Chapter 24.2, Problem 25PP

To determine

The direction of a force on electron for given magnetic field.

Answer to Problem 25PP

The direction of a force on electron for given magnetic field will be downwards.

Explanation of Solution

Given:

The electron in moving towards east.

Direction of magnetic field is towards north.

Formula used:

Lorentz Force Law:

The force on a moving charge in magnetic field is given by the relation,

  $\begin{array}{l}F=q\overrightarrow{v}\times \overrightarrow{B}\text{or}\\ F=qvB\sin \theta \end{array}$

Where, $F$ is the force on charge, $q$ is the charge, $v$ is the velocity of charge, $B$ is the magnetic field strength and $\theta$ is the angle between direction of motion and magnetic field.

The force is always perpendicular to magnetic field and velocity.

The direction of force is given by right hand rule.

If the charge is negative the direction of force will be opposite.

For the given condition as velocity is towards east and magnetic field is directed towards north. Therefore, by right hand rule

On moving the palm from direction of motion to the direction of field (east to north), the thumb directs towards up.

But, as the charge is negative on electron, so the direction of force will be downward.