There are many similarities between the electric and magnetic forces – in fact both forces are aspects of a single force, called the electromagnetic interaction. The following table summarises their properties.
|
|
Electric Field |
Magnetic Field |
|
Symbol |
|
|
|
Cause |
Electric charge |
Magnets or electric currents |
|
Affects |
Charges |
Magnets or electric currents |
|
Types of 'Charge' |
Positive and negative |
North and south poles |
|
Force rule |
Like charges repel, unlike charges repel |
Like poles repel, unlike poles attract |
or
or
To visualise magnetic fields, as for the electric field we can use the concept of field lines. The field lines of the magnetic field are called flux lines and show the direction of the force on a magnetic north pole, so magnetic flux lines point from north pole to south pole. As for the electric field, the strength of the magnetic field is indicated by the closeness of the flux lines. Densely packed flux lines indicate a strong magnetic field.
In addition, a magnet does not feel a force when it is placed in an electric field, nor does a stationary charge experience a force when it is placed in a magnetic field.
There are however some differences. The electric field arises from the presence of electric charges. We can have a single, isolated positive or negative charge, and this will give rise to an electric field. It is believed however that there is no such thing as an isolated magnetic charge – magnetic monopoles do not exist.
The magnetic force arises directly or indirectly from charges moving in a magnetic field. The equation that describes this force is
This force depends on the velocity of the electric charge, and is perpendicular to the magnetic field. The corresponding equation that describes the electric force is
This does not depend on the velocity of the charge, nor on any magnetic charge and is parallel to the electric field.