(c) Dr Paul Kinsler. [Acknowledgements & Feedback]
This is part of an information maze -- see the index-file for the full picture.
The theory of electro-magnetism is the theory of electro-magnetic-fields, which unifies electric-fields and magnetic-fields.
Electro-magnetic-waves can be though of as being in many different sorts of "shapes". Electro-magnetic-waves emitted outwards from a point are spherical, and continue travelling outwards, and do not contract again. It is simplest (although not necessarity most accurate) to create a distinction between two things:
An electro-magnetic-wave (see waves) is made of real photons that can be detected and absorbed and so on. A radio station broadcasts a powerful electro-magnetic-wave, and some of the (radio-frequency) photons in the wave excite electrons in the antenna of your radio receiver, which causes oscillating currents (see electricity) which the radio amplifies and converts into the sound you can hear.
An electro-magnetic-field is caused by unequal distributions of electric charge (see electricity, electron), and is static and unchanging. It is made not of real photons, but "virtual" ones.
In practise these terms are not used systematically; and the situation is further confused by the fact that an electro-magnetic-wave as I described it above consists of rapidly oscillating electro-magnetic-fields (which average over time to zero). Fast oscillations make electro-magnetic-waves that carry a lot of energy. So the two cases are the two ends of a range, and any given electro-magnetic effect will lie somewhere between the two extremes.
XINDEX: electro-magnetic-field, index-file.
XKEYWORD: electro-magnetic-waves
20000205(c) Paul Kinsler
Email Feedback: Dr.Paul.Kinsler@physics.org
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