Magnetic Properties of Matter

JEE Main level, single correct, medium

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Question 31 of 57

The length of a magnet is large compared to its width and breadth. The time period of its width and breadth. The time period of its oscillation in a vibration magnetometer is 2 s2 \mathrm{~s}. The magnet is cut along its length into three equal parts and three parts are then placed on each other with their like poles together. The time period of this combination will be

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Answer: (B) 2/3 s2 / 3 \mathrm{~s}

Time period of vibration, T=2π1MBT=2 \pi \sqrt{\frac{1}{M B}}
Where ℓ=\ell= moment of inertia of magnet, M=\mathrm{M}= magnetic moment I=mℓ212\mathrm{I}=\frac{\mathrm{m} \ell^2}{12} and M=\mathrm{M}= pole strength ×ℓ\times \ell
I′=112(m3)(ℓ3)2×3=l9I^{\prime}=\frac{1}{12}\left(\frac{m}{3}\right)\left(\frac{\ell}{3}\right)^2 \times 3=\frac{\mathrm{l}}{9}
and M′=M^{\prime}= pole strength (will remain the same) ×(ℓ/3)×3=M\times(\ell / 3) \times 3=M.
T′=T9=29 s.\mathrm{T}^{\prime}=\frac{\mathrm{T}}{\sqrt{9}}=\frac{2}{9} \mathrm{~s} .

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