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Questions  

A pivoted magnetic needle of length 2l  and pole strength  'm' is at rest in magnetic meridian. It is held in equilibrium at an angle θ  to a magnetic induction field BH  by applying a force  F at a distance 'r'  from the pivot along a direction perpendicular to the field, then F=?

a
F=(2l)mBHtanθr
b
F=r2lmBHtanθ
c
F=mBHtanθ2r
d
F=2rmBHtanθ

detailed solution

Correct option is A

In equilibrium, torque due to F is balanced by torque due to BH  ∴τF=τBH ⇒ ( Fcosθ)r=MBHsinθ ⇒F=MBHsinθcosθ×1r ⇒F=m(2l)BHtanθr

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