# A point charge Q(=3×10−12C) rotates uniformly in a vertical circle of radius R =1 mm. The axis of the circle is aligned along the magnetic axis of the earth. At what value of the angular speed ω, the effective magnetic field at the center of the circle will be reduced to zero ? (Horizontal component of Earth's magnetic field is 30 micro Tesla)

1011 rad/s

109 rad/s

1013 rad/s

107 rad/s

## The correct Answer is:A

## μ02R(ωq2π)=30×10−6

So ω=1011 rad/s

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