Solving for Pressure in Four Cylinder Thermodynamic Problem

[alex1261]

Homework Statement

http://www.docstoc.com/docs/14364187/Heat_-Temperature_-Thermodynamics-and-Heat-Engines-Problem-Set

This question I am trying to figure out is Number eight with the four cylinders.

Homework Equations

P=F/A
PV=nRT

The Attempt at a Solution

I'm stuck at trying to find the pressure. I use the P=F/A formula but the pressure doesn't seem to work.

 

[rock.freak667]

At state 2, when the mass is in equilibrium, you need to do a force balance. Remember F=PA. Then look at the temperatures of states 1 and 2. What type of process is that?

 

[alex1261]

Thanks ! i have managed to solve a and b but now i am stuck on c I believe the answer to c is isobaric, because P=F/A is not affected by the growing volume nor the temperature. and it can't be isothermal because the temperature is changing. and I don't quite understand how adiabatic works.

 

[alex1261]

Ok my answer to the volume are wrong, do I use P1V1=P2V2 or do I use PV=nRT equation ?

 

[alex1261]

can anyone help please ? I've figured out pressure, but I'm stuck on how to find volume

 

[rock.freak667]

I think you should use P1V1=P2V2, that is supposed to work when T=constant.

 

[alex1261]

yeah I was considering that. And to find the volume in state 4, I would have to use PV=nRT correct ? since T isn't constant ?

 

[rock.freak667]

yeah I was considering that. And to find the volume in state 4, I would have to use PV=nRT correct ? since T isn't constant ?

From state 3 to 4, it looks like T=100°C for both 3 and 4. So while you can use PV=nRt at 4, you should be able to use P₃V₃=P₄V₄ as well