Move the bar magnet away from the screen. Now you will get a clear picture. Repeat this to confirm that the motion of electrons is affected by the field produced by the bar magnet. This must be due to the fact that the magnetic field exerts a force on the moving charges. This force is called magnetic force.

• Generalize the equation for magnetic force on charge when there is an angle ‘θ’ between the directions of field B and velocity v. It is experimentally proved that when there is an angle between direction of field and velocity, the magnetic force experienced by the charge is given by,

     F = q v B sinθ.


• What is the magnetic force on the charge moving parallel to a magnetic field?

When charge moves parallel to the magnetic field (along the magnetic field or against the field) the value of θ becomes zero. In the above equations θ is 0⁰ so that sinθ = 0.

Thus the charge experiences no force when it is moving parallel to the magnetic field (along field direction or against field direction).


• What is the direction of magnetic force acting on a moving charge?

We have an easy method to find out the direction of magnetic force acting on a charge moving in a magnetic field. Keep your right hand fingers along the direction of velocity of moving charge and next curl your fingers towards the direction of magnetic field then the thumb gives the direction of magnetic force as shown in figure 8(a). This rule is applied for any case of angle between directions of velocity and field. The direction of magnetic force is always perpendicular to the direction of both velocity and magnetic field then the thumb gives the direction of magnetic force as shown in figure 8(a). This rule is applied for any case