Question

A bar magnet is held at right angle to a uniform magneitc field. The couple acting on the acting on the magnet is to be halved by rotating it form this position. The angle of rotation is
A. `60^(@)`
B. `45^(@)`
C. `30^(@)`
D. `75^(@)`

Answer 1

As, `tau=MB sintheta`
Where, `theta=90^(@) Rightarrow tau=MB`
`tau_(2)=(1)/(2)tau_(1) Rightarrow MB sintheta=(1)/(2)MB Rightarrow sin theta =(1)/(2) Rightarrow theta=30^(@)`
`therefore "Angle of rotation"=90^(@)-30^(@)=60^(@)`