GREPhysics.NET: GR9277 #72

GR9277 #72

Problem

GREPhysics.NET Official Solution

\prob{72}
A system in thermal equilibrium at temperature T consists of a large number $N_0$ of subsystems, each of which can exist only in two states of energy $E_1$ and $E_2$ , where $E_2-E_1=\epsilon \gt 0$ . In the expressions that follow, k is the Boltzmann constant.

The internal energy of the system at any temperature T is given by $E_1N_0+\frac{N_0\epsilon}{1+e^{\epsilon/kT}}$ . The heat capacity of the system is given by which of the following expressions?

$N_0k \left(\frac{\epsilon}{kT}\right)^2\frac{e^{\epsilon/kT}}{\left(1+e^{\epsilon/kT}\right)^2}$
$N_0k \left(\frac{\epsilon}{kT}\right)^2\frac{1}{\left(1+e^{\epsilon/kT}\right)^2}$
$N_0k\left(\epsilon/kT\right)^2e^{-\epsilon/kT}$
$N_0k\left(\epsilon/kT\right)^2$
$3N_0k/2$

Statistical Mechanics $\Rightarrow$ }Heat Capacity

The heat capacity is just $dU/dT$ , where ETS generously supplies U, the internal energy. Since $E_1$ and $N_0$ are constants, the first term is trivial.

The temperature-derivative of the second term is $N_0\epsilon^2/(kT^2)e^{\epsilon/kT} /(1+e^{\epsilon/kT})^2=$ , as in choice (A).

(The temperature derivative is easily done if one applies the chain-rule $\frac{df}{dy}\frac{dy}{du}\frac{du}{dT}$ where $f=1/y$ , $y=1+e^u$ , $u=\epsilon/(kT)$ .)

Alternate Solutions

jmason86
2009-09-03 19:19:22

I did this one by limits and general test taking strategy.

Limits: Q --> finite value as T --> $\infty$ . Eliminates (D) and (E)

ETS will generally force you to choose between very similar answers. Eliminates (C)

With only (A) and (B) left, it is a good idea to guess. But if you can get a hunch, that is even better. It's probably totally flawed thinking, but I saw the lack of an exponential in the numerator for (B), which made it look similar to a Bose-Einstein distribution. The full expression in (A) reminded me of the Maxwell-Boltzmann distribution that comes from the problem statement. Choose (A)

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Comments

jmason86
2009-09-03 19:19:22

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RebeccaJK42
2007-03-23 10:27:19

Where does the k in the numerator come from?

alpha
2007-03-30 22:37:37

k is Boltzmann constant.. The k on the numerator is actually canceled out by the $k^2$ in the denominator

Richard
2007-09-14 00:05:03

The fact of the matter is, you have a factor of
$\epsilon/k$ introduced by the derivative.
To make it look pretty, they multiplied the top and bottom by $k$ giving (with the extra $\epsilon$ ) the factor $k(\frac{\epsilon}{k})^2$ . Then of course, you have the $T^2$ .

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Andresito
2006-03-29 10:05:29

I could not obtain T^2 in the denominator. Is there an error in the solution provided by the ETS?

radicaltyro
2006-10-31 14:19:25

Hi Andresito,

Review your calculus and try again. The answer is correct.

Blue Quark
2007-11-01 08:00:11

Andresito you made the same mistake I did. The T is in the denominator, not the numerator. Thus when you differentiate you get a (-T^(-2)) in front of the exponential in addition to the normal e/k

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