    Question

# Two bar magnets of same length and breadth having magnetic moments M and 4M respectively are joined with likes poles together and suspended in a uniform magnetic field B, it oscillates with time period T=√3 s. If the polarity of one of the magnet is reversed and the combination is again made to oscillate in the same field. Find the new time period of this combination.

A
3 s
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B
5 s
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C
2 s
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D
2 s
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Solution

## The correct option is B √5 sGiven: M1=M; M2=4M; T1=√3 s In a combination, like poles placed together, so net dipole moment will be Mres=M1+M2=M+4M=5M And we know that, the time period of oscillation of bar magnet in uniform magnetic field is T=2π√IMB So, for the combination of like poles, T1=2π√I(5M)B .......(1) In a combination, unlike poles are together, resultant moment will be M′res=|M1−M2|=|M−4M|=3M So time period for this combination will be T2=2π√I(3M)B ........(2) On dividing eq. (2) by (1), we get T2T1=√5√3 T2=√5√3×T1=√5 s Hence, option (B) is correct.  Suggest Corrections  0      Similar questions  Related Videos   Ampere's Law
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