Ideal gas temperature/pressure/volume problem

[colbyrandell]

Homework Statement

A mass of gas at an initial pressure of 38 bar, and with an internal energy of 1400kJ, is contained in a perfecty insulaed cylinder of volume 0.06 meters cubed. the gas is allowed to expand behind a piston until its internal energy is 1300kJ. the law of expansions is PV^1.8=C. find:
work done
the final volume
the final pressure

Homework Equations

PV^1.8=c
Uinital + Q = U final + W
W = (PiVi - PfVf)/(n-1)
PiVi^1.8=PfVf^1.8

The Attempt at a Solution

Ui - Uf = W - Q
1400-1300= W - 0
1000 = W[/B]

 

[ehild]

Welcome to PF!

Homework Statement

A mass of gas at an initial pressure of 38 bar, and with an internal energy of 1400kJ, is contained in a perfecty insulaed cylinder of volume 0.06 meters cubed. the gas is allowed to expand behind a piston until its internal energy is 1300kJ. the law of expansions is PV^1.8=C. find:
work done
the final volume
the final pressure

Homework Equations

PV^1.8=c
Uinital + Q = U final + W
W = (PiVi - PfVf)/(n-1)
PiVi^1.8=PfVf^1.8

Your equation for the work is not correct. The work of the gas when it expands from volume V_i to volume V_f is equal to the integral $$W=\int _{V_i}^{V_f}{PdV}$$

In case of this gas, $$P = \frac{P_iV_i^{1.8}}{V^{1.8} }$$

The Attempt at a Solution

Ui - Uf = W - Q
1400-1300= W - 0
1000 = W[/B]

1400-1300=100 kJ

ehild