34 Week 1: Discrete Charge and the Electrostatic Field
1.1: Charge
In nature we can readily observe electromagnetic forces. In fact, we can do little else. In a very
fundamental sense, weareelectromagnetism. Electromagnetic forces bind electrons to atomic nuclei,
bond atoms together to form molecules, mediate the interactions between molecules that allow them
to change and organize and, eventually, live. The energy that is used to support life processes is
electromagnetic energy. The objects that we touch, or hear, ortaste, or smell, the light that we see,
the organized pattern of neural impulses that we use to think about the input from our senses – all
are electromagnetic.
Given its ubiquity, it should come as no surprise that the directed observation and study of
electricity is quite ancient. It was studied, and written about, at least 3000 years ago, and artifacts
that may have been primitive electrical batteries have been discovered in the Middle East that
date back to perhaps 250 BCE. It is revealing that the verywordelectricity and the name of the
elementary particle most visibly responsible for its transport is derived from the greek word for
amber,electron. One of the firstrecordedobservations of electrical force was the static electrical
force created between amber, charged by rubbing it with wool, andsmall bits of wool or hair.
However, it took until the Enlightenment (roughly 1600) and the invention of physics and calculus
for the scientific method to develop to where systematic studies ofthe phenomenon could occur, and
it wasn’t until the middle 1700s that the correct model forelectrical charge^24 was proposed. From
that point rapid progress was made over a period of 250 years, culminating in our contemporary
understanding of electromagnetic forces as one aspect of a unified field theory.
As pointed out above, even our prehistoric ancestors no doubt knew about “charge”. The
experience of rubbing one’s body against fur on a cold, dry day and thereby picking up enough
charge to generate a spark is probably tens of thousands of years old. By the historic time of
the Greeks, it was known that rubbing amber with wool or fur would charge the amber, and the
term electricity is derived from the Greek word for amber,elektron. We now know that the charge
produced on the amber is negative.
During the Enlightenment much more systematic studies were made of this phenomenon. It is
possible to chargemanyobjects by rubbing them against other objects. For example, if one rubs
glass with silk, one literally rubs electrons off of the molecules that make up the glass and transfer
them to the silk. The silk becomes negatively charged and the glass becomes positively charged.
The study of this continues today where this sort of charge transfer due to friction is called the
Wikipedia: http://www.wikipedia.org/wiki/Triboelectric effectTriboelectric effect. Recall that the
study of friction is called “Tribology”, so that this makes sense.
In order to do the experimental work that led to the identification of the two kinds of charge and
our ability to manipulate electrostatic charges and measure forcesquantitatively, it was necessary
to find ways ofsystematicallycharging up conductors with specific increments of charge. One could
use the triboelectric effect to charge up a piece of glass or amber orbone or metal, but the amount
and even the sign of the charge produced was not always consistent. Charge also has a habit of
“leaking away” from anything that is charged because same-sign charge is always repulsive.
It is difficult to properly and completely summarize all of the people that contributed to the formal
discoveries. Otto von Guericke almost by accident built the first triboelectric electrostatic generator.
Charge generated in this way could be stored inLeyden Jars^25. Benjamin Franklin conducted a
series of experiments in the mid-1700’s (long before the American revolution!) that determined
that lightning was electrical in nature, that charging an object generally involved moving charge
of a single sign from one object that otherwise contained equal, balanced amounts of both signs of
(^24) Wikipedia: http://www.wikipedia.org/wiki/electric charge.
(^25) Wikipedia: http://www.wikipedia.org/wiki/Leyden Jar. ALeyden Jar is a primitive capacitor, which we will
study in more detail in three more weeks.