alleryThe conductivity of a metal in the free-electron model is $\sigma = ne^2\tau/m$. The mobility $\mu$ of the charge carriers is
A$\mu = ne\tau/m$
B$\mu = e\tau/m$
C$\mu = m/(e\tau)$
D$\mu = e\tau E/m$
Answer & Solution
Correct answer: B. $\mu = e\tau/m$
1. Mobility is the drift speed per unit field: $\mu = v_d/E$.
2. Since $v_d = e\tau E/m$, dividing by E gives $\mu = e\tau/m$.
3. The field E cancels, so D (which keeps E) is wrong.
4. A inserts an extra $n$; C inverts the expression.
_Source: syllabus Physics Ch 3 "Current Electricity", p.8_
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