- #1
anothersivil
- 8
- 0
Homework Statement
*1* The Gibbs function G(T; P) of a certain gas is:
G = nRT ln P + A + BP + (1/2)*CP^2 + (1/3)DP^3
where A, B, C and D are constants. Find the equation of state of the gas.
Homework Equations
G = nRT ln P + A + BP + (1/2)*CP^2 + (1/3)*DP^3
The Attempt at a Solution
I think I have a solution for this, but it seemed to be too easy.
The definition for Gibbs free energy is defined to be:
1) G = U - TS + PV and 2) dG = -SdT + vdP
solving 2) for dG/dP yields: dG/dP = -SdT/dP + V 3)
I thought to take 3) at constant temperature yielding: dG/dP = V
Now, taking dG/dP at constant T from the given formula yields:
dG/dP =nRT/P + B + CP + DP^2 4)
Setting 3) = 4) and solving for PV yields:
PV = nRT + PB + CP^2 + DP^3 That just seemed too easy for me >.>
The second part of this homework is:
Homework Statement
Consider a spring which follows Hookes law; namely the displacement x from equilibrium
position is proportional to the tension X when it is pulled at a constant temperature. The spring constant is temperature dependent, k = k(T). Determine the free energy F, the internal energy U, and the entropy S, as a function of T and x. Neglect the thermal expansion of spring. Use F0(T) ´ F(T; x = 0); U0(T) ´ U(T; x = 0); S0(T) ´ S(T; x = 0) where necessary.
Homework Equations
k = k(T)
F[0](T) = F(T; x = 0)
U[0](T) = U(T; x = 0)
S[0](T) =S(T; x = 0)
where the brackets denote subscripts
The Attempt at a Solution
The force of the spring can be obtained by: F = -k(T) * x
And the work: W = int(-k(T) * x) dx = -(1/2)*k(T)*x^2
and dW = k(T) *xdx
Using the first law of thermodynamics: dU = dQ - dW, so
dU = dQ + k(T)*xdx
Last edited: