Magnetic Effects of Current and Magnetism Part 1

According to question, a charge Q is moving d distance in magnetic field B.
As we know that force acting on a point charge by magnetic field is always perpendicular to the velocity of particle v.
∴F=Q(v×B).....(i)
The above, statement means that F is perpendicular to v.
∵ Work done, W=F⋅ds
=Q(v×B)⋅dl  [using Eq. (i)]
=Q(v×B)dl‌cos‌θ
=Q(v×B)dl‌cos‌90∘    [∵θ=90∘F⟂v or dl]
=0
∴ The work done by the magnetic field will be zero.