An alpha particle is moving along a circle of radius R with a constant angular velocity w. Point A lies in the same plane at a distance 2R from the centre. Point A records magnetic field produced by a alpha particle.If the minimum time interval between two successive times at which A records zero magnetic field is "t", the angular speed w, in terms of t is 

Dear Student,

Right hand rule suggest over the entire loop the magnetic field at A will be zero but for half the circular path, there is going to be some magnetic field component, so when it suggest t is the interval it means its the time taken to cover a semi circular arc, thus total time period = 2t

we know 
ω=2πT2π2tπt
Regards.

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