Why is angle between m and B in moving coil galvanometer =90°?

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Solution

n uniform magnetic field , Torque on a coil will vary if it rotates at an certain angle...or we change Theta i.e, angle between B and A..then we get variation in Torque..but By Creating Radial Magbetic field...angle between B and A is costant and equal to 90 degree...and we know that sin 90 = 1 so torque on a coil becomes NBA...

So here Torque become independent of Theta...which mean no matter if you rotate coil in radial magnetic field , The angle between B and A remain constant i.,e 90 which is not in the case of uniform magnetic field...

This is why Radial magnetic field is required...and such magnetic fields are produced by Using U-Shaped magnets...

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