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January 12, 1999.... Physics 201B... Spring 1999


Welcome!! I would like each mentor to send an email with the names of those in attendance at this first class.

We will need to fill out forms sent from Charleston and take the mandatory pre-test which will cut down on the time to do the exploratory listed for today. I suggest that each one do them at home and report back with results next week. Exploratory is available on Website:


A very opportune year to study both electricity and light....

celebrate the contributions... began century with the discovery of the electron and photon which led to understanding atoms and structure of matter... add Relativity theory and Quantum theory and the total change of thinking ... probability and uncertainty which have led us from the industrial age into the age of technology and information... it was in this century that the linkages between light and electricity were exploited... we hope to explore an understanding of electricity and magnetism and their linkage which leads to light and its understanding.


Electricity starts with an understanding of the atom... recall the structure of the atom... nucleus contains the positive charge within a region of diameter 10-15 m within which the protons are held together by the strong nuclear force. The electrons are in clouds governed by quantum numbers n, l, ml , ms . We can't directly interact with the protons but we can cause interactions with the electrons...

chemical interactions... bonding...

excite electrons... light emission... lasers

remove electrons...

So what do we mean when we say a body is charged? We mean it has an EXCESS of electrons... total electrons > total protons ... which we call a NEGATIVE charge.

or a DEFICIENCY of electrons... total electrons < total protons ... which we call a POSITIVE charge.

The sign of charge was defined by Franklin... it could have been "chosen" in opposite direction.

We use the letter Q or q to represent how much excess or deficiency exists. I like to use q as "my" charge... the charge on "my" body of interest ... while Q is the "other" charge which is affecting "my" charge.

Units of charge... Well before the electron was discovered (1895), the unit of charge was defined as the Coulomb. Once the electron was discovered it was found that

1 Coulomb = 6 x 1018 electrons.

Usually, we don't achieve charges this large... rather we have charges of "micro" (m) coulombs or "micro micro" (mm) ... where micro = 10-6... micro micro = 10-12

To get an idea of what fraction of atoms on a surface have lost an electron...

In an area of 10 square centimeter, given atoms about 5 x 10-8 cm apart... there are about (10 cm)2 / (5 x 10-8)2 cm2/atom = 4 x 1012 atoms (electrons assuming one electron is normally stripped from each atom)... a charge of 2 micro micro coulombs = 12 x 106 electrons... which would mean 12 x 106 electrons / 4 x 1012 available electrons which is 3 atoms per 1 million have been stripped of an electron.

Properties of Materials:

Insulators... eg... plastic, glass, ceramics, paper, etc...

atoms bound atom to atom... electrons only move between adjacent atoms...

Semiconductors... eg... silicon, germanium... column 4 elements

electron shared with each of 4 nearest neighbor atoms. electrons freed by raising temperature of material OR by "doping" ... replacing column 4 by column 3 (+) or column 5 (-) atoms.

Metals... eg... copper, silver, aluminum...

one (or two) electrons released from each atom to be "free" to flow throughout the material.

Bringing a charge near materials:

Thus the charges on the rod cause the atoms to "distort" such that electron cloud has more density on the side away from the charge on the rod... we say the atom becomes a DIPOLE...

Note that some molecules are naturally dipoles... eg... water

Thus the charges on the rod cause free electrons in the metal to move to the opposite end of the metal...


Force between charges:

Compare to:

Force between masses:


G = 6.67 x 10-11 N m2 / kg2

k = 9 x 109 N m2 / Coul2

So how much charge must we put on two 1 kg masses to have the electric force = gravitational force? M=m= 1 kg; Q=q

G M m = k Q q

6.67 x 10-11 N m2 / kg2 * (1 kg)2 = 9 x 109 N m2 / Coul2 * Q2

Q = 8.6 x 10-11 Coul = 5 x 108 electrons

Of course, 1 kg copper has approximately 3 x 1026 electrons.

But the biggest difference between the gravitational force and the electrical force is that M is always the earth so we can only vary m and its position r from the center of the earth ----- whereas in electricity, nature has not provided a single large charge Q so we are able to change BOTH Q and q as well as the distance r between them.


The earth's role in charging bodies... a reservoir for charge...

Charging by induction...