Re: Coulomb's constant is a CGS kludge

From: Eugene S. (user_at_host.com)
Date: 02/25/05 

Date: Fri, 25 Feb 2005 04:37:01 GMT

"John C. Polasek" <jpolasek@cfl.rr.com> wrote in message
news:gt8t11p0bugm5jpv6itnuia6t4ufq8jr26@4ax.com...
> On Fri, 25 Feb 2005 03:22:58 GMT, "Eugene S." <user@host.com> wrote:
>
> >You got it all backwards.
> >
> >In cgs we start with Coulomb law
>
> No, Coulombs law is defined as force F

Coulomb's law says that force is proportional to product of charges,
divided by square of distance between them. Electric field is defined as
force acting on a unit charge.

>
> > and define a unit of charge so that
> >E = Q/4pi*r^2 in vacuum.

Actually, I stand corrected, it should be just Q/r^2.

>
> Then E must be coulombs per meter^2, divide it out. And that doesn't
> make sense. It needs to be a forcing function in volts per meter. See,
> we are already losing grip with reality.

Not coulombs per meter^2, but cgs charge units per cm^2.
Volt is an artificial unit, it has no room in cgs. So is coulomb.

>
> >D is secondary to E and represents the
> >electric field, corrected for the polarization of the matter.
> >D = E + 4*pi*P;
>
> Then D is also coulombs/m^2. That's OK.

units cgs per cm^2. Incidentally, so are E and H, but let's not go into that
yet.

>
> >in case of isotropic dielectric, P ~ E, D ~ E.
> > In vacuum, E=D, because there is no polarization.
>
> Vacuum does polarize, vacuum capacitors are proof of that.

By 'polarization' we mean that there are electric charges in the dielectric,
they get displaced by an external electric field and, as a result, they
create
secondary electric field. Total electric field inside the dielectric is
equal to
external field plus field created by polarized charges. In the vacuum there
are no charges -> no polarization.

Vacuum capacitors work simply because there is electric field in the vacuum,
it has nothing to do with polarization.

>
> >SI / mks was designed to be backwards compatible with preexisting unit
> >system.
> >Units like volt, ampere, coulomb existed long before the physics behind E
&
> >D was
> >perfectly understood. In order to keep backwards compatibility, we had to
> >introduce
> >artificial constants eps0 and mu0.
>
> Multiply eps0 by area/gap and you get electric capacity. You could not
> sell a 100 centimeter capacitor.

CGS unit of capacity is a centimeter. One centimeter is a capacity of a pair
of metal plates in the vacuum, separated by the distance of one centimeter,
provided that plates are large enough that we can ignore the boundary
effects.

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