A small magnet of magnetic moment M, is placed at a distance ‘r’ from the origin ‘O’ with axis parallel to x – axis as shown in fig.
A small coil of one turn is placed on the x – axis, at the same distance from the origin with the axis of the coil coinciding with x-axis. For what value of the current in the coil does a small magnetic needle, kept at origin remains undeflected?

Question

A small magnet of magnetic moment M, is placed at a distance ‘r’ from the origin ‘O’ with axis parallel to x – axis as shown in fig.

A small coil of one turn is placed on the x – axis, at the same distance from the origin with the axis of the coil coinciding with x-axis. For what value of the current in the coil does a small magnetic needle, kept at origin remains undeflected?

Infinity Learn · Accepted Answer

The small magnetic needle will remain undeflected when magnetic field of Bar magnet is equal and opposite to the magnetic field of the coil. Bm¯$$=Bc$$¯⇒μ$$0M4$$$$π$$$$r3=$$μ$$0Ia22r3$$              Coil is small, a<

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