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Books by David Michalets

Mass and Gravity

3 Coulomb's Force Between Charges

This is section 3 of 11 in the web-book.

With the publication of "A Dynamical Theory of the Electromagnetic Field" in 1865, [James Clerk] Maxwell demonstrated that electric and magnetic fields travel through space as waves moving at the speed of light.


Maxwell's equations define several properties of "free space" and those values define the rate of propagation of light through that free space.

Now, they can be considered properties of the medium, aka,  the aether, which is whatever unknown "stuff" permeates the universe.

The medium defines the rate of propagation of the synchronized electric and magnetic fields within light.

Most know light travels slower through glass or water than through air or space.

The diffraction index is the factor defining the change in light velocity by the medium.

Essentially, the medium has a measurable resistance to the changing of electric and magnetic fields. During the propagation of light, both fields are oscillating or in continuous change.
Light is more complicated then that simple statement because different wave lengths have different behaviors like X-rays which can be either penetrating or shielded by different media. The color violet is slower than red through a glass prism.

At the foundation of Maxwell's equations are 2 constants which define how the medium affects changes in an electric field or a magnetic field:

the permittivity of free space, epsilon-nought
the permeability of free space, mu

These factors become Coulomb's constant.

The Electric force is described by Coulomb's law.

F = ke * (q1 * q2) / r2

where ke is Coulomb's constant (ke ? 8.99×10^9 Nm^2?C^2), q1 and q2 are the signed magnitudes of the charges, and the scalar r is the distance between the charges. The force of the interaction between the charges is attractive if the charges have opposite signs (i.e., F is negative) and repulsive if like-signed (i.e., F is positive).

In very simple terms, there is a force between any 2 charges. This electric force is reduced by 2 factors:

1) ke from the medium,

2) r from the distance.

The units of ke are essentially a ratio of force in an area relative to charge.

Free space defines a factor within ke resulting in a force reduction between charges.

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last change 03/21/2022