Question

A cubical region of space is filled with some uniform electric and magnetic fields. An electron enters the cube across one of its faces with velocity `vecv` and a positron enters via opposite face with velocity `-vecv`. At this instant,
A. the electric forces on both the particles cause identical accelerations
B. the magnetic forces on both the particles cause equal acceleration.
C. Both particles gain or loose energy at the same rate
D. the motion of the centre of mass (CM) is determined by B alone.

Answer 1

Correct Answer - B::C::D
The magnetic forces`F=q(vxxB)` on charge particle is either zero or F is perpendicular to v (or component of v) which in turn revolves particles on circular path with uniform speed. In both the cases particles have equal accelerations.
Both the particles gain or loss energy at the same rate as both are subjected to the same electric forces `(F=qE)` in opposite direction.
Since, there is no change of the centre of mass (CM) of the particles, therefore the motion of the centre of mass (CM) is determined b B alone.