Optional Unit VII: Electromagnetism
A. Magnetism

Key Concepts

Similarities which scientists observed between electricity and magnetism led them to suggest that magnetic properties are possibly the result of forces between electric charges in motion.

Substances which can be induced to become magnetized in a magnetic field are called ferromagnetic. Soft ferromagnetic materials become demagnetized spontaneously when removed from a magnetic field. Hard ferromagnetic materials can retain their magnetism, making them useful in the production of permanent magnets.

A compass is a magnet which can align itself within the earth's magnetic field.

A magnet contains a north-seeking pole (north pole) and a south-seeking pole (south pole). (The possibility of having a single monopole is being investigated.)

Similar magnetic poles repel. Opposite magnetic poles attract. (Law of Magnetic Poles)

A magnetic field is a region in space where a magnetic force can be detected.

The magnetic field is strongest at the poles of a magnet.

Magnetic lines of force are a way of representing a magnetic field.

By convention, magnetic lines of force point from north to south outside a magnet (and from south to north inside a magnet).

Magnetic lines of force form complete loops. They never cross.

The magnetic poles of the earth are not located at the geographic poles. The angle between the geographic North Pole and the magnetic "north" pole is called the magnetic declination.

The angle of declination depends on one's location on earth.

The earth's magnetic field does not run parallel to the earth's surface. The angle of magnetic dip is the measure from the horizontal plane to the magnetic lines of force. This also varies depending on one's position on the surface of the earth.

The angle of magnetic dip is very large in the vicinity of the earth's magnetic poles, making navigation difficult.

The earth's magnetic field moves very slightly over long periods of time. Plate tectonics may help to account for this phenomenon.

Ore bodies in the Earth can influence the strength of the Earth's magnetic field.

The units for magnetic field strength are the weber/m², called the tesla. More familiar units representing the same thing are N/(A.m)

Learning Outcomes

1. Define the following terms: ferromagnetic, soft ferromagnetic, hard ferromagnetic, compass, magnetic field, magnetic lines of force, north- seeking pole, angle of declination, south- seeking pole, angle of magnetic dip.

2. Describe some of the similarities that exist between electricity and magnetism.

3. Explain how a compass works.

4. State the Law of Magnetic Poles.

5. Recognize that conventions are determined by arriving at a consensus among scientists.

6. Recognize the importance of using conventions to communicate information in science.

7. State some important properties of magnetic lines of force.

8. Suggest a plausible explanation for the movement of the earth's magnetic field over time.

9. Apply an understanding of magnetism to common experiences and practical applications.

10. Investigate important uses of magnets in different technological applications.

Teaching Suggestions, Activities and Demonstrations

1. Perform an activity to investigate the magnetic field near a magnet.

2. Illustrate the conventions used to show the direction of magnetic lines of force.

3. Suggest important navigational implications regarding differences in the positions of the earth's geographic and magnetic poles, and the angle of magnetic dip.