A magnetic needle shows identical deflections in two cases, first when one magnet is placed normal to the earth's magnetic field with its centre at a distance of $20$ cm from the centre of the magnetic needle, and second when another one is placed with its centre at a distance of $30$ cm from the centre of the needle. The correct ratio of the magnetic moments of the two magnets is?

2 : 3

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4 : 9

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8 : 27

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9 : 4

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Solution

The correct option is C $8 : 27$
Magnetic moment $\propto (d i s t a n c e f r o m c e n t r e)^{3}$
$∴ \frac{M_{1}}{M_{2}} = {(\frac{20}{30})}^{3} = \frac{8}{27}$

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The correct option is C 8:27Magnetic moment ∝(distancefromcentre)3∴M1M2=(2030)3=827

The correct option is C $8 : 27$
Magnetic moment $\propto (d i s t a n c e f r o m c e n t r e)^{3}$
$∴ \frac{M_{1}}{M_{2}} = {(\frac{20}{30})}^{3} = \frac{8}{27}$