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

Generally right hand rule is used when velocity and field are perpendicular to each other. This law states that, “If the fore-finger points towards the direction of velocity of charge or current, middle finger points to the direction of field (B) then thumb gives direction of force when the three fingers are stretched in such a way that they are perpendicular to each other as shown in figure 8(b).”

This rule is applicable to positive charge.

• What is the direction of force acting on a negative charge moving in a field? First find the direction of force acting on a positive charge. Next reverse its direction. This new direction is the direction of force acting on the negative charge. Let us see an example of force acting on a charged particle.

Example 1

A charged particle ‘q’ is moving with a speed ‘v’ perpendicular to the magnetic field of induction B. Find the radius of the path and time period of the particle.

Solution

: Let us assume that the field is directed into the page as shown in figure E-1. Then the force experienced by the particle is F = q v B. We know that this force is always directed perpendicular to velocity. Hence the particle moves along a circular path and the magnetic force on a charged particle acts like a centripetal force

Let r be the radius of the circular path.

We know that centripetal force = mv³/r
q v B = mv²/r

Solving this equation, we get; r = mv/Bq

Time period of the particle; T =2πR/v

Substituting r in above equation, we get T =2πm/Bq

• What happens when a current carrying wire is placed in a magnetic field?