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Consider a rectangular current-carrying coil PQRS having N turns and area A, placed in a uniform field 87290.png, in such a way that the normal 87296.png to the coil makes an angle θ with the direction of 87302.png (Fig. 3). the coil experiences a torque given by τ = NBiA sinθ, Vectorially, 87314.png
Fig. 3
  • τ is zero when θ = 0, i.e., when the plane of the coil is perpendicular to the field.
  • τ is maximum when θ = 90°, i.e., the plane of the coil is parallel to the field τmax = NBiA

Work done

If coil is rotated through an angle θ from its equilibrium position, then the required work is. W = MB(1 – cos θ). It is maximum when θ = 180° W­max = 2MB.

Potential energy

U = –MB cosθ  87320.png

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