A proton moving with a constant velocity passes through a region of space without any changing its velocity. If `E` and `B` represent the electric and magnetic fields, respectively. Then, this region of space may have

Options

A. E=0 and B=0
B. `E=0` and `B!=0`
C. `E!=0` and `B=0`
D. `E!=0` and `B!=0`

Answer

A, B, D

Explanation

Since there is no change of velocity there may be the following three cases :
I. Electric and magnetic fields cancel each other i.e. `qE=q(barvxxbarB)`, so that the net force exerted =0, in the case `E!=0,B!=0` but vel. Remains constant.
II. If `E=0, B!=0`, and if the dir. of magnetic field is `||` to the dir. of velocity then also the force on particle =0 so no change in velocity (i.e. `V||B` so `q(barvxxbarB)=0`.
III. If `E=0,B=0`, then no force acts on the particle and particle’s velocity remains constant.