Minimal of free energy equation

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SUMMARY

The discussion centers on demonstrating that the Helmholtz free energy, defined as F = SUM(E_i*p_i) + T*SUM(p_i*ln(p_i)), reaches a minimum when the probabilities p_i are given by the formula p_i = e^(-E_i/T) / SUM(e^(-E_i/T)). The key insight is that while F is minimized under these conditions, the probabilities must also be normalized to 1. The relationship between the Helmholtz free energy and entropy is crucial, as the entropy must remain fixed during the minimization process.

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  • Understanding of Helmholtz free energy and its definition (F = U - TS)
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  • Basic concepts of entropy and its role in thermodynamics
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Homework Statement


Show that the free energy, F=SUM(E_i*p_i)+T*SUM(p_i*ln(p_i)) is minimal when p_i=e^(-E_i/T)/SUM(e^(-E_i/T))

Where E is energy, T is temperature

Homework Equations


Nothing really

The Attempt at a Solution


So if I just try and find the derivative of the force I arrive at something along the lines of p_i=e^((-E_i/T)-1).
I'm assuming I'm supposed to use some constraints to show that the entropy is fixed but I'm not sure how to go about doing that.
 
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Hi. Note that F is not a force but the Helmholtz free energy defined as F = U–TS;
Also note that you do have a minimum when pi is proportional to exp(–1–Ei/T) as you vary F and set δF = 0, but pi are probabilities and therefore they need to be normalized to 1...
 

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