Question

A positively-charged particle (alpha-particle) projected towards the west is deflected towards the north by a magnetic field. The direction of the magnetic field is,

• A. towards south
• B. towards east
• C. downward
• D. upward

Answer 1

The correct option is D

upward

Fleming left-hand rule

The rule used is the Fleming left-hand rule which states that if we stretch our thumb, forefinger and middle finger of our left-hand perpendicular to each other in such a way that the forefinger points in the direction of the magnetic field and the middle figure points in the direction of the current, then the thumb will represent the direction of the force.

The explanation of the correct option

In case of option D

1. As given, a positively-charged particle (alpha-particle) projected towards the west is deflected towards the north by a magnetic field.
2. As indicated by Fleming's Left Hand Rule, if the middle finger focuses on the direction of motion of positive charge (west), the thumb focuses toward force (north), and at that point, the forefinger focuses on the magnetic field (upwards).
3. In this way, the magnetic field is pointing upwards (out of the place containing north and west).

Hence, option D is the correct answer.