Question

Cathode rays gain kinetic energy when accelerated by an electric field. If they are subjected to a uniform magnetic field, then their

• A. energy increases
• B. momentum increases
• C. energy and momentum decrease
• D. energy and momentum remain unaffected.

Answer 1

The correct option is D energy and momentum remain unaffected.

In case of electric force on a particle is given by qE in the direction of electric field.
But the magnetic force on a particle is $qV\times B$. i.e. it is a cross product of velocity and magnetic field vector.

So, the resultant is always perpendicular to velocity and thus only acts to change direction of velocity.

There fore, energy and momentum are unchanged.