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This is my earlier message (with corrections)
Mass m is a 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 systematically
investigated electric phenomenon was mutual attraction and
mutual repulsion of light objects under the influence of
something which was not a 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, and many
other, observation, and 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 Coulombs 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 not at all different. 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.
http://alpha.montclair.edu/~kowalskiL/SI/si_page.html
Note that F in Coulombs 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).
Here is my first "electricity problem" for the next semester:
1) Two drops of water (0.01 grams each) are separated by a
distance of 1 cm. Calculate the force of their mutual
gravitational attraction. (The answer is 6.67*10^-17N)
2) Suppose that a superman removed 1% of electrons from
one drop (making it positive) and transferred them to
another drop (making it negative). Calculate the force of
mutual electrical attraction. (Answer: 2.35*10^+14 N).
3) What is the weight of one cubic mile of water ?
(Answer: 4.08*10^+13 N)