Magnets and Electricity
Magnetic and electrical phenomena have a number of
similarities but also some key differences. First, a
In the same way that the region of electrical influence
around a positive electric charge is called an electric
field, the region of magnetic influence around
a pair of north
and south magnetic poles is called a magnetic
field. Either of these fields can be diagrammed
as a system of lines of force that spread out
from a positive charge (for electric lines of force)
or a north pole (for magnetic lines of force) and come
together at a negative charge, or a south pole, respectively.
Such lines of force become visible when iron filings
are placed on a sheet of paper immediately above a magnet.
Now, a difference: While positive and negative electric
charges can occur separately, magnetic poles are always
found in north-south pairs.
The relation between electric systems of
charges and fields and magnetic systems of poles and
fields is dynamic and three-dimensional, and took many
years to discover even after people were looking for
it (see history
of magnets). Stationary electric charges
and stationary magnet poles have no effect on each other.
But moving charges (such as current in a wire),
or fluctuating electric fields, create
a magnetic field and will exert forces on magnets,
and moving or fluctuating magnetic fields create electric
fields and will exert forces on electric charges. The
first of these effects is used to make electromagnets,
and the second is used to make electrical generators.
When an electric current in a wire creates a magnetic
field, the magnetic field lines do not line up with
the electric field lines that are driving the charges
along the wire -- instead, the magnetic field lines
run in circles around the wire, at right
angles to the electric field. If the wire is bent
into a circle, the magnetic field lines run around it
in circles that form a doughnut enclosing the wire --
funneling through the middle of the wire circle and
flaring out to wrap back around the outside and funnel
through again. If the wire is wound in multiple turns
around its circle, the magnetic field strength of all
the turns adds up to form a field of the same doughnut
shape, but stronger by as many times as the number of
additional turns of wire.
All this can be done — creating a magnetic field
that will deflect compass needles or, with enough turns,
will pick up paper clips — with a magnetic field
established only by current in a wire, with no iron.
But when iron is put within a coil that carries
a current, the iron will act as a magnet while also
intensifying the magnetic effect many times. It is therefore
customary to wrap the wire around iron instead of air.
The device that results is called an electromagnet.
Electromagnets have many uses and are extremely important
in our lives. Every electric machine that does anything
mechanical depends on them. Electric motors are made
up either partly or completely of electromagnets, and
most electric generators — all large ones —
have electromagnets as a fundamental component. Electromagnets
are also used in many automatic switches and valves,
and in buzzers, doorbells, and similar devices. Compared
to permanent magnets, they can easily be made much stronger,
and also have the big advantage that they can be turned
off or even reversed.