A very long straight wire carries a current I. At the instant when a charge `+Q` at point `P` has velocity `vecV`, as shown, the force on the charge is

Options

A. Opposite to `ox`
B. along `ox`
C. opposite to `oy`
D. along `oy`

Answer

D

Explanation

According to Maxwell's cork screw rule, the direction of magnetic field at point `P` in plane of paper is perpendicular to the plane of paper inwards, i.e, along `z`-direction. Now since charge is moving along `x`-direction therefore, from Fleming's left hand rule stretching the forefinger in direction of paper inwards the thumb will indicate the direction of force `F` acitng on the charged particle along `oy`.
Alternative: According to Maxwell's cork screw rule the direction of magnetic field at point `P,B` acts inwards into plane of the paper, i.e., along `z`-direction and `v` is along `ox` hence,
`F=Q(vec(v)xxvec(B))=Q|(hati,hatj,hatk),(upsilon,0,0),(0,0,-B)|`
`=Q[i(0)-hatj(-upsilonB)+hatk(0)]`
`hatjQupsilonB`
That is force is along `oy` direction.