kJ
At state point 1
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 Summer '08
 Sherif
 Thermodynamics, Heat, piston cylinder device, constant pressure process, Wother, deg F.

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