Question

An electron moving to the east in a horizontal plane is deflected towards south by a magnetic field. The direction of this magnetic field is
A. Towards north
B. Towards west
C. Downwards
D. Upwards

Answer 1

Hint: Use Fleming’s left hand rule.
This rule gives the relation between the directions of the magnetic field, motion of the conductor and the electric current.

Complete step by step answer:
Fleming’s left hand rule can be used to determine the direction of the induced current in a conductor moving in the magnetic field.
According to Fleming’s left hand rule, when the forefinger, the middle finger and the thumb are held perpendicular to each other and if the forefinger represents the direction of magnetic field and the thumb represents the direction of motion of the conductor then the middle finger represents the direction of the electric current.
In the present question, an electron is moving in the east direction in a horizontal plane and then the direction of motion changes towards the south direction.
Now determine the direction of the magnetic field using Fleming’s left hand rule.
Keep the forefinger, middle finger and the thumb of the left hand in three mutually perpendicular directions.
Now change the position of the left hand in such a way that the middle finger points towards the east direction as the middle finger represents the direction of the electric current and the thumb points towards south direction as the thumb represents the direction of motion.
Then one can conclude that the forefinger will be directed downwards.
Since the forefinger represents the direction of the magnetic field, the magnetic field is directed downwards.

So, the correct answer is “Option C”.

Note:
While determining the direction of the magnetic field, use proper directions for the forefinger, middle finger and the thumb.
In the present question, an electron is moving in the east direction in a horizontal plane and then the direction of motion changes towards the south direction.