Question

A straight conductor carries a current. Assume that all free electrons in the conductor move with the same drift velocity v. A and B are two observers on a straight line XY parallel to the conductor. A is stationary. B moves along XY with a velocity v in the direction of the free electrons.

(a) A and B observe the same magnetic field.

(b) A observes a magnetic field, B does not.

(c) A and B observe magnetic fields of the same magnitude but opposite directions.

(d) A and B do not observe any electric field.

Answer 1

Correct Answer is: (a, d)

A is stationary and observes the current I. B observes the free electrons to be at rest, but the unbalanced positive charges in the conductor will appear to move in the direction opposite to that of v. Thus, A and B observe the same current and hence the same magnetic field.