Question 8: Uniform electric and magnetic fields are produced in the same direction. An electron is projected in the direction of the fields. What will be the effect on the K.E of the electron due to the fields?

Consider the fig. Both electric and magnetic fields are uniform and in the same direction. Electron is also projected in the same direction. The magnetic force on the electron is given by

Thus the magnetic force does not affect the motion of the electron.

However, the force due to electric field does not depend on the inclination of the velocity of the charged particle. The force due to the electric field is given by

F = -eE

The negative sign accounts for the fact that this force is opposite to the direction of motion of the electron and therefore, decelerates the motion of the electron. So under the influence of this force, the electron would be decelerated to stop. It then starts motion in the opposite direction (against the electric field) after it comes to stop. The acceleration is thus given by

In the same way, the initial kinetic energy of the electron would be used in doing work against the opposing electric force.