1.

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 v and a positron enters via opposite face with velocity – v. At this instant,

Answer»

the electric forces on both the particles cause identical accelerations.
the magnetic forces on both the particles cause equal accelerations.
both particles gain or loose energy at the same rate.
the motion of the Centre of Mass (CM) is DETERMINED by `vecB` alone.

Solution :1. Magnetic force on charges `vecF=q(vecvxxvecB)` bezero. So, both particle gain equal acceleration.
2. Both particle gain and LOST equal amount of rate of energy because force on both particles are equal in magnitude and OPPOSITE in directions.
3. C.M. of system does not change so we can find it by only `vecB`. So, (B, C and D) OPTIONS are TRUE.


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