Magnetism
Anyone who has
played with magnets
knows that the magnet
force is concentrated
at the magnet's poles.
A magnetic pole is a
mysterious thing.
You can't seem to
isolate one.
Magnetism
Furthermore, a
magnetic pole's
influence seems to
extend far beyond the
surface of the magnet.
This region of
influence is called
a magnetic field.
Magnetic Forces
Magnetic poles also show attraction and repulsion.
Like poles repel.
Unlike poles attract.
In spite of their
similar behavior,
magnetic poles are
not the same things
as electric charges,
but they are related.
A Magnetic Compass
A magnetic compass
is nothing more than
a small magnetized
pointer or “needle”
that pivots around a
center. The needle is
really a miniature
bar magnet.
The Magnetic Compass
These magnetic compasses point toward the
bar magnet's poles they are attracted to.
The Magnetic Effect
Nineteenth century investigators found that
moving electric charges can exert a magnetic
force on a nearby compass.
No electric current in
the wire.
Electric charges are
flowing here.
The Magnetic Force Law
Electrically charged objects that are moving
exert and feel an additional force beyond the
electric force that exists when they are at rest.
This additional force is called the magnetic
force. All magnetic forces are caused by the
motion of charged objects.
This means that even the magnetic effects of a bar
magnet are caused by moving electric charges.
But how could that be? You don't plug in a bar magnet!
The answer has to do with the fact that there are moving
electric changes inside of each and every atom.
Forces Acting at a Distance
The Earth holds the Moon in its orbit through the
force of gravity.
How does it do it, when there is no physical
connection?
Magnets can attract (or repel) other magnets
without physically touching them.
To help explain these mysteries, scientists now
talk of the existence of force fields.
Force Field Theory
A gravitational field is a region of influence
around a mass.
An electric field is a region of influence
around an electric charge.
A magnetic field is a region of influence
around a magnet or a moving electric charge.
Fields may seem to be just an abstract idea, but they
are very real and they can store energy.
Magnetic Fields
Here is a drawing of the
magnetic field around a
bar magnet. A magnetic
compass would line up its
needle with the blue lines.
Compare with this photo
of a real bar magnet.
Iron filings have lined up
in a pattern that is very
much like the drawing.
Electric Fields
Here is a drawing of the
electric field between
two unlike electric
charges.
Compare with this photo
of two electric charges
surrounded by fine
pieces of thread.
Force Field Laws
Charged objects
create electric fields
and feel forces due to
the electric fields of
other objects.
Moving charged objects
create magnetic fields and
feel forces due to the
magnetic fields of other
moving charged objects.
Electric Potential Energy
Fields can store energy!
In this example: two
metal plates hold opposite
electric charges separated
by a small air space.
It is possible to store
energy in the electric
field between the plates
Electric Potential Energy
The air gap between the metal plates prevents
the unlike charges from moving together.
Also many like charges (which
repel each other) are crowded
together on the surface of the
metal plates.
You have to put a lot of energy
into creating these conditions.
That energy is stored by the
field until you need it.
Electric Potential Energy
When you compress a
spring, you store elastic
potential energy in it.
When you squeeze
together two
repelling electric
charges, you store
electric potential
energy in their field.
Capacitors
Devices that store electric potential energy are
called capacitors.
Uses for
capacitors would
include:
providing energy
for the flash
lamp in a
camera and for a
defibrillator.
Batteries
Although similar in function, batteries store
chemical potential energy.
Capacitors are
used when a
quick burst of
energy is needed.
Batteries are
more useful
when energy is
needs to be
released slowly and steadily over a period of time.