Imagine you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the from wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?

Options

A. Vertically downwards
B. Vertically upwards
C. Towards Right
D. None of these

Answer

A

Explanation

Here the electron beam is moving from our back wall to the front wall, so the direction of current will be in the opposite direction, from front wall towards back wall towards back wall or towards us. The direction of deflection (or force) is towards our right side. We now know two things :
(i) direction of current is from front towards us, and
(ii) direction of force is towards our right side.
Let us now hold the forefinger, centre finger and thumb of our left hand at right angles to one another. We now adjust the hand in such a way that our centre finger points towards us (in the direction of current) and thumb points towards right side (in the direction of force). Now, if we look at our forefinger, it will be pointing vertically downwards. Since the direction of forefinger gives the direction of magnetic field, therefore, the magnetic field is in the vertically downward direction.