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Chapter 21
Electric Charge
In this chapter we will introduce a new property of
matter known as
“electric charge” (symbol
q
).
We will explore the charge of
atomic constituents.
Moreover, we will describe the following properties of
charge:
 Types of electric charge
 Forces among two charges
(Coulomb’s law)

Charge quantization
 Charge conservation
(211)
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View Full Document Empirically it was known since ancient times that if
amber is rubbed on cloth, it acquires the property
of attracting light objects such as feathers.
This
phenomenon was attributed to a new property of
matter called “electric charge.”
(Electron is the
Greek name for amber.) More experiments show
that there are two distinct types of electric charge:
positive (color code: red)
and
negative (color
code: black).
The names “positive” and “negative”
were given by Benjamin Franklin.
When we rub a glass rod with silk cloth, both
objects acquire electric charge. The sign on the
charge on the glass rod is defined as
positive.
In a similar fashion, when we rub a plastic rod with
fur both objects acquire electric charge. The sign
on the charge on the plastic rod is defined as
negative.
Q:
Do we have enough information so as to be
able
to determine the sign of all other charges
in nature?
To answer this question we need one
more piece of information.
Further experiments on charged objects showed
that:
1. Charges of the same type (either both positive
or both negative) repel each other (fig.
a
).
2. Charges of opposite type on the other hand
attract each other (fig.
b
).
The force direction allows us to
determine the sign of an unknown electric
charge.
Charges of the same sign repel each
other.
Charges of opposite sign attract
each other.
(213)
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force
attractive
force
The recipe is as follows:
We charge a glass rod by rubbing it with silk cloth.
Thus we know that the charge on the glass rod is
positive.
The rod is suspended in such a way so
that it can keep its charge and also rotate freely
under the influence of a force applied by charge
with the unknown sign.
We approach the
suspended class rod with the new charge whose
sign we wish to determine.
Two outcomes are possible. These are shown in
the figure to the left:
Fig.
a
: The two objects repel each other.
We then
conclude that the unknown charge has a
positive
sign.
Fig.
b
: The two objects attract each other.
We
then conclude that the unknown charge has a
negative
sign.
(214)
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This note was uploaded on 08/26/2008 for the course PHY 2049 taught by Professor Any during the Summer '08 term at University of Florida.
 Summer '08
 Any
 Charge

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