4.Main Content
guidance
Electric fields
Students will know how magnets behave in a magnetic field, and may know what happens to a current-carrying wire or a beam of electrons placed across a magnetic field. Magnetic fields are used in ammeters and in electric motors and loudspeakers.
Gravitational fields cause masses to attract each other. At its surface, the Earth pulls with a force of 9.8 newtons on every kilogram of matter; this is its field strength, g. In general, field strength describes the relationship between Weight, W and mass, m, that is, W = mg
At an introductory level, students may ask what makes electrons run along a wire. At more advanced level, they may ask what makes the random motion of electrons in a metal wire drift in one direction when connected to a power supply.
You could say that a power supply piles up electric charges at its terminals, plus and minus, and those charges exert forces which make electrons move through the wire. You could also say that there is an electric field, E around any charge, q, a state of readiness to push or pull, F, on other charges, that is F=qE.
A link between electric and magnetic fields
The theory of relativity tells a more complicated story about electric and magnetic fields. It is possible to describe everything in terms of electric charges, electric fields and relative motion.
A moving magnetic field generates an electric field, and a moving electric field generates a magnetic field. Imagine a small electric charge (a ‘test’ charge) being moved around to explore the fields resulting from other charges. You will see no magnetic field resulting from any charge that is at rest relative to the test charge. But any charge moving relative to the test charge will create a magnetic field in the region of the test charge; you will see this magnetic field exerting a force on the test charge.
In the introductory stages of teaching about fields, however, it will be better to describe electric and magnetic fields as entirely different entities.
Updated 21 May 2009