Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Since the particle deflects towards Right side,
The force is in towards right side
Now, if negatively charged particle moves from back wall to front wall.
Then, current is the opposite direction,
i.e. Current is from front wall to back wall (Towards Inside our bod)
Applying Fleming’s left hand rule,
Taking the forefinger, middle finger and thumb of the left hand in mutually perpendicular direction
- Pointing thumb towards right side (direction of force)
- Middle-finger towards our body (direction of current)
- We get that the direction of magnetic field is downwards (into the ground)