Q.

A magnet is parallel to a uniform magnetic fleld. If it is rotated by 60o, the work done is 0.8 J. How much work is done in moving it 30o further?

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a

0.8 x 107 ergs

b

0.4 J

c

8 J

d

0.8 ergs

answer is A.

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Detailed Solution

W=MB(cos θ1-cos θ2)When the magnet is rotated from 0o to 60o, then work done is 0.8 J.0.8=MB(cos 0o-cos 60o)=MB2⇒MB = 1.6Nm In order to rotate the magnet through an angle of 30o further, i.e., from 60o to 90o, the work done isW'=MB(cos 600-cos 900)=MB12-0=MB2=1.62=0.8 J=0.8 x 107 ergs
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