I am building a worksheet to show the difference in isothermal and
adiabatic expansion. Using Q = dU + W as first law. For adiabatic dU = -W.
Plotting the process on a PV diagram I used P*V^gamma = constant. For air
gamma = 1.40....according to one reference.
Now I calculate dU (really it is deltaU) as = 1.5nR(T2 - T1). The the work
should be the area bounded by the PV graph (I hope). The problem is that
for the numbers I am using dU and W do not agree.
P1 = 200 kPa ; V1=1 m^3; n = 0.04 mol; T1 = 600K
P2 = 103 kPa ; V2 = 1.6 m^3; n = 0.04 mol ; T2 = 496K
delta U = -51 J area under graph (counting squares ) 88J
I am sure I am missing something important or have a gross misconception.
Can anyone help?
Thanks in advance!
Ken Fox
Science Department Coordinator
IB Physics Teacher
Smoky Hill High School
Aurora, CO
kfox@mail.ccsd.k12.co.us