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Movement of a Charge in a Magnetic Field

An analysis of the movement of an electric charge in a magnetic field, also discussing the particular case when the charge travels at right angles relative to the field

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Contents

Overview
Page Comments

Key Facts

Gyroscopic Couple: The rate of change of angular momentum ( $\tau$ ) = $I\omega\Omega$ (In the limit).

= Moment of Inertia.
$\omega$ = Angular velocity
$\Omega$ = Angular velocity of precession.

Overview

Key facts

The force acting on a charge

traveling at a speed

in a magnetic field

is given by:

If a charged particle of mass

travels with a speed

at right angles relative to a magnetic field

, then the charge will move in a circle of radius

given by:

$r = \frac{mv}{qB}$

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:

$v = \frac{dl}{dt}$

(2)

from which:

$dt = \frac{dl}{v}$

(3)

Also, we know that the electric current

can be calculated as:

$i = \frac{q}{t}$

(4)

where

is the electric charge transfered during a time

. Using the expression of time from (3), equation (4) becomes:

$i = \frac{qv}{dl}$

(5)

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:

$F = i \cdot dl \cdot B$

(6)

By using the expression form of the electric current from (5) in (6), we get that:

$F = \frac{qv}{dl} \cdot dl \cdot B$

(7)

which leads to:

(8)

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).