Magnetism and Electromagnetism

Magnetism and Electromagnetism

The ancient Greeks knew that a type of rock with magnetic properties known as lodestone or magnetite attracted iron. The compass, an important device for navigation, has a suspended magnet which aligns parallel to the magnetic field produced by the Earth and as a result points to the North (seeking) pole.

The compass was documented as early as 1040. It is known that the Vikings used Lodestone to navigate. By the end of the twelfth century, Europeans were using this simple compass to aid navigation.

The earth's south magnetic pole is located near the geographic north pole and is called the geomagnetic north or north seeking pole.

Properties of Magnets

• A magnet will always have two poles which we call arbitrarily North and South
• If the magnet is broken in two this will create two new magnets with North and South poles.
• Like poles repel each other. If a north pole is brought close to the north pole of a second magnet a repulsive force will be felt. Similarly if a south pole is brought close to the south pole of another magnet, the two magnets will repel each other.
• Unlike poles attract and will stick together.
• Magnets attract iron rich materials

The Domain Theory of Magnetism

The domain theory states that inside a magnet there are small regions in which the magnetic alignments of all the atoms are aligned in the same directions.

The Magnetic Field

The image shows the field lines produced by a bar magnet. Iron fillings are sprinkled on a piece of paper and the bar-magnet is placed under the paper. The iron-fillings line up and show the intensity and direction of the magnetic field.

Magnetic flux density is given the symbol B and has the unit Tesla.

The strength of the magnetic field is determined by the number of field lines passing a unit area. The more field lines the stronger the magnetic field. The direction of the field line can be determined by using a compass needle.

By convention the arrow tip on magnetic field lines points towards the south magnetic pole and away from the north magnetic pole.

Source / Magnitude of B (T)
A hair dryer / 10-3
Sunlight / 10-6
Colour TV / 10-6
A small bar magnet will produce / 10-2
MRI body scanner magnet / 2
Reseach Physics Labs produce up to / 50
The field at the surface of a neutron star / 108

Magnetic fields obey the inverse field law - field strength decreases inversely with distance.

Note on representing three dimensions on diagrams

In electromagnetism the pattern of a magnetic field in the space around a conductor, or the connection between directions for forces, fields and currents, are often important.

We have to consider three dimensions.

In art, the illusion of three dimensions is produced by the use of perspective.

In physics, diagrams are drawn flat and symbols used to show vectors which have directions into and out of the page, that is:

a circle with a dot inside it (the tip of an arrow coming towards you) shows a vector out of the page , and

a circle with a cross inside it (the tail of an arrow going away from you) shows a vector into the page .

The strength and direction of the magnetic field at any point is defined in terms of the force on a moving charged particle such as an electron. The force created the magnetic field comes from the Lorentz:

(This simplified equation only works when the motion of the charged particles is at right angles to the magnetic field).

Magnetism and electricity

Magnetism and electricity were originally studied as separate areas.

Last century it was found that the two are intimately connected. Those connections are central to the study of electric power.

Around a current flowing in a long straight wire there is a circular magnetic field. Its direction can be found from the right hand grip rule as illustrated:

This is known as the Right Hand Grip Rule.

When current flows in a wire, a magnetic field is created around the wire. To visualize the magnetic field, take your right hand, curl the fingers, and stick the thumb straight out.Point your thumb in the direction of the current flowing in the wire (using conventional current).

The direction your fingers are curved around the wire is the direction of the magnetic field around the wire. For example, if the current wasgoing straight into this page your thumb would be pointing away from you and your fingers would indicate a clockwise direction to the magnetic field around the wire.

The field strength decreases inversely with distance from the current carrying wire.

When two wires are placed close to each other with the current flowing in opposite directions, a more complex magnetic field pattern is created.

A solenoid (a current carrying coil of wire) creates a magnetic field that looks much like a bar magnet:

For a wire wound into a flat coil or over the surface of a cylinder, the magnetic field direction can be found by applying the RH grip rule or using the S N rule. Wire wound over the surface of a cylinder forms a solenoid. Its magnetic field is of importance. Inside the solenoid the field is uniform (the same everywhere), parallel to the solenoid axis. The right hand grip rule for solenoids gives the direction of the magnetic field inside the solenoid: