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Re: electric charge



Herbert H Gottlieb wrote:

***Most of us will agree that it is VERY MUCH easier
to find errors in a written passage than to actually write
such a passage without any errors.

How can I deny this? Of course, the rods were not stolen.
And, as pointed out by Michael Bowen, the defining constant
should have been k=9,000,000,000 and not 1,000,000,000.
What follows is a corrected version of what was posted last
night. I will think about other comments and will probably
respond to them later. Thanks to both of you.
Ludwik Kowalski

***************************************
Mass m is the mechanical attribute of an object. It determines
how the object accelerates (F=m*a) and how it is attracted by
another mass (F=G*m*M/d^2). The first electric phenomenon
discovered was mutual attraction and mutual repulsion of light
objects under the influence of something which was not mass
or magnet. That something was named charge. A glass rod
rubbed with silk, for example, acquires a property of repelling
another glass rod rubbed with silk. That property, named charge,
was initially modeled as a fluid. An object containing that fluid
was said to be electrified.

Likewise, a plastic rod rubbed with wool repels another plastic
rod rubbed with wool. But an electrified glass and an electrified
plastic attract, rather than repel, each other. This observation,
and many others, lead to a realization that there are two kinds of
electric fluids, positive and negative. The term charge used to be
interpreted as the "amount of electric fluids" or "amount of
electricity" which an object can acquire or lose.

A modern interpretation is based on the realization that
submicroscopic particles, protons and electrons, are permanently
charged with positive and negative electricity. A macroscopic
object is charged when the number of electrons and the number
of protons are not identical. An excess of protons results in a net
positive charge while an excess of electrons results in a net
negative charge. The net charge, like the total mass, becomes an
attribute of an object. It is an attribute responsible for forces
between electrified objects. Two similar charges (both positive or
both negative) always repel but two dissimilar charges (positive
and negative) always attract. This was the first qualitative
observation about electric forces.

It turns out that the magnitude of an electric force between two
charges (q1 and q2) is proportional to the product q1*q2 and
inversely proportional to the square of the distance (d^2)
between their centers. This observation, made by Coulomb,
is known of Coulomb’s law. It can be written as:

F = k*q1*q2 / d^2

where k is the proportionality constant. The value of that
constant can be chosen arbitrarily in order to define a unit of
electric charge. For the purpose of this introduction the unit
of electric charge, one coulomb, C, we will defined by
declaring that k=9,000,000,000. This is equivalent to saying
that the electric charge is one coulomb if it attracts or repels an
identical charge with a force of nine billion newtons when the
distance between the centers of two charges is one meter. One
coulomb is a very large charge; charges produced on rubbed
rods and plates are usually expressed in microcoulombs or in
nanocoulombs. Ignoring sign differences we can say that the
charge of one electron and the charge of one proton are
identical (1.6*10^-19 C).

The so-called "official" SI definition of the unit of charge is
conceptually different from the one presented above. But in
practical terms it is nearly identical. In SI the ampere, A, is the
first unit; all other electrical units are defined in terms of kg, m,
s and A. The unit of charge, coulomb, C, is defined as A*s. In
our sequence C is the first electric unit and A will be defined as
C/s. Other nuances associated with electrical and magnetic SI
units will be discussed later. Note that F in Coulomb’s law is
positive when two charges repel (q1 and q2 have the same sign)
and negative when they attract (signs of q1 and q2 are different).