Electric Charges and Forces
Objects, including elementary particles like the electron and proton, have a number of
The one we have studied the most so far is mass.
The mass of an object is important
in two of the basic laws of mechanics.
First, from Newton's second law of motion, the mass of
an object determines its response to a force: the acceleration
that results from the application of
to an object is related to its mass
Second, the mass of the object is
involved in gravitational forces through Newton's law of gravitation: the magnitude of the
gravitational force between two objects of masses
separated by a distance
Another property of objects is their electric charge.
What leads us to think that there are
such things as "charges," anyway?
Think about what you have learned about "forces." A force
can be thought of as a push or a pull that may act on an object. Usually, the force that acts on
some object "A" is caused by some other object which is in direct contact with object "A." For
instance, a hammer might strike a nail; then the hammer is exerting a force on the nail.
According to Newton's third law, the nail also exerts a force on the hammer (equal in magnitude,
opposite in direction). This type of force is often referred to as a "contact" force.
another type of force, in which direct contact between objects is not required. Suppose you hold
a book up in the air, and then let it fall. What made it fall? You know that a gravitational force
exerted by the earth on the book was responsible for the book's fall. (By Newton's third law,
there is also a gravitational force exerted by the book on the earth.) You have also observed
magnets appearing to exert forces on each other, even though they are not in direct contact.
These forces have been called "action-at-a-distance" forces.
Now consider some other phenomena you have encountered in the past. When you rub a
comb through your hair, you may have noticed the comb pick up small bits of paper. You may
have seen balloons appearing to "stick" to walls, without any glue. You have probably seen
various types of plastic materials (such as thin plastic wrap) adhere to other objects, as if they
were "attracted" to each other across space. Magnets don't seem to be involved in this type of
phenomena. Could it be due to gravitational forces?
Absolutely not, because gravitational forces
between these small objects is much too small to be noticeable.
In experiments using certain simple materials such as plastic and glass rods, when they
are rubbed with fur or silk, certain phenomena consistently recur: (1) when two of these objects
made from identical materials are prepared the same way (e.g., plastic rubbed with fur), the
objects appear to exert small repulsive forces on each other. If the objects are suspended by a
string, or mounted on a sensitive pivot, this force may result in a visible motion; (2) when two of
these objects made from different materials are held near each other (e.g., a plastic rod held near