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Homework Statement
Hi I am new to Physics Forums and desperately need some help! This is my first class back since undergrad and my math skills have gotten a little rusty. I think that's where I am failing. Okay here goes:
http://tinypic.com/r/2zqf04k/8
There is a 4 subsystem Isoentropic system filled with ideal gases. It contains: a heat reservoir (r), which contributes heat to systems 1 and 2. Systems 1 and 2 which have a movable wall between them but do not share temperature or concentration. And the system (m) which is a pulley system with a weight on the end with mass "m" and is a measurable height off the ground.
let "s" be the cross section of the wall separating systems 1 and 2.
(1) Derive the conditions for equilibrium of only systems r, 1 and 2.
(1A) Now Derive a relationship between N1,N2 and the equilibrium volumes V1°,V2°.
(2) Derive the conditions for equilibrium of all 4 systems.
(2A) Using the initial conditions found in 1A derive a relationship between m and the new equilibrium volume of system 1.
(3) Linearize the relation between m and V derived in 2A by assuming that V1=V1° +ΔV1. Express as m=αΔV1
Homework Equations
F=MA
F=P(area)
PV=NRT
E=cNRT
E(S,V,N,T)
The Attempt at a Solution
(1) Since the system is isoentropic, using Esys=Er+E1+E2 I took the partial derivatives of E with respect to S,V,N and T for each system and then simplified to:
0=((dE1/dS1/)v1N1 - (dEr/dSr))
0=((dE2/dS2/)v2N2 - (dEr/dSr))
0=((dE1/dV1)N1S1 -(dE2/dV2)N2S2)
Then from the diagram:
T1(S1,V1,N1)=Tr=T2(S2,V2,N2)
P1(S1,V1,N1)=P2(S2,V2,N2)
(1A)
PV=NRT substituted into P1(S1,V1,N1)=P2(S2,V2,N2) gives: (N1R1T1/V1)=(N2R2T2/V2)
The T1 and T2 can be substituted as Tr since T1(S1,V1,N1)=Tr=T2(S2,V2,N2)
So I get: N1/V1=N2/V2
Then since the total volume of system 1 and 2 is constant:
V°(1,2)=V°1+V°2
(2)
Esys= Er +Em + E1 +E2 I took the partaials of Er,E1 and E2 with respect to S,V,N like before and then Em with respect to Height and S. Is that correct?
I got back:
0=((dE1/dS1/)v1N1 - (dEr/dSr))
0=((dE2/dS2/)v2N2 - (dEr/dSr))
0=((dE1/dV1)N1S1 -(dE2/dV2)N2S2)
0=((dEm/dSm)mH-(dEr/DS4))
0=(dEm/dH)m,Sm
P°(1,2)= P1 +P2
V(1,2)= V1+V2
(2A)
This is where I really get stuck. I am very tempted to set up the equation F(1,2,m)= F1 -F2 +Fm based off of free body diagrams I drew in the system. From there using F=MA for the mass system and F=P(area of the wall denoted by s.)
Substituting in I get: 2(P1)(s)- (P°(1,2))(s)+mg
Substituting in PV=NRT and V1= V°+ΔV and T1=T2=Tr I get : TrR((2N1/V°)+2(N1/ΔV)-(N°1,2/V°(1,2))+mg
Am I on the right track here? Should I have used and Energy equation instead? I tried that with combining PVT=NRT with E=cNRT and E=mgH and got an Equation:
E= cP1V1 +cP2V2+ mgH
and after taking the derivative an answer of m=-2ΔP1ΔV
but I wasn't sure if this was correct either. Please help!