# Thermodynamics of heating water in a cylinder with piston

1. Sep 19, 2014

### yy205001

1. The problem statement, all variables and given/known data
Consider water contained in a cylinder at 25C with a frictionless piston with some weights on it. Initially the pressure inside the piston is 100kPa and then the water is heated such that the water does 290kJ/kg of work on the piston. Find the final temperature of the water.

2. Relevant equations
First Law: Ein-Eout=ΔEsystem

3. The attempt at a solution
Since heat is added and work in done by the system, by the first law,
qin-wout=Δesystem=Δu=cv(T2-T1)

q-290=cv(T2-T1)

But how can I get q in order to calculate T2?

Thank you

2. Sep 19, 2014

### BvU

You will need more equations than just the one under 2.
cv requires v constant. How can the water do work if the piston doesn't move ?

3. Sep 19, 2014

### yy205001

Can I assume the pressure is constant during the process?

4. Sep 19, 2014

### haruspex

Why would the pressure change? What determines the pressure in this set-up?

5. Sep 19, 2014

### yy205001

The pressure is 100kPa in state1 but did not mention about the pressure about state2?

6. Sep 19, 2014

### DEvens

What would "a frictionless piston with some weights on it" mean about the pressure before and after?

7. Sep 19, 2014

### Staff: Mentor

How can the pressure on the water be 100 kPa if the piston has weights on it? Isn't that just atmospheric pressure? Or is the 100 kPa gage pressure?

Chet