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Question: Answered & Verified by Expert
A coil in the shape of an equilateral triangle of side $l$ is suspended between the pole of a permanent magnet such that $B$ is in plane of the coil. If due to current $i$ in the triangle a torque $\tau$ acts on it, the side $l$ of the triangle is:
PhysicsMagnetic Effects of CurrentNEETNEET 2005
Options:
  • A $\frac{2}{\sqrt{3}}\left(\frac{\tau}{B i}\right)$
  • B $2\left(\frac{\tau}{\sqrt{3} B i}\right)^{1 / 2}$
  • C $\frac{2}{\sqrt{3}}\left(\frac{\tau}{B i}\right)^{1 / 2}$
  • D $\frac{1}{\sqrt{3}} \frac{\tau}{B i}$
Solution:
1085 Upvotes Verified Answer
The correct answer is: $2\left(\frac{\tau}{\sqrt{3} B i}\right)^{1 / 2}$
Since $I=M B=(I A) B=I \frac{\sqrt{3}}{4} l^2 B$
$$
\Rightarrow \quad l=2\left(\frac{I}{\sqrt{3} B I}\right)^{1 / 2}
$$

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