Thermodynamics 1st law question - closed system

[chuffy]

Homework Statement

1kg of helium in a closed system has 40kj added by heating & 100kj removed by work during a reversible polytropic process.
The initial temp of the gas is 300K & the initial pressure is 100kN/m2

Q. find the final temp of the gas & pressure

Homework Equations

given: Cp=14.57 kgK & M=4

The Attempt at a Solution

Hi guys, I can't get my head around how to work out the final pressure (P2)

I got the Temp correct (T2) bu doing the following:

R=Ru/M
R =2.0785

Cv = R +Cp
Cv = 7.2715

closed system so V= const

Qnet = -100 -40 =-140
qnet = Q/m

qnet = Cv(T2-T1)
T2 = 281K

Can't think how to work out P2 when I don't have n

any help?

 

[nil1996]

closed system doesn't mean that volume is constant instead it means that the system cannot exchange matter but it can exchange energy.

 

[physics&math]

I'm not entirely sure if this would work, but you could try applying the ideal gas law.

P1V1/T1 = P2V2/T2

Then you would only have to solve for P2. (Since the system is closed, V1 and V2 would cancel).

 

[nil1996]

You have calculated Qnet wrong.Qnet is the net heat change of the system.

 

[nil1996]

I'm not entirely sure if this would work, but you could try applying the ideal gas law.

P1V1/T1 = P2V2/T2

Then you would only have to solve for P2. (Since the system is closed, V1 and V2 would cancel).

Volume is not constant

 

[chuffy]

yeah I've got the const vol bit wong

 

[Chestermiller]

You don't know P2 and you don't know n, but you have enough information to solve for both of these. What you need is two equations in these two unknowns.

What is the equation for W in terms of P2 and n for a reversible polytropic process?

What is the equation relating pressure, temperature, and n for a polytropic process?

You already know W and T2.

Chet