# Heat Capacity Power Series Approximation

• Fuzzletop

## Homework Statement

"Derive a more accurate approximation for the heat capacity at high temperatures, by keeping terms through $x^{3}$ in the expansions of the exponentials and then carefully expanding the denominator and multiplying everything out. Throw away terms that will be smaller than $(\frac{ε}{kT})^{2}$ in the final answer. When the smoke clears, you should find $C = Nk(1 - \frac{1}{12}(\frac{ε}{kT})^{2})$"

## Homework Equations

The "exact" formula for the heat capacity was found in an earlier part to the question:
$C = \frac{Nε^{2}e^{\frac{ε}{kT}}}{kT^{2}(e^{\frac{ε}{kT}} - 1)^{2}}$

## The Attempt at a Solution

I used the Power Series expansion for small x:

$e^{x} ≈ 1 + x + \frac{x^{2}}{2} + \frac{x^{3}}{6}$

where $x = \frac{ε}{kT}$

I expanded all this out, factoring out $x^{2}$ on bottom to cancel the same on top, removed powers of x greater than 2:

$C = N k \frac{x^{2}(1 + x + \frac{x^{2}}{2} + \frac{x^{3}}{6})}{(x + \frac{x^{2}}{2} + \frac{x^{3}}{6})^{2}}$

$C = N k \frac{x^{2}(1 + x + \frac{x^{2}}{2} + \frac{x^{3}}{6})}{x^{2} + x^{3} + \frac{7 x^{4}}{12} + \frac{x^{5}}{6} + \frac{x^{6}}{36}}$

$C = Nk\frac{1 + x + \frac{x^{2}}{2}}{1 + x + \frac{7 x^{2}}{12}}$

I'm not sure how to take this any further. I have gotten something that almost resembles what I want:

$C (1 + x + \frac{7x^{2}}{12}) = N k (1 + x +\frac{7x^{2}}{12} - \frac{x^{2}}{12})$

But clearly, I can't just divide by the part attached to C and call it a day. Any tips to help get to the target answer?

Welcome to PF, Fuzzletop!

You can use: ${1 \over 1 - x}=1+x+x^2+x^3+...$

But before you do so, you should expand your denominator with one extra term in the first step.
And then you need to keep all powers up to power 3 (power 5 in the denominator since that reduces to power 3).

Thanks!

I did manage to get something of an answer. I did so while still ignoring the powers of 3 (I tried to use them at first, but I couldn't get it to work, so I switched back to just up to powers of 2, and I tried this below; I'm sure including the powers of 3 would work equally as well):

$C = Nk\frac{(1 + x + x^{2} - \frac{5x^{2}}{12} - \frac{x^{2}}{12})}{(1 + x + x^{2} - \frac{5x^{2}}{12})} = Nk\frac{(\frac{1}{1 - x} - \frac{5x^{2}}{12} - \frac{x^{2}}{12})}{(\frac{1}{1 - x} - \frac{5x^{2}}{12})}$

Which eventually comes very close to the goal equation, but for a term $\frac{5x^{2}}{12}$ which I removed as it's "smaller than $(\frac{ε}{kT})^{2}$" (a bit less than half), and I got the answer, $C = Nk(1 - \frac{1}{12}(\frac{ε}{kT})^{2})$

Thanks a lot! I'm not usually one to think of using series expansions unless I've been told to do so explicitly, so that was a great help!

You're welcome!

Actually, I intended (but with terms up to the 3rd power):
$$C = Nk\frac{1 + x + \frac{x^{2}}{2}}{1 + x + \frac{7 x^{2}}{12}} = Nk(1 + x + \frac{x^{2}}{2})(1 - (x + \frac{7 x^{2}}{12}) + (x + \frac{7 x^{2}}{12})^2 - ...)$$