Question

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

(a) E=0, B=0

(b) E=0, B≠0

(c) E≠0, B=0

(d) E≠0, B≠0

Answer 1

If both E and B are zero, then vector F_e and vector F_m both are zero. Hence, velocity may remain constant. Therefore, option (a) is correct.

If E = 0, B ≠ 0 but velocity is parallel or  antiparallel to magnetic field, then also vector F_e and vector F_m both are zero. Hence, option (b) is also correct.

If E ≠ 0, B ≠ 0 but vector (F_e + F_m) then also velocity may remain constant or option (d) is also correct.