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Chapter 15
College Physics, 6th Edition
Wilson / Buffa / Lou
© 2007 Pearson Prentice Hall
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Figure 15-1
Simplistic model of atoms
The so-called solar
system model of (a) a
hydrogen atom and (b) a
beryllium atom views the
electrons (negatively
charged) as orbiting the
nucleus (positively
charged), analogously to
the planets orbiting the
Sun. The electronic
structure of atoms is
actually much more
complicated than this.
Figure 15-2
The charge–force law, or law of charges
(a) Like charges repel. (b) Unlike charges attract.
Table 15-1
Sub-atomic Particles and Their Electric Charge
Figure 15-3
Conductors, semiconductors,
and insulators
A comparison of
the relative
magnitudes of
the electrical
conductivities of
various materials
(not drawn to
scale).
Figure 15-4
The electroscope
An electroscope can be used to determine whether an object is electrically charged.
When a charged object is brought near the bulb, the leaf moves away from the metal
piece.
Figure 15-5
Charging by conduction
(a) The electroscope is
initially neutral (but the
charges are separated),
as a charged rod touches
the bulb.
(b) Charge is transferred
to the electroscope.
(c) When a rod of the
same charge is brought
near the bulb, the leaf
moves farther apart.
(d) When an oppositely
charged rod is brought
nearby, the leaf collapses.
Figure 15-6 Charging by induction
(a) Touching the bulb with a finger provides a path to ground for charge transfer. The
symbol e- stands for “electron.”
(b) When the finger is removed, the electroscope has a net positive charge, opposite
that of the rod.
Figure 15-7 Polarization
(a) When the balloons are charged by
friction and placed in contact with the
wall, the wall is polarized. That is, an
opposite charge is induced on the
wall’s surface, to which the balloons
then stick by the force of electrostatic
attraction. The electrons on the balloon
do not leave the balloon because its
material (rubber) is a poor conductor.
(b) Some molecules, such as those of
water, are polar by nature; that is, they
have permanently separated regions of
positive and negative charge. But even
some molecules that are not normally
dipolar can be polarized temporarily by
the presence of a nearby charged
object. The electric force induces a
separation of charge and,
consequently, temporary molecular
dipoles.
Figure 15-8 Coulomb’s law
a) The mutual electrostatic forces on two point charges are equal and opposite.
(b) For a configuration of two or more point charges, the force on a particular charge is
the vector sum of the forces on it due to all the other charges. (Note: In each of these
situations, all of the charges are of the same sign. How can we tell that this is true? Can
Figure 15-9
Comb and paper
Figure 15-10
Coulomb’s law and electrostatic forces
Figure 15-11
Electric field direction
a) The mutual
electrostatic forces on
two point charges are
equal and opposite.
(b) For a configuration of
two or more point
charges, the force on a
particular charge is the
vector sum of the forces
on it due to all the other
charges. (Note: In each
of these situations, all of
the charges are of the
same sign. How can we
tell that this is true? Can
you tell their sign? What
is the direction of the
force on q2 due to q3?)
Figure 15-12 Electric field
(a) The electric field points away from a positive point charge, in the direction a force
would be exerted on a small positive test charge. The field’s magnitude (the lengths
of the vectors) decreases as the distance from the charge increases, reflecting the
inverse-square distance relationship characteristic of the field produced by a point
charge.
(b) In this simple case, the vectors are easily connected to give the electric field line
pattern due to a positive point charge.
Learn by Drawing 15-1
Using the Superposition Principle to Determine the Electric Field
Direction
To estimate the direction of the electric field at any point P, draw the individual electric
fields vectors and add them, taking into account the relative field magnitudes if you can.
In this situation, E1 is much smaller than E2 because of both distance and charge
factors. Can you explain why the vector representing E2, if drawn accurately, would be
about eight times as long as that of E1? The final step is to complete the vector addition.
