Moving Charges and Magnetism

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 the region of space may have : Choose the most appropriate answer from the options given below :

1. (A), (B) and (C) only
2. (A), (C) and (D) only
3. (A), (B) and (D) only
4. (B), (C) and (D) only

Answer

(3) (A), (B) and (D) only

Solution

No change in velocity net force zero: q E+q( v B)=0

(A): no field, no force (A) is possible

(B): v B gives v B=0 (B) is possible

(C): q E acts alone and cannot be balanced (C) is not possible

(D): crossed fields with E=- v B (velocity selector, v= EB) (D) is possible

(A), (B) and (D) only

Answer: (3)