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# The dipole in a magnetic field

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#### The dipole in a magnetic field - Lesson Summary

The magnitude of the magnetic field can be accurately determined by placing a small compass needle in the magnetic field. Consider a compass needle of pole strength m, magnetic length 2l , moment of inertia I and magnetic moment M. Magnetic pole strength is a physical quantity used to measure the strength of a pole of a bar magnet or the hypothetical magnetic monopole. The magnetic moment is the product of pole strength and magnetic length. Place the compass needle in a uniform magnetic field of magnetic induction B such that, the axial line of the compass needle makes an angle �� with the magnetic field. The magnetic field exerts a force F, on the poles of the compass. Two equal and opposite forces acting on the compass needle constitute a couple. Thus, the moment of couple or torque acting on the compass needle is equal to the magnitude of the force into the perpendicular distance between the two forces: τ = F x d. The torque acting on a compass needle is τ = M x B.

The analogy between an electric dipole in an external electric field and a magnetic dipole in an external magnetic field is given in the following table.