Fundamentals of Magnetism

• A magnet sets up a field of force.
• Magnetic lines of force form closed loops that flow from North to South.
• The space through which magnetic lines of force flow is called the magnetic field.
• The magnetic field is strongest closer to the magnet and becomes weaker as it gets further away.
• Magnetic lines of force never cross each other.
• There is no known insulator against magnetism.
• Magnetic lines pass more easily through iron and steel than air.
• Opposing forces will occur at opposite ends of the magnet (Polarity). One end is the North Pole (+), the opposite end is the South Pole (-).
• Like poles repel each other, unlike poles attract each other.
• Some materials (wood, ceramics, and some metals) cannot be magnetized.

There are two common types of magnets:

• Permanent Magnets  - made from materials such as hardened steel that become magnetic when subjected to an outside magnetizing force and remain magnetic even after the outside force has been removed.
• Temporary Magnets  - made from materials such as soft iron that remain magnetic only as long as an outside magnetic force is present.

The lines of force of all magnets, either permanent or temporary flow from the North Pole of the magnet to the South Pole. The magnetic lines of force or "flux" are stronger closer to the magnet and get weaker as the distance from the magnet increases. (Fig. 19/1)

Polarity refers to the opposing forces occurring at opposite ends of the magnet. All magnets have a North Pole and a South Pole. Like poles will repel each other and unlike poles will attract. (Fig. 19/2)

Most temporary magnetic fields are produced by electricity flow. Whenever current flows through a conductor magnetic lines of force develop around the conductor.

These lines of force form a circular pattern. The lines can be visualized as a magnetic cylinder extending the entire length of the conductor. (Fig. 19/3)

The lines of force have direction and change dependent on direction of current flow.

The density of the lines of force are dependent on current flow through the conductor.

The greater the current flow, the stronger the magnetic field that will be around the conductor.

Passing a current flow through a conductor will not generate a magnetic field strong enough to perform any work.

If the conductor is coiled, the lines of force combine and become more dense forming a stronger field (Fig.20/1).

The greater the number of turns of the conductor or the stronger the current flowing through the conductor the stronger the magnetic field.

Inserting an iron core in the coiled conductor increases the magnetic field even more as iron makes a better path for the magnetic lines than air (Fig.20/2).

This conductor wound around an iron bar is an "Electromagnet". A coil with an air core is a "Solenoid".