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fluidistic
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Homework Statement
I would like some assistance to solve the following problem.
A magnetic system satisfies Curie's law ##M=nDB/T##, has a specific heat capacity at constant magnetization ##C_M=\text{constant}##. It is used in a Carnot engine that operates between temperatures ##T_h## and ##T_c## (##T_c<T_h##).
M is the magnetization and B is the external magnetic field.
1)Sketch a qualitative (B,M) diagram of a complete cycle.
2)Calculate the work done by the engine after 1 cycle.
3)Calculate the efficiency of the engine.
Homework Equations
##C_M=\frac{T}{n} \left ( \frac{\partial S}{\partial T} \right ) _{M,n}=\left ( \frac{\partial U}{\partial T}\right ) _{M,n}## (1)
The Attempt at a Solution
1)I've done the sketch, I don't think there's anything particular about it.
2)This is where I'm stuck.
From equation (1) I've determined that ##S(T,M,n)=C_Mn \ln T+f(M,n)## and that ##U(T,M,n)=C_MT+g(M,n)##. Not sure this can help.
I know that the work done is the area enclosed by the sketch in the B-M diagram, namely ##W=\oint B dM##.
I know that in a Carnot cycle there are 2 adiabatic and 2 isothermal processes. Also the 1st law of Thermodynamics states that ##\Delta U =Q+W## so after a cycle ##\Delta U=0## and so ##Q=-W##. In other words the heat toward the auxiliary system is equal to the work done BY the system. And since there are 2 adiabatic processes (no heat is being absorbed by the system), I get that the work done after 1 cycle is equal to the heat absorbed by the system during the 2 isothermal processes.
Now if I think of M as a function of B, T and n and assuming that n is constant for the auxiliary system then ##dM=\frac{nDdB}{T}-\frac{nDBdT}{T^2}##. Note that for the 2 isotherms, ##dM=\frac{nDdB}{T}##.
I also know that ##dW=BdM## and so for the isotherms, ##dW=\frac{nDBdB}{T}##. I know that there's 1 isotherm at a temperature of ##T_h## and the other at a temperature of ##T_c##. My problem is that I don't know what are the limits of the integral if I integrate this expression. So I can't really get W via this expression.
I don't really know how to proceed further.
Any tip is appreciated!