Question

An electron enters a chamber in which a uniform magnetic field is present as shown below.

An electric field of appropriate magnitude is also applied, so that the electron travels undeviated without any change in its speed through the chamber. We are ignoring gravity. Then, the direction of the electric field is

• A. opposite to the direction of the magnetic field
• B. opposite to the direction of the electron's motion
• C. normal to the plane of the paper and coming out of the plane of the paper
• D. normal to the plane of the paper and into the plane of the paper

Answer 1

Answer: (C)

Using right hand rule, direction of magnetic force can be found. It acts outward from the plane of paper in given case.

To avoid deflection of electron, electric field must be applied normal to the plane of paper ($XOY$-plane in diagrams) and pointing outward.

Magnitude of electric field applied is such that $F_m=F_e$.