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The energy density (energy per unit volume) of a magnetic field $B$ in free space is:
A$2 B^2 / \mu_0$
B$B^2 / \mu_0$
C$\mu_0 B^2 / 2$
D$B^2 / (2\mu_0)$
Answer & Solution
Correct answer: D. $B^2 / (2\mu_0)$
$u_B = B^2/(2\mu_0)$. Mirrors the electric-field energy density $u_E = \tfrac{1}{2}\varepsilon_0 E^2$. Holds for any magnetic field, not just inside a solenoid.
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