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 $\ne$ 0
• C. E $\ne$ 0, B = 0
• D. E $\ne$ 0, B $\ne$ 0

Answer 1

Answer: (D)

E $\ne$ 0, B $\ne$ 0

Answer 2

If both E and B are zero, then $F_e$ and $F_m$ both are zero.
Hence, velocity may remain constant. Therefore, option (a) is correct.
If E = 0, B $\ne$ 0 but velocity is parallel or antiparallel to magnetic field, then also $F_e and F_m$ both are zero. Hence, option (b) is also correct.
If $E \ne 0, 5 \ne 0 \ but\ F_e + F_m = 0,$ then also velocity may remain constant or option (d) is also correct.