Lesson 1: Charge and Charge
Suppose that you rubbed a balloon with a sample of
animal fur such as a wool sweater or even your own hair.
The balloon would likely become charged and its charge
would exert a strange influence upon other objects in its
vicinity. If some small bits of paper were placed upon a
table and the balloon were brought near and held above
the paper bits, then the presence of the charged balloon
might create a sufficient attraction for the paper bits
to raise them off the table. This influence - known as an
electric force -
occurs even when the charged balloon is held some
distance away from the paper bits. The electric force is
a non-contact force. Any charged object can exert this
force upon other objects - both charged and uncharged
objects. One goal of this unit of The Physics Classroom
is to understand the nature of this electric force. In
this part of Lesson 1, two simple and fundamental
statements will be made and explained about the nature of
the electric force.
Perhaps you have heard it said so many
times that it sounds like a cliché.
Opposites attract. And
These two fundamental principles of charge
interactions will be used throughout the unit to explain
the vast array of static electricity phenomena. As
mentioned in the previous section of
Lesson 1, there are two types of electrically charged
objects - those which contain more protons than electrons
and are said to be positively-charged and those which
contain less protons than electrons and are said to be
negatively-charged. These two types of electrical charges
- positive and negative - are said to be opposite types
of charge. And consistent with our fundamental principle
of charge interaction, a positively-charged object will
attract a negatively-charged object. Oppositely-charged
objects will exert an attractive influence upon each
other. In contrast to the attractive force between two
objects with opposite charges, two objects which are of
like charge will repel each other. That is, a
positively-charged object will exert a repulsive force
upon a second positively-charged object; this force will
push the two objects apart. Similarly, a
negatively-charged object will exert a repulsive force
upon a second negatively-charged object. Objects with
like charge repel each other.
Electric Force and Newton's Third Law
This electric force exerted between two
oppositely-charged objects or two like-charged objects is
a force in the same sense that friction, tension, gravity
and air resistance are forces. And being a force, the
same laws and principles which describe any force
describe the electrical force. One of those laws was
Newton's law of
action-reaction (discussed in Unit
2 of The Physics Classroom). According to Newton's
third law, a force is simply a mutual interaction between
two objects which results in an equal
and opposite push or pull upon those objects. Let's apply
Newton's third law to describe the interaction between
Object A and Object B, both having positive charge.
Object A exerts a rightward push upon
Object B. Object B exerts a leftward push upon Object A.
These two pushing forces have equal magnitudes and are
exerted in opposite directions of each other. Each object
does its own pushing upon the other. The push upon Object
B (by Object A) is directed away from Object A; and the
push upon Object A (by Object B) is directed away from
Object B. Because of the away from nature of the
mutual interaction, the force is said to be
let's apply the same action-reaction principle to two
oppositely charged objects - Object C (positive) and
Object D (negative). Object C exerts a leftward pull upon
object D. Object D exerts a rightward pull upon Object C.
Again, each object does its own pulling of the other.
Just as before, these two forces have equal magnitudes
and are exerted in opposite directions of each other.
However in this instance, the direction of the force on
Object D is towards Object C; and the direction of the
force on Object C is towards object D. Because of the
towards each other nature of the mutual
interaction, the force is described as being
Between Charged and Neutral Objects
The interaction between two like-charged objects is
repulsive. The interaction between two oppositely-charged
objects is attractive. What type of interaction is
observed between a charged object and a neutral object?
The answer is quite surprising to many students of
physics. Any charged object - whether positively-charged
or negatively-charged - will have an attractive
interaction with a neutral object. Positively-charged
objects and neutral objects attract each other; and
negatively-charged objects and neutral objects attract
third interaction between charged and neutral objects is
commonly observed in the physics classroom. In the Static
Electricity Lab, a charged balloon was held above neutral
paper bits; the force of attraction for the paper bits
was strong enough to overwhelm the downward force of
gravity and raise the bits of paper off the table. In a
classroom demonstration, a charged tube was held above
some bits of paper and the paper was sent airborne. And
in another demonstration, a charged rubber balloon was
attracted to a wooden cabinet. Any charged object -
plastic, rubber, or aluminum - will exert an attractive
force upon a neutral object. And in accordance with
Newton's law of action-reaction,
the neutral object attracts the charged object.
