Question

Let `vecE and vecB` denote the electric and magnetic field in a certain region of space. A proton moving with a velocity along a straight line enters the region and is found to pass through it undeflected. Indicate which of the following statement are consistent with the observation.
A. `vecE=0 and vecB=0`
B. `vecE!=0 and vecB=0`
C. `vecE!=0 and vecB!=0` and both `vecE and vecB` are parallel to `vecv`
D. `vecE` is parallel to `vecv but vecB` is perpendicular to`vecv`

Answer 1

Correct Answer - (a,b,c)
If a charge particle experiences no electromagnetic force, then
electric field must be zero (as `vecF=qvecE`). The magnetic field may
or may not be zero as a charge particle at rest or moving in the
direction of magnetic field experience no force.
(a) When both `vecE and vecB` are 0, it is undeflected.
(b) If `vecE` is non-zero, it will move in the direction of E and
remains undeflected.
(c) `vecE` is non-zero, force will be in direction of E. B is non-zero
and V is parallel to `vecB`.
(d) `vecF=qvecvxxvecB` will become zero due to `vecB` and proton will
move in a direction of `vecE` only.