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Physics LibreTexts

Electricity and Magnetism

Magnetostatics.

1. There are two types of poles, "North" and "South".

2. Like poles repel: unlike poles attract.

3. The behavior of compasses can be explained by imagining that the Earth is a large magnet.

4. Around each system of magnets a "magnetic field" exists (Faraday).

5. Magnetic poles, unlike electric charges, always appear in pairs on North and South "dipoles"; isolated poles (monopoles) do not exist (have not been found).

Electrostatics.

1. There are two types of charges, "positive" and "negative".

2.. Like charges repel: unlike charges attract.

3. The force between two isolated point charges varies inversely as the square of the distance between them (Coulomb's Law).

4. Around each system of charges an "electric field" exists (Faraday).

5. Materials are made of atoms, which consist of massive nuclei made of neutrons, which have zero charge; protons, which are positively charged; and the very much lighter electrons which are negatively charged. Normal matter contains equal amounts of both types of charge, and is therefore neutral (has no net charge).

6. It is the electrons which are mobile. Negative charges on bodies are caused by an excess of electrons, and positively charged bodies have a deficit of electrons.

7. In materials which are conductors, the electrons can be easily moved by electric fields. In insulators, the electrons are more closely bound to their atoms, and can be moved only with difficulty.

8. A source of electric charges is called a battery.(Volta)

9. When a conductor is connected to a battery electrons will flow along the conductor to produce what is called an "electric current".

The Generation of Electromagnetic Waves

1. A moving charge produces a magnetic field (Oersted) yields A uniformly moving electric field produces a constant magnetic field and finally ... An oscillating electric field produces an oscillating magnetic field.

2. A moving magnet produces an electric field (Faraday) yields A uniformly moving magnetic field produces a constant electric field and finally .....an oscillating magnetic field produces an oscillating electric field.

3. SO ... An oscillating electric field will produce an oscillating magnetic field ....which, in turn, will produce an oscillating electric field ....which, in turn, will produce an oscillating magnetic field ..... and so on .... and so on..... These fields thus become self-sustaining and self-generating; they then disengage themselves from the source of oscillating electric field, and move off at high speed into space, oscillating madly!

By the way ... the easiest way to get an source of oscillating electric field is to wave an electric charge around - which is what radio stations (and TV) stations do when they broadcast their signals. The sound is transformed into electrical impulses in the station's antenna that thus sends off electromagnetic waves (radio waves) which, in turn, make the electrons in your antenna move to produce an electrical impulse that is transformed by your sound system back into sound.

James Clerk Maxwell showed that the speed with which the waves move was directly calculable from the electric and magnetic properties of the vacuum, and turned out to be 3x108 meters per second - the speed of light.