Question

A uniform electric field and a uniform magnetic field are produced, pointed in the same direction. An electron is projected with its velocity pointing in the same direction:
(A) The electron will turn to its left
(B) The electron will turn to its right
(C) The electron velocity will increase in magnitude
(D) The electron velocity will decrease in magnitude

Answer 1

Hint
Approach the solution to this question as first thinking about the condition for interaction between a magnetic field and a moving charge and check whether it’ll affect the motion of the electron or not, also check the effect the electric field will have on the electron.

Complete step by step answer
We will approach the solution to the question exactly as told in the hint.
The necessary condition for any interaction between any moving charge and magnetic field is that magnetic field and the velocity of the moving charge, both should never be along the same line. But as you can also read in the question, both, the velocity of the moving charge and the magnetic field are along the same direction and line. So, we can easily say that magnetic will have no effect on the motion of the electron.
As for the interaction between electrons and the electric field, know that the charge is negatively charged, so the force exerted by the electric field will be in the opposite direction as the electric field. This will result in an acceleration in a direction opposite to the electric field, which will result in decreasing the magnitude of the velocity of the electron.
So, the correct option is option (D), i.e. the electron velocity will decrease in magnitude.

Note
Many students forget that the question is about electrons which are negatively charged particles and thus take the wrong direction of the force applied by the electric field. Many students forget that the magnetic field will have no effects in the motion of the electron because its velocity is in the same direction as that of the magnetic field and thus consider its effect while they should not.