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?

Solution:

The direction of the magnetic field can be determined using Fleming’s Left-hand rule. The direction of the magnetic field will be perpendicular to the direction of the current and the direction of deflection, i.e., either upward or downward. The direction of the current is from the front wall to the back wall because negatively charged electrons move from the back wall to the front wall. The direction of the magnetic force is rightward. Hence, using Fleming’s left-hand rule, it can be concluded that the direction of the magnetic field inside the chamber is downward.