Topic 4. Electromagnetic origins of light

a. Static electric force equation: Coulomb’s Law

b. Static interaction between magnetic materials (compass needles, etc.)

Neither static interaction requires a medium between the source and the receiver – the effect works just as well between charges or magnets in a vacuum.

c. Moving charges create magnetic fields: magnetism and electrical effects are related

d. Changes in velocity produce varying magnetic fields which produce electric fields, which produce …

e. wave are generated that travel with velocity defined by parameters from static force equations: k_e/k_m = c² !!

f. energy radiated increases very rapidly with acceleration; acceleration increases with frequency of velocity variations

g. fluctuating electric fields can interact with charges to transfer energy and momentum to them

The fields do not require a medium to propagate. This was very surprising 100 years ago, but it probably should not have been given that the static interactions between charges and magnets don’t require a medium either. People were so convinced that a medium had to exist that they created one called the “ether” whose sole function was to support transmission of electromagnetic waves. The existence of the ether was finally disproved by a number of experiments; the most famous is the Michelson-Morley experiment in 1887.

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