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.