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Physics · 2025 · 3 marks
CBSE 2025 · Region 5 · Set 3 · Q24
The magnetic moment ( 5 J / T ) of a bar magnet points along a uniform magnetic field 0 · 4 T. (a) Calculate (i) the potential energy of the bar…
The magnetic moment ( $\displaystyle 5 \mathrm{~J} / \mathrm{T}$ ) of a bar magnet points along a uniform magnetic field $\displaystyle 0 \cdot 4 \mathrm{~T}$.
(a)
Calculate (i) the potential energy of the bar magnet, and (ii) the work done in turning the magnet by $\displaystyle 180^{\circ}$.
(b)
In which case is the potential energy of the magnet minimum ?
Marking-scheme solution
(a)(i)
$\displaystyle U=-M B \cos \theta$
$\displaystyle =-5 \times 0.4 \times 1$
$\displaystyle =-2 \mathrm{~J}$
(ii)
$\displaystyle W=M B\left(\cos \theta_{2}-\cos \theta_{1}\right)$
$\displaystyle =-5 \times 0.4(-1-1)$
$\displaystyle =4 \mathrm{~J}$
(b)
Potential energy is minimum in case (i).
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