Lesson 2: Methods of
Charging
Charging by
Friction
In Lesson 1, it was explained
that atoms are the building blocks of matter.
Furthermore, it was explained that material objects are
made of different types of atoms and combinations of
atoms. The presence of different atoms in objects provide
different objects with different electrical properties.
One such property is known as
electron affinity.
Simply put, the property of electron affinity refers to
the love which a material has for electrons. If atoms of
a material have a high electron affinity, then that
material will have a relatively high love for electrons.
This property of electron affinity will be of utmost
importance as we explore one of the most common methods
of charging - charging by friction or rubbing.
Suppose that a rubber balloon is rubbed
with a sample of animal fur. During the rubbing process,
the atoms of the rubber are forced into close proximity
with the atoms of the animal fur. The electron clouds of
the two types of atoms are pressed together and are
brought closer to the nuclei of the other atoms. The
protons in the atoms of one material begin to interact
with the electrons present on the other material. Amidst
the sound of crackling air, you might even be able to
hear the atoms saying "I like your electrons." And of
course, the atoms of one material - in this case, the
atoms of rubber - are more serious about their claim for
electrons. As such, the atoms of rubber begin to take
electrons from the atoms of animal fur. When the rubbing
has ceased, the two objects have become charged.
The procedure of rubbing a rubber
balloon against your hair is quite easily performed. You
might try it now if you've never performed it. When done,
you will likely notice that the rubber balloon and your
 hair
will attract each other. On a dry day, you might even be
able to let go of the balloon and have it adhere to your
hair. (You will also probably notice that the procedure
will initiate a bad hair day. Sorry.) This attraction
between the two charged objects is evidence that the
objects being charged are charged with opposite charge.
One is positively-charged and the other is
negatively-charged. How does this happen? How does the
simple rubbing together of two objects cause the objects
to become charged and charged oppositely?
The frictional charging process results
in a transfer of electrons between the two objects which
are rubbed together. Rubber has a much greater attraction
for electrons than animal fur. As a result, the atoms of
rubber pull electrons from the atoms of animal fur,
leaving both objects with an imbalance of charge. The
rubber balloon has an excess of electrons and the animal
fur has a shortage of electrons. Having an excess of
electrons, the rubber balloon is charged negatively.
Similarly, the shortage of electrons on the animal fur
leaves it with a positive charge. The two objects have
become charged with opposite charges as a result of the
transfer of electrons from the least electron-loving
material to the most electron-loving material.
Frictional charging was demonstrated in
class on several occasions. Two rubber balloons were
suspended from the ceiling and hung at approximately head
height. When rubbed upon the teacher's head, the balloons
became charged as electrons were transferred from the
teacher's fur to the balloons. Since the teacher's fur
lost electrons, it became positively-charged and the
subsequent attraction between the two rubbed objects
could be observed. Of course, when the teacher pulled
away from the balloon, the balloons experienced a
repulsive interaction for each other.
 As
mentioned, different materials have different affinities
for electrons. By rubbing a variety of materials against
each other and testing their resulting interaction with
objects of known charge, the tested materials can be
ordered according to their affinity for electrons. Such
an ordering of substances is known as a
tribolectric series.
One such ordering for several materials is shown in the
table at the right. Materials shown highest on the table
tend to have a greater affinity for electrons than those
below it. Subsequently, when any two materials in the
table are rubbed together, the one which is higher can be
expected to pull electrons from the material which is
lower. As such, the materials highest on the table will
have the greatest tendency to acquire the negative
charge. Those below it become positively-charged.
The
Law of Conservation of Charge
The frictional charging process (as well as any
charging process) involves a transfer of electrons
between two objects. Charge is not created from nothing.
The appearance of negative charge upon a rubber balloon
is merely the result of its acquisition of electrons. And
these electrons must come from somewhere; in this case,
from the object it was rubbed against. Electrons are
transferred in any charging process. In the case of
charging by friction, they are transferred between the
two objects being rubbed together. Prior to the charging,
both objects are electrically neutral. The
net charge of the
system is 0 units. After the charging process, the more
electron-loving object may acquire a charge of -12 units;
the other object acquires a charge of +12 units. Overall,
the system of two objects has a net charge of 0 units.
Whenever a quantity like charge (or momentum
or energy or
matter) is observed to be the same prior to and after the
completion of a given process, we say that the quantity
is conserved. Charge is always conserved. When all
objects involved are considered prior to and after a
given process, we notice that the total amount of charge
amidst the objects is the same before the process starts
as it is after the process ends. This is referred to as
the law of conservation of
charge.
Check
Your Understanding
Use your understanding of charge to answer the
following questions. When finished, depress the mouse on
the "pop-up menu" to view the answers.
1. During a physics lab, a plastic strip was rubbed
with cotton and became positively charged. The correct
explanation for why the plastic strip becomes positively
charged is that ...
a. the plastic strip acquired extra protons
from the cotton.
b. the plastic strip acquired extra protons during
the charging process.
c. protons were created as the result of the
charging process.
d. the plastic strip lost electrons to the cotton
during the charging process.
2. Saran Wrap has a larger electron affinity than
Nylon. If Nylon is rubbed against Saran Wrap, which would
end up with the excess negative charge? ____________
Explain.
3. A physics teacher rubs a glass object and a felt
cloth together and the glass becomes positively charged.
Which of the following statements are true? Circle all
that apply.
a. The glass gained protons during the
rubbing process.
b. The felt became charged negatively during this
rubbing process.
c. Charge is created during the rubbing process; it
is grabbed by the more charge-hungry object.
d. If the glass acquired a charge of +5 units, then
the felt acquires a charge of -5 units.
e. This event violates the law of conservation of
charge.
f. Electrons are transferred from glass to felt;
protons are transferred from felt to glass.
g. Once charged in this manner, the glass object
and the felt cloth should attract each other.
h. In general, glass materials must have a greater
affinity for electrons than felt materials.
4. When a rubber rod is rubbed with fur, the rubber
rod becomes negatively charged. Thus, it can be concluded
that ____.
a. rubber is a better insulator than fur.
b. rubber is a better conductor than fur.
c. molecules of rubber have a stronger attraction
for electrons than the molecules of fur.
d. molecules of rubber have a weaker attraction for
electrons than the molecules of fur.
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