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 10^{5} 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