Question

Imagine that you are sitting in a room with your back to one wall and that an electron beam traveling horizontally from the back wall towards the front one is deflected towards the right. What is the direction of the magnetic induction field that exists in the room?
A. Vertically upwards.
B. Vertically downwards.
C. Horizontal and perpendicular to the direction of motion of the electron beam.
D. Horizontal and parallel to the direction of motion of the electron beam.

Answer 1

To solve this question, we must draw a diagram to understand the directions of the beam properly. The direction of current will be opposite to the direction of electron flow. Any charge moving in a magnetic field will be experiencing a force and the direction of this force is given by Fleming’s left hand rule.

Complete answer:
First of all, we must know what Fleming’s left hand rule is. Fleming’s left hand rule gives the direction of force acting on a current carrying conductor. Here, the forefinger represents the direction of the field, the second finger represents that of current, and the thumb gives the direction of force.
i.e.

Now, we will try to create the diagram for a given situation and we must know that, current direction will be opposite to that of the electron beam direction, i.e. from front wall to back. Also, the electron beam is experiencing a force towards the right side as it is deflected towards right.

So, by applying Fleming’s left hand rule in this scenario we can understand that the magnetic field will be directed downwards to the floor.
Therefore, we can conclude that the direction of the magnetic field will be directed vertically downwards.

So, option B is the correct answer.

Note:
In these types of questions, rules for finding direction of different parameters is the key. So, we must be very familiar with those. Also, the current direction is always taken in the opposite direction of electron flow. According to Fleming’s rule, Magnetic field, force and current will be always perpendicular to each other.