The magnetic moment of a circular coil carrying current is

CONCEPT:

• Magnetic moment (M): It is defined as the quantity that finds the force which a magnet is able to exert on electric currents and the torque that the magnetic field will exert on it.
  • Magnetic moments can be demonstrated by a lot of things such as a loop of electric current, an electron, a bar magnet, a molecule, or a planet.

Formula:

M = n i A.....(for circular loop)

where, n = number of turns, i = current flowing through , A =area of cross section.

EXPLANATION:

• As per the above discussion, it's clear that M α n, M α i, M α A.

M α A=> M α r² (As l = 2πr , i.e  l α r)

Therefore, M α l²

Hence the correct option is 4.