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Q.

A magnetic needle with dipole moment M, is attached to a wooden bar of length L (see the below figure) and placed in a uniform magnetic field of intensity B. The bar and needle can revolve around point O. Find the magnitude of the moment of force that will cause the rotation of the bar around point O if the bar makes an angle α with the direction of the lines of force of the magnetic field.

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a

MB sin α

b

MB sin 2α

c

MB ( sin α + cos α)

d

2MB tan α

answer is A.

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Detailed Solution

The moment of forces about O, acting upon each pole of the needle will be equal to mB(l+L)sinα  and -mBLsinα.

Hence, the net moment of the force acting on the bar is

                              ζ =  mB(l+L)sinα-mBLsinα

                                 =  mBlsinα = (ml)Bsinα

                                 =  MBsinα

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