Q191 / 200 · Manipal (MET) 2012 · Physics

The ratio of magnetic moments of two short magnets which give null deflection in $\tan B$ position at 12 cm and 18 cm from the centre of a deflection magnetometer is

  • A
    $2: 3$
  • Correct answer
    B
    $8: 27$
  • C
    $27: 8$
  • D
    $4: 9$

Solution

According to question, $\begin{aligned} & \frac{M_1}{d_1^3}=\frac{M_2}{d_2^3} \\ \Rightarrow \quad & \frac{M_1}{M_2}=\left(\frac{d_1}{d_2}\right)^3=\left(\frac{12}{18}\right)^3 \\ \therefore \quad & \frac{M_1}{M_2}=\frac{8}{27} \end{aligned}$

Hence, the correct answer is option: 'B'.

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