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23
Electric Fields
CHAPTER OUTLINE
23.1

Properties of Electric
Charges
Charging Objects by
Induction
Coulomb’s Law
The Electric Field
Electric Field of a
Continuous Charge
Distribution
Electric Field Lines
Motion of Charged Particles
in a Uniform Electric Field

ANSWERS TO QUESTIONS

23.3
23.4
23.5

23.6
23.7

Q23.1A neutral atom is one that has no net charge. This means that it
has the same number of electrons orbiting the nucleus as it has
protons in the nucleus. A negatively charged atom has one or
more excess electrons.

Q23.2

When the comb is nearby, molecules in the paper are polarized,
similar to the molecules in the wall in Figure 23.5a, and the
paper is attracted. During contact,charge from the comb is
transferred to the paper by conduction. Then the paper has the
same charge as the comb, and is repelled.

Q23.3

23.2

The clothes dryer rubs dissimilar materials together as it
tumbles the clothes. Electrons are transferred from one kind of
molecule to another. The charges on pieces of cloth, or on
nearby objects charged by induction, can produce strong
electricfields that promote the ionization process in the
surrounding air that is necessary for a spark to occur. Then you
hear or see the sparks.

Q23.4

To avoid making a spark. Rubber-soled shoes acquire a charge by friction with the floor and could
discharge with a spark, possibly causing an explosion of any flammable material in the oxygenenriched atmosphere.

Q23.5

Electrons are lessmassive and more mobile than protons. Also, they are more easily detached from
atoms than protons.

Q23.6

The electric field due to the charged rod induces charges on near and far sides of the sphere. The
attractive Coulomb force of the rod on the dissimilar charge on the close side of the sphere is larger
than the repulsive Coulomb force of the rod on the like charge on the far side of thesphere. The
result is a net attraction of the sphere to the rod. When the sphere touches the rod, charge is
conducted between the rod and the sphere, leaving both the rod and the sphere like-charged. This
results in a repulsive Coulomb force.

Q23.7

All of the constituents of air are nonpolar except for water. The polar water molecules in the air quite
readily “steal” charge from a chargedobject, as any physics teacher trying to perform electrostatics
demonstrations in the summer well knows. As a result—it is difficult to accumulate large amounts of
excess charge on an object in a humid climate. During a North American winter, the cold, dry air
allows accumulation of significant excess charge, giving the potential (pun intended) for a shocking
(pun also intended) introduction tostatic electricity sparks.
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Electric Fields

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Q23.8

Similarities: A force of gravity is proportional to the product of the intrinsic properties (masses) of
two particles, and inversely proportional to the square of the separation distance. An electrical force
exhibits the same proportionalities, with charge as the intrinsicproperty.
Differences: The electrical force can either attract or repel, while the gravitational force as
described by Newton’s law can only attract. The electrical force between elementary particles is
vastly stronger than the gravitational force.

Q23.9

No. The balloon induces polarization of the molecules in the wall, so that a layer of positive charge
exists near the balloon. This isjust like the situation in Figure 23.5a, except that the signs of the
charges are reversed. The attraction between these charges and the negative charges on the balloon
is stronger than the repulsion between the negative charges on the balloon and the negative charges
in the polarized molecules (because they are farther from the balloon), so that there is a net attractive
force toward the wall....