OJEE Energy Density — practice questions

Explore 2 free Energy Density questions in OJEE Physics. Open each question or exercise to read its content, reference answers and available explanation.

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Energy Density questions and exercises with answers

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The energy density (energy per unit volume) of a magnetic field $B$ in free space is:Read the answer and worked solution →The energy stored per unit volume in a magnetic field $B = 0.1$ T in vacuum (take $\mu_0 = 4\pi \times 10^{-7}$ N/A²) is approximately:Read the answer and worked solution →

Worked practice questions

The energy density (energy per unit volume) of a magnetic field $B$ in free space is:

$2 B^2 / \mu_0$
$B^2 / \mu_0$
$\mu_0 B^2 / 2$
$B^2 / (2\mu_0)$

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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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The energy stored per unit volume in a magnetic field $B = 0.1$ T in vacuum (take $\mu_0 = 4\pi \times 10^{-7}$ N/A²) is approximately:

$4.0 \times 10^{3}$ J/m³
$8.0 \times 10^{0}$ J/m³
$1.59 \times 10^{-3}$ J/m³
$1.0 \times 10^{-6}$ J/m³

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Correct answer: A. $4.0 \times 10^{3}$ J/m³

$u_B = B^2/(2\mu_0) = (0.1)^2 / (2 \times 4\pi \times 10^{-7}) = 0.01 / (2.51\times10^{-6}) \approx 3.98 \times 10^3$ J/m³ ≈ 4 kJ/m³.