Consider a metal bar moving through a uniform magnetic field, as shown below. (Assume that there is no electric field in this diagram's implied reference frame.)

Observers in all reference frames will agree (perhaps by tracking single electrons) that the bar's left side becomes negatively charged and its right end becomes positively charged, meaning that just after the bar begins to move (but before the charges have time to pile up at the ends) some leftward electromagnetic force must have been acting on the bar's electrons. What type of force is this
-(b) in the rest frame of the bar?
A) The force is purely electric.
B) The force is purely magnetic.
C) The force is both electric and magnetic.
D) There is no force on the bar's electrons in this frame.

Question

Consider a metal bar moving through a uniform magnetic field, as shown below. (Assume that there is no electric field in this diagram's implied reference frame.)
  
Observers in all reference frames will agree (perhaps by tracking single electrons) that the bar's left side becomes negatively charged and its right end becomes positively charged, meaning that just after the bar begins to move (but before the charges have time to pile up at the ends) some leftward electromagnetic force must have been acting on the bar's electrons. What type of force is this
-(b) in the rest frame of the bar?
A) The force is purely electric. 
B) The force is purely magnetic. 
C) The force is both electric and magnetic. 
D) There is no force on the bar's electrons in this frame.

Quizplus · Accepted Answer

In the rest frame of the bar, the electrons are not moving relative to the bar, so there is no magnetic force acting on them. However, if there is a force acting on the electrons in another frame (such as a laboratory frame where the bar might be moving), this force must be accounted for by an electric field in the rest frame of the bar, leading to a purely electric force on the electrons.

Consider a Metal Bar Moving Through a Uniform Magnetic Field