Chapter 15
Electricity
The phenomenon ofelectricityhas been known since ancient times. Long ago people discovered that rubbing
fossilized tree resin (calledamber) with fur could cause it to attract bits of light material. (In fact, the Greek
word for amber, "
o, is where we get our word “electricity”.)
Experiments by French scientist Charles du Fay in the early 18th century showed that there were two
types of electricity: one he called “vitreous”, acquired by glass when rubbed with silk; and the other he called
“resinous”, acquired by amber when rubbed with fur. He also discovered that two objects with vitreous charge
repelled each other, as did two resinous-charged objects, but that a vitreous-charged object and resinous-
charged object attracted each other.
Another of many early scientists studying electricity was the American scientist and statesman Benjamin
Franklin. Franklin held the view that electricity was a fluid, and that the two types of electricity were actually
an excess of electric fluid in one material and a deficiency of fluid in the other. But which was which? Franklin
took a 50-50 shot in the dark—and missed! He called the vitreous charge “positive”, and the resinous charge
“negative”, believing these to be an excess and deficiency of electric fluid (respectively). We now know
it’s the other way around. What Franklin thought of as an electric fluid is actually a flow of particles called
electrons, and it is an excess of electrons that is what we call “negative” charge; positive charge is a deficiency
of electrons. Franklin’s unfortunate choice continues to be a source of some confusion in discussing electric
current, as we’ll see later.
Benjamin Franklin is also famous for his (quite dangerous) “kite experiment”, in which he flew a kite into
an electrically charged storm cloud. Electricity from the cloud conducted down the wet kite string to a key at
the other end, and Franklin was able to produce sparks by bringing his knuckle near the key. The experiment
showed that lightning is a form of electricity. (For his contributions to the theory of electricity, the unit of
charge in electrostatic units is named thefranklinin Benjamin Franklin’s honor.)
15.1 Electric Charge
Our modern understanding of electricity may be summarized as follows:
- There are two types of electricity, calledpositive(C) andnegative().
- Like-charged bodies (CandC,orand) repel; unlike-charged bodies (Cand) attract.
- Electric charge isquantized; that is, the charge on a body must always be a multiple of the so-called
elementary chargee. No charge can ever be smaller thane. - Electric charge isconserved: that is, the total charge in a closed system is always constant.