Q.

An infinitely long positively charged straight thread has a linear charge density λ Cm-1. An electron revolves along a circular path having axis along the length of the wire. The graph that correctly represents the variation of the kinetic energy of electron as a function of radius of circular path from the wire is    [2024]

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Ans.

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Electric field E at a distance r due to infinite long wire is E=2kλr

Force of electron, F=eEF=e(2kλr)=2kλer

This force will provide the required centripetal force. 

F=mv2r=2kλerv=2kλem

KE=12mv2=12m(2kλem)=kλe=constant