Electric Charges, Forces, and Fields
Answers to Even-numbered Conceptual Questions
Giving an object a negative charge means transferring electrons to the object.
turn, increases its mass.
No, the basic physics of electric charges would not have been affected at all by an
opposite assignment of positive and negative labels.
The use of + and – signs, as opposed
to labels such as A and B, has the distinct advantage that it gives zero net charge to an
object that contains equal amounts of positive and negative charge.
Referring to Table 19-1, we see that rubbing rabbit fur against glass will result in a
positive charge for the rabbit fur, and a negative charge for the glass.
For glass and
rubber, we see that the rubber acquires a negative charge.
Note that in this case, the
charge on the glass is positive; hence, the charge acquired by a material depends not only
on the material itself, but also on the material that it is rubbed against.
rabbit fur and glass are adjacent in Table 19-1, whereas glass and rubber are widely
separated, we conclude that the magnitude of triboelectric charge is greater in the glass-
Initially, the bits of paper are uncharged and are attracted to the comb by polarization
(See Figure 19-5 and the accompanying discussion.)
When one of the bits of
paper comes into contact with the comb, it acquires charge from the comb.
Now the piece
of paper and the comb have charge of the same sign, and hence we expect a repulsive
force between them.
Even uncharged objects will be attracted to a charged rod, due to polarization effects.
See Figure 19-5 and the accompanying discussion.
Both force laws depend on the product of specific properties of the objects involved; in
the case of gravity it is the mass that is relevant, in the case of electrostatics it is the
In addition, both forces decrease with increasing distance as 1/
extremely important difference between the forces, however, is that gravity is always
attractive, whereas electrostatic forces can be attractive or repulsive.
The answer is (b), the center of the square.
At the center of the square, the forces exerted
by the charges +
are in the same direction – regardless of the sign of the third
charge – giving a large net force.
At an empty corner of the square, the third charge is
farther from the other charges, and the two forces acting on it are in different directions.
Both of these effects tend to decrease the magnitude of the net force.
If the ball is displaced slightly upward from the equilibrium position, the attractive
electrostatic force will be larger than the gravitational force, which will displace the ball
Similarly, if the ball is displaced slightly downward, the gravitational
force is now stronger than the electrostatic force, and the ball will move farther
Therefore, the equilibrium is unstable.