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Old December 24th 17, 09:28 PM posted to uk.tech.digital-tv,uk.d-i-y
Graham Nye
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On 24/12/2017 20:43, Bill Wright wrote:
What are the parameters that set the speed of electromagnetic
transmission in a vacuum? I've googled everywhere but I can't find the
answer. It's easy enough to find the figure but WHY? Why not
29,979,245.8 metres per second or 2,997,924,580 metres per second?


To save you wading through a load of vector partial differential
equations have a look at the equation under the text "which identify"
in the section:
https://en.wikipedia.org/wiki/Maxwell's_equations#Vacuum_equations,_electromagne tic_waves_and_speed_of_light

The summary is:
Speed of light (or other EM radiation) is one over the square root
of (μ (permeability) times ε (permittivity)).

Permeability and permittivity are measures of the magnetic and
electric properties (respectively) of a material.

A common practice is to take the permeability and permittivity of a
material and divide them by the permeability and permittivity of a
vacuum. The resulting ratios are known as the relative permeability
and relative permittivity respectively.

For people familiar with electronic components relative permittivity
is also known as dielectric constant in capacitors. It's the factor
by which the capacitance is multiplied due to using a given dielectric
material as an insulator rather than vacuum (or, more practically, air).

Some more reading:
https://en.wikipedia.org/wiki/Permeability
https://en.wikipedia.org/wiki/Permittivity

The number you get will depend on the units you use. If you use something
other than SI units (metres, kilograms, seconds, etc) you'll get a
different number, e.g. 186,282 miles per second. (But it should be the
same speed, just expressed in different units.)


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Graham Nye
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