Q. A short bar magnet of magnetic moment 20 Am² has a cross sectional area of 1.5 × 10-4 m². If the intensity of magnetisation of the magnet is 105 A/m. The length of magnet is
(a) 0.33 m
(b) 0.13 cm
(c) 1.33 m
(d) 1.33 cm
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Ans: (c)
Sol: Let l = Length of magnet
$ \displaystyle I =\frac{M}{V}=\frac{M}{A\times l}$
$ \displaystyle l =\frac{M}{A \times I}$
$ \displaystyle l =\frac{20}{1.5\times 10^{-4} \times 10^5}$
= 1.33 m