Because charged objects interact with their
surroundings, an observed interaction provides possible
that an object is charged. Suppose that you enter the
physics classroom and observe two balloons suspended from
the ceiling. Rather than hanging straight down
vertically, the balloons are hanging at an angle,
exhibiting a repulsive interaction as shown at the right.
The only way that two objects can repel each other is if
they are both charged with the same type of charge. Thus,
the repulsion of the balloons provides conclusive
evidence that both balloons are charged and charged with
the same type of charge. One could not conclude that the
balloons are both positively-charged or both
negatively-charged. Additional information or further
testing would be required to make a conclusion about the
type of excess charge present upon the balloons.
Nonetheless, one can be convinced that both balloons
possess an excess charge - either positive or
Now let's contrast the observation of
repulsion with that of attraction. Suppose that you now
enter the physics
classroom and observe two balloons suspended from the
ceiling and exhibiting an attractive interaction as shown
at the right. There are two underlying reasons for two
objects attracting each other. One could be neutral and
the other charged or both objects could be charged with
the opposite type of charge. Thus, your only conclusion
could be that at least one of the objects is charged. The
other object is either neutral or charged with the
opposite type of charge. You cannot draw a conclusion
about which one of the objects is charged or what type of
charge (positive or negative) the charged object
possesses. Additional information or further testing
would be required to make these conclusions. For example,
if you could take each balloon and individually bring
them near some neutral bits of paper, you could test to
see if each individual balloon is charged or neutral. If
a balloon is charged, then it would exhibit an attractive
interaction with the neutral paper bits. On the other
hand, an uncharged balloon would not interact at all with
neutral paper bits.
The above thought experiments
illustrate the conclusive nature of a repulsive
interaction. When objects repel each other, one can be
certain that both objects are charged. On the other had,
the observation of an attractive interaction leads to
limited conclusions. At best, one can conclude that at
least one of the objects is charged.
We'll conclude this part of Lesson 1
by asking the question "How can a charged object and a
neutral object attract?" As you've read this page, you
might have been thinking something like "But I've only
heard of two fundamental charge interactions - opposites
attract and likes repel. Where did this third charge
interaction come from?"
In all likelihood, most of us have
only heard of two types of charge interactions (opposites
attract and likes repel); and both of these charge
interactions are fundamental interactions. The third
statement - any charged object and a neutral object will
attract each other - is simply an observable fact that
can be explained by the two fundamental charge
interactions. How? The explanation of this third charge
interaction will be saved for the
last page of Lesson 1. But first, the subject of
conductors and insulators must
be explored in order to understand our third type of
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. Electrical forces ____.
a. can cause objects to only attract each
b. can cause objects to only repel each other
c. can cause objects to attract or repel each
d. have no affect on objects
2. On two occasions, the following charge interactions
between balloons A, B and C are observed. In each case,
it is known that balloon B is charged negatively. Based
on these observations, what can you conclusively confirm
about the charge on balloon A and C for each
Upon entering the room, you observe two balloons
suspended from the ceiling. You notice that instead of
hanging straight down vertically, the balloons seems to
be repelling each other. You can conclusively say ...
a. both balloons have a negative charge.
b. both balloons have a positive charge.
c. one balloon is charge positively and the other
d. both balloons are charged with the same type of
Explain your answer.
4. Jean Yuss is investigating the charge on several
objects and makes the following findings.
Jean knows that object A is negatively charged and
object B is electrically neutral. What can Jean Yuss
definitively conclude about the charge on objects C, D,
E, and F? Explain.
Two objects are charged as shown at the right. Object X
will ____ object Y.
c. not affect
Two objects are shown at the right; one is neutral and
the other is negative. Object X will ____ object Y.
c. not affect
Balloons X , Y and Z are suspended from strings as shown
at the right. Negatively-charged balloon X attracts
balloon Y and balloon Y attracts balloon Z. Balloon Z
____. List all that apply.
a. may be positively-charged
b. may be negatively-charged
c. may be neutral
d. must be positively-charged
e. must be negatively-charged
f. must be neutral
(NOTE: This is an exercise in logic and reasoning as
much as it is an exercise in physics.)