Question

A uniform magnetic field and a uniform electric field are produced pointed in the same direction. an electron is projected with its velocity in the same direction

(A) the electron will turn to its right

(B) the electron will turn to its left

(c) the electron's velocity will increase in magnitude

D) the electrons velocity will decrease in magnitude

Ans given is option D

Answer 1

A particle of charge q moving with velocity v in the presence of an electric field E and a magnetic field B experiences a force

F = q.E + q (v x B)

Both the fields can act on the charged electron ;

the force experienced by electron due to electric field will be = (charge). E

where E is the field strength of electric field.

and the force by the magnetic field { charge .(v X B)}

1. as the electron’s charge is negative the electric field will produce retardation on the electron

2. and the cross product in the effect of magnetic field vxB will yield zero effect as the angle between the two vectors is zero and a cross product gives a factor of sine of the angle between them.

therefore the electron will move on with reduced velocity as time advances and ultimately stop. and the magnetic field will not produce any effect whatsoever.
so the electron velocity will decrease in magnitude so option D