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Two point masses m each carrying charge ...

Two point masses m each carrying charge `-q and +q` are attached to the ends of a massless rigid non-conducting rod of length l. The arrangement is placed in a uniform electric field E such that the rod makes a small angle `5^(@)` with the field direction. The minimum time needed by the rod to align itself along the field is ..................

A

`t=pisqrt((2ml)/(3qE))`

B

`t=(pi)/(2)sqrt((ml)/(2qE))`

C

`t=sqrt((ml)/(qE))`

D

`t=2pisqrt((ml)/(E))`

Text Solution

Verified by Experts

The correct Answer is:
B

`tau=qE(lsintheta), tau=qEl theta`
`I=m((l)/(2))^(2)+m((l)/(2))^(2)=(ml^(2))/(2)`
`tau=Ialphaimpliesalpha=(tau)/(I)omega^(2)=(2qE)/(ml)`
`T=(2pi)/(omega)=(2pi)/(sqrt((2qE)/(ml)))=2pisqrt((ml)/(2qE)), t=(T)/(4)=(2pi)/(4)sqrt((ml)/(2qE))=(pi)/(2)sqrt((ml)/(2qE))`
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