Question

A uniform magnetic field exists in the plane of paper as shown in the diagram. In this field, an electron (e−) and a positron (p+) enter as shown. The electron and positron experience forces :

• A. both pointing into the plane of the paper.
• B. both pointing out of the plane of the paper.
• C. pointing into the plane of the paper and out of the plane of the paper respectively.
• D. pointing out of the plane of the paper and into the plane of the paper respectively.

Answer 1

Answer: (A)

both pointing into the plane of the paper.

Answer 2

We can use the right-hand rule (or Fleming’s left-hand rule) to determine the direction of the force. For a positive charge (positron), the direction of the magnetic field is to the right, and the direction of motion is upward. Using the right-hand rule, the force is out of the plane of the paper.
For a negative charge (electron), the direction of the magnetic field is to the right, and the direction of motion is upward. Applying the right-hand rule and reversing the direction due to the negative charge gives us a force into the plane of the paper.