Re: Since k varies but not G suggests an Eather


guskz@xxxxxxxxxxx wrote:
Randy Poe wrote:
guskz@xxxxxxxxxxx wrote:
Well according to Randy in another thread k in F= k Qq/r^2 remains the
constant permittivity of space

Yes.


Your words from another thread:

" The electrostatic force felt by a particle q in a medium....using the
standard, unchanging value of k."

Yes.

But the link below specifies that "k" does vary with the medium for
ELECTROSTATIC FIELDS?:

http://www.plus2physics.com/electrostatics/study_material.asp?chapter=2

Electrostatic field: (The electric field stength is E= k Qq/r^2)

Quote:
"Lines of force are a convenient way of visualizing an electric field
.....The total number of lines of force is
inversely proportional to the ***PERMITTIVITY(thus k) OF THE MEDIUM****
in which the charge is located."

The reason for your confusion is that there are two
equivalent ways of looking at what happens with
a medium.

A "medium" is a convenient fiction, an approximation.
It can be described in terms of such things as
dielectric constant and index of refraction, and
light "slowing down".

However, all of these quantities are DERIVED from
the vacuum equations. You calculate how the charges
in the medium respond to electromagnetic fields
(by polarizing for instance), and then what fields
result from those actions of the charges in the medium. These
calculations are all done using vacuum values,
as those are the fundamental equations which are
always true.

When you do those calculations, you can summarize
the effects due to charges in the medium as "medium
properties".

Thus, it is true that the force on a charge is always,
always, ALWAYS the sum of kQq/r^2 where k is
the vacuum value and the sum is over ALL OTHER
CHARGES, including those in the medium.

It is also true that you don't have to look at the
individual charges in the medium, but can approximate
their effects by introducing the idea of a "permittivity
of a medium". When you do that, you are doing
the process I am saying: summing up VACUUM
kQq/r^2 over all the charges of the medium. Note
that when people do this, they no longer consider
the individual charges in the medium. That's just an
approximation, albeit a good one.

But things like "permittivity of a medium" can actually
be calculated, by considering those individual charges
and using the vacuum equations. I know. I had to do
it for exams.

- Randy

.

Relevant Pages