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understand both the concept and the unit by working with them.
TABLE 6.1
Consider, for example, that an object has a net electric charge (q)
–
Electrical conductors and insulators because it has an unbalanced number (n) of electrons (e ) and
+

protons (p ). The net charge on you after walking across a carpet
Conductors Insulators
depends on how many electrons you rubbed from the carpet. Th e
Silver Rubber net charge in this case would be the excess of electrons, or
Copper Glass
quantity of charge = (number of electrons)(electron charge)
Gold Carbon (diamond)
Aluminum Plastics or
Carbon (graphite) Wood
q = ne
Tungsten
equation 6.1
Iron
18
Lead Since 1.00 coulomb is equivalent to the transfer of 6.24 × 10
Nichrome particles such as the electron, the charge on one electron must be
q
_
e =
n
18
tends to remain. In fact, your body is a poor conductor, which is where q is 1.00 C, and n is 6.24 × 10 electrons,
why you become charged by friction. __

Materials vary in their ability to conduct charges, and this e = 1.00 coulomb

18
ability is determined by how tightly or loosely the electrons are 6.24 × 10 electrons
held to the nucleus. Metals have millions of free electrons that can –19 coulomb
_
= 1.60 × 10
take part in the conduction of an electric charge. Materials such electron
as rubber, glass, and plastics hold tightly to their electrons and are –19

good insulators. Thus, metal wires are used to conduct an electric This charge, 1.60 × 10 coulomb, is the smallest common charge

–19
current from one place to another, and rubber, glass, and plastics known (more exactly 1.6021892 × 10 C). It is the funda-
are used as insulators to keep the current from going elsewhere. mental charge of the electron. Every electron has a charge of
–19
–19

There is a third class of materials, such as silicon and ger- –1.60 × 10 C, and every proton has a charge of +1.60 × 10 C.
manium, that sometimes conduct and sometimes insulate, To accumulate a negative charge of 1 C, you would need to accu-
depending on the conditions and how pure they are. Th ese mulate more than 6 billion billion electrons. All charged objects
materials are called semiconductors, and their special properties have multiples of the fundamental charge, so charge is said to be
make possible a number of technological devices such as the quantized. An object might have a charge on the order of about
–8
–6
electrostatic copying machine, solar cells, and so forth. 10 to 10 C.
EXAMPLE 6.1
MEASURING ELECTRICAL CHARGES
Combing your hair on a day with low humidity results in a comb with
As you might have experienced, sometimes you receive a slight
–8
a negative charge on the order of 1.00 × 10 coulomb. How many
shock after walking across a carpet, and sometimes you are

electrons were transferred from your hair to the comb?
really zapped. You receive a greater shock when you have accu-
mulated a greater electric charge. Since there is less electric
charge at one time and more at another, it should be evident SOLUTION

that charge occurs in different amounts, and these amounts can The relationship between the quantity of charge on an object (q), the

be measured. The size of an electric charge is identified with the number of electrons (n), and the fundamental charge on an electron

–
number of electrons that have been transferred onto or away (e ) is found in equation 6.1, q = ne.
q
from an object. The quantity of such a charge (q) is measured –8 _

q = 1.00 × 10 C q = ne ∴ n =
in a unit called a coulomb (C). A coulomb unit is equivalent e
18
–8

to the charge resulting from the transfer of 6.24 × 10 of the e = 1.60 × 10 –19 C _ n = __

1.00 × 10 C

e

charge carried by particles such as the electron. The coulomb is 1.60 × 10 –19 C _

n = ? e
a metric unit of measure like the meter or second. –8
e _
1.00 × 10

The coulomb is a unit of electric charge that is used with = __ ×

C

–19
1.60 × 10 C
other metric units such as meters for distance and newtons for –10
force. Thus, a quantity of charge (q) is described in units of cou- = 6.25 × 10 e

lomb (C). This is just like the process of a quantity of mass (m) Thus, the comb acquired an excess of approximately 62.5 billion elec-

being described in units of kilogram (kg). The concepts of charge trons. (Note that the convention in scientific notation is to express an

and coulomb may seem less understandable than the concepts of answer with one digit to the left of the decimal. See appendix A for
mass and kilogram, since you cannot see charge or how it is mea- further information on scientifi c notation.)
sured. But charge does exist and it can be measured, so you can
6-5 CHAPTER 6 Electricity 143