Question

A point charge q is revolving in a circle of radius ′r′ around a fixed infinite line charge with positive charge λ per unit length. Now the point charge is shifted and it revolves in a circle of radius 2r. Then :

A
speed of the point charge q remain constant
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B
speed of the point charge q will be change
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C
angular velocity of the point charge q about line charge does not change
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D
work done by all forces is non-zero
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Solution

The correct option is A speed of the point charge q remain constantElectric field at distance r from the infinite line charge is E=k2λrThe necessary centripetal force to the point charge of mass m is provided by electrostatic attraction.∴mv2r=qE=qk2λr or v=√2kλqmThe speed of charged particle is independent of orbital radius. Hence the K.E. of point charge in orbits of radius r and 2r is same. As the work done is the change of K.E, so the work done by all forces is zero.

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