Movement of a Charge in a Magnetic Field

In order to define the force acting on an electric charge placed in a magnetic field, consider a charge traveling at a speed in a magnetic field . Also, assume that in a time the charge traveled the distance , as highlighted in Figure 1.

We can thus write that:

from which:

Also, we know that the electric current can be calculated as:

where is the electric charge transfered during a time . Using the expression of time from (2), equation (3) becomes:

The force which acts on an electric charge responsible for generating a current , and which is placed in a magnetic field , can be written as:

By using the expression form of the electric current from (4) in (5), we get that:

which leads to:

This force acts in the same direction as that of the lines of force of the magnetic field . If the charge travels at right angles relative to the magnetic field, as for example in Figure 1, then the force will act perpendicular to the direction of the charge's movement, and thus the charge will move in a circle (see Figure 2).

As in this case the force is oriented inwards, it equivalates the centripetal force given by:

where is the mass of the charged particle, and is the radius of the circle. Thus, we get that:

from which we obtain the radius of the circle on which the charge moves: