Write the expression, for the force acting on a charge moving in a uniform magnetic field. Mention the nature of a trajectory of the charged particle which is moving (i) parallel and (ii) perpendicular to the magnetic field.

Explanation

Force acting on a charge q moving a with velocity `vecV` at an angle `theta` to the magnetic field `vecB` is given by
`vecF=q(vecV xx vecB)`
The direction of `vecF` is perpendicular to the plane containing `vecV` and `vecB` .The magnitude of `vecF` is given by
`vecF=qVB sin theta`

1.If `theta=0` or `theta=180^@` , F=0
Hence the charge will move in a straight line,
2. if `theta=90^@`, F=qV B and the charge will move in a circular path.
Hence when `theta` has a value between `0^@` to `90^@` , the trajectory of the charge will be hellical.