[geocentrism] Re: SI units.
- From: "Robert Bennett" <robert.bennett@xxxxxxx>
- To: <geocentrism@xxxxxxxxxxxxx>
- Date: Mon, 26 Feb 2007 11:51:43 -0500
Quantitatively measuring the static charge from Coulomb’s Law is more
difficult than electronically counting charge carriers in a current flow, or
measuring the magnetic force between two wires resulting from a standard
current.
The ampere is chosen as the standard, because it’s easier to verify by
experiment and more precise than finding the charge in coulombs…….imho.
Robert
-----Original Message-----
From: geocentrism-bounce@xxxxxxxxxxxxx
[mailto:geocentrism-bounce@xxxxxxxxxxxxx]On Behalf Of Martin Selbrede
Sent: Monday, February 26, 2007 12:37 AM
To: geocentrism@xxxxxxxxxxxxx
Subject: [geocentrism] Re: SI units.
Let's be careful here: the "four times pi" factor in the following expression,
, is based on the number of steradians in a sphere: the E field is integrated
over an entire sphere. This accounts for the presence of the "four times pi"
factor in the denominator or numerator of other equations governing
electromagnetism.
Moreover, you could derive this from the electrical permittivity with equal
ease, given the algebraic relationship of the magnetic permeability and
electrical permittivity of free space to the speed of light in a vacuum.
Finally, it should be noted that in the SI system, the Ampere is a fundamental
unit and not a derived unit (which makes the Coulomb a derived unit, namely, an
Ampere-Second). Most of us would find it more logical to define the Coulomb in
terms of a integral number of charge carriers (e.g., electrons) and make the
Ampere a derived unit defined from that quantity of charge carriers passing
through a fixed region within one second of time, but the SI Committee,
well.... What can I say?
Martin
On Feb 25, 2007, at 4:47 PM, philip madsen wrote:
I know that some of you might consider me irrational when it comes to modern
physics in general, and its math in particular. I got particularly angry with
the new SI units system.. and I hear often here and elsewhere, where math is
use to fake reality.
To some extent here is supporting proof of my case.
Since 1948, the SI <http://en.wikipedia.org/wiki/SI> Ampere
<http://en.wikipedia.org/wiki/Ampere> has been defined by choosing μ0 to be
H <http://en.wikipedia.org/wiki/Henry_%28inductance%29> /m. Similarly, since
1983 the SI metre <http://en.wikipedia.org/wiki/Metre> has been defined by
choosing the speed of light c to be 299792458 m/s. Consequently
Z0 = μ0c = 119.9169832πΩ ?
exactly, or
Z0 = 376.73031346177...Ω
approximately. This situation may change if the the Ampere is redefined in 2011.
[ edit
<http://en.wikipedia.org/w/index.php?title=Vacuum_impedance&action=edit§ion=3>
] 120π-approximation
It is very common in textbooks and learned papers to substitute the approximate
value 120π for Z0. This is equivalent to taking the speed of light to be 3
\times 10^8 m/s. For example, Cheng 1998 states that the radiation resistance
<http://en.wikipedia.org/wiki/Radiation_resistance> of a Hertzian dipole
<http://en.wikipedia.org/wiki/Hertzian_dipole> is
R_{r} = 80 \pi^{2} \left( \frac{l}{\lambda}\right)^{2} [not exact]
This practice may be recognized from the resulting discrepancy in the units of
the given formula. Consideration of the units, or more formally dimensional
analysis <http://en.wikipedia.org/wiki/Dimensional_analysis> , may be used to
restore the formula to a more exact form - in this case to
R_{r} = \frac{2 \pi}{3} Z_{0} \left( \frac{l}{\lambda}\right)^{2}
Ampere is the unit of electric current. It can be precisely defined from
Faradays laws, and in particular from the amount of silver deposited on an
electrode during electrolysis.
Perhaps they will go back to that in 2011.. Fat chance.
Philip.
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