Question

A positively-charged particle is moving in the +x-direction. A uniform magnetic field in the +z direction is then applied. What is the direction of the magnetic force on the charge?

Answer 1

The direction can be found out using Right-hand Rule, which states that,

In order to find the direction of the magnetic force (F) on a positive moving charge, the thumb of the right hand must point in the direction of the velocity vector,v (in the direction of motion), the fingers in the direction of the magnetic field (B), then the force (F) is directed perpendicular to the right-hand palm.

NB : The direction of the force (F) on a negative charge is in the opposite sense to that stated above (so pointed away from the back of your hand).

It is as depicted in the picture.

So in the given case,

• A positively-charged particle is moving in the +x-direction. So the direction of velocity is along +X axis. So your right-hand thumb must point in the direction of +X axis (to your right side).
• The magnetic field is in the +z direction. So your fingers must point in the direction of +Z axis (towards you).

So the plane of the palm will be now facing downwards, so the direction of force is downwards, ie, along -Y axis