Isobaric process. Find magnitude of heat transfer

[tigertan]

Hi there,

I'm having a great deal of trouble with this thermodynamics question.

Cylinder filled with steam at 350 kN/m2. Initial volume is 0.05 m3. Sufficient heat transfer is effected to raise the internal energy in each cylinder by 50 kJ. The piston is moved slowly to maintain the pressure constant. Piston is of area of 0.1 m2, and the piston travels 0.15 m. Find the magnitude of the heat transfer.

So my attempt at the question:

dQ = dH (for isobaric case)
therefore:
dQ = dU + P (V2-V1)

and therefore
dQ = 50kJ + 350(1.55-0.05)
=575 kJ


What am I doing wrong? What am I missing?

 

[Chestermiller]

Hi there,

I'm having a great deal of trouble with this thermodynamics question.

Cylinder filled with steam at 350 kN/m2. Initial volume is 0.05 m3. Sufficient heat transfer is effected to raise the internal energy in each cylinder by 50 kJ. The piston is moved slowly to maintain the pressure constant. Piston is of area of 0.1 m2, and the piston travels 0.15 m. Find the magnitude of the heat transfer.

So my attempt at the question:

dQ = dH (for isobaric case)
therefore:
dQ = dU + P (V2-V1)

and therefore
dQ = 50kJ + 350(1.55-0.05)
=575 kJ


What am I doing wrong? What am I missing?

You miscalculated the volume change.

 

[Andrew Mason]

Also, there appears to be only one cylinder but the problem says that the Q increases ΔU by 50KJ for EACH cylinder.

AM