Figure 15-13
Electric field in one dimension
Figure 15-14
Finding the electric field
Learn by Drawing 15-2
Sketching Electric Lines of Force
How many lines should be drawn for 1½ q, and what should their direction
be?
Figure 15-15 Mapping the electric field due to a dipole
(a) The construction of one electric field line from a dipole is shown. The electric
field is the vector sum of the two fields produced by the two
ends of the dipole. (See Example 15.8 for details.)
(b) The full electric dipole field is determined by following the procedure in part (a)
at other locations near the dipole.
Figure 15-16 Electric field due to very large parallel plates
(a) Above a positively charged plate, the net electric field points upward. Here, the
horizontal components of the electric fields from various
locations on the plate cancel out. Below the plate, E points downward.
(b) For a negatively charged plate, the electric field directions (shown on both sides of
the plate) are reversed.
(c) Superimposing the fields from both plates results in cancellation outside the plates
and an approximately uniform field between them.
Insight 15-1 Lightning and Lightning Rods
Cloud
polarization
induces a
charge on the
Earth’s surface.
Insight 15-2 Electric Fields in Law Enforcement and Nature: Stun Guns
and Electric Fish The Taser stun gun
A sketch of the electric field between the electrodes. (The charges change sign
periodically, producing an oscillating electric field.)
Insight 15-2 Electric Fields in Law Enforcement and Nature:
Stun Guns and Electric Fish Electric Fish
(left) At an instant in time, the approximate electric field produced by the elephant nose
fish’s electric organ, located near its tail. (The field actually oscillates.)
(right) At an instant in time, the approximate electric field produced by an electric eel.
The electric organ in the eel is capable of producing fields that can kill and stun as well
as locate and communicate.
Figure 15-17 Electric fields
Electric fields for (a) like point
charges and (b) unequal like
point charges.
Figure 15-18 Electric fields and conductors
(a) Under static conditions, the electric field is zero inside a conductor. Any excess
charge resides on the conductor’s surface. For an irregularly shaped conductor, the
excess charge accumulates in the regions of highest curvature (the sharpest points), as
shown. The electric field near the surface is perpendicular to that surface and strongest
where the charge is densest.
(b) Under static conditions, the electric field must not have a component tangential to
the conductor’s surface.
Figure 15-19
Concentration of charge on a
curved surface
(a) On a flat surface, the repulsive forces
between excess charges are parallel to the
surface and tend to push the charges apart.
On a sharply curved surface, in contrast,
these forces are directed at an angle to the
surface. Their components parallel to the
surface are smaller, allowing
charge to concentrate in such areas.
(b) Taken to the extreme, a sharply pointed
metallic needle has a dense concentration
of charge at the tip. This produces a large
electric field
in the region above the tip, which is the
principle of the lightning rod.
Figure 15-20
An ice pail experiment
Figure 15-21
Various Gaussian surfaces and lines of force
(a) Surrounding a single positive point charge, (b) surrounding a single negative
point charge, and (c) surrounding a larger negative point charge. (d) Four different
surfaces surrounding various parts of an electric dipole.
Figure 15-22 Water analogy to Gauss’s law
A net outward flow of water indicates a source of water inside closed surface 1. A net
inward flow of water indicates a water drain inside closed surface 2.
Figure 15-23
Gauss’s law: Excess charge on a conductor
Figure 15-24
Hydrogen atom
See Exercises 36 and 37.
Figure 15-25
Charge triangle
See Exercises 38, 59, and 60.
Figure 15-26
Charge rectangle
See Exercises 39, 61, and 65.
Figure 15-27
Repelling pith balls
See Exercise 40.
Figure 15-28
A point charge inside a thick metal spherical shell
See Exercise 47.
Figure 15-29
Electric dipole field
See Exercise 66.
Figure 15-30
Safe inside a car?
See Exercise 70.
Figure 15-31
Electric field
See Exercise 91.
Figure 15-32
Electron in a computer monitor
See Exercise 93.