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PowerPoint Lectures for
Universi ty Physics , Thir teenth Edi t ion
Hugh D. Young and Roger A. F reedman
Lectures by Wayne Anderson
Chapter 21
Electric Charge andElectric Field
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Goals for Chapter 21
To study electric charge and charge conservation
To see how objects become charged
To calculate the electric force between objectsusing Coulombs law
To learn the distinction between electric force andelectric field
To calculate the electric field due to many charges To visualize and interpret electric fields
To calculate the properties of electric dipoles
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Introduction
Water makes life possible
as a solvent for biological
molecules. What electrical
properties allow it to do
this?
We now begin our study
ofelectromagnetism, one
of the four fundamental
forces. We start with electric
charge and look at electric
fields.
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Electric charge
Two positive or two negative charges repel each other. A positivecharge and a negative charge attract each other.
Figure 21.1 below shows some experiments in electrostatics.
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Electric charge and the structure of matter
The particles of the atom
are the negative electron,
the positiveproton, and the
uncharged neutron.
Protons and neutrons make
up the tiny dense nucleuswhich is surrounded by
electrons (see Figure 21.3
at the right).
The electric attraction
between protons and
electrons holds the atom
together.
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Atoms and ions
A neutral atom has the same number of protons as electrons.
Apositive ion is an atom with one or more electrons removed.A negative ion has gained one or more electrons.
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Conservation of charge
The proton and electron have the same magnitudecharge.
The magnitude of charge of the electron or proton is anatural unit of charge. All observable charge isquantizedin this unit.
The universalprinciple of charge conservation statesthat the algebraic sum of all the electric charges in anyclosed system is constant.
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Conductors and insulators
A conductorpermits theeasy movement of charge
through it. An insulatordoes not.
Most metals are goodconductors, while most
nonmetals are insulators.(See Figure 21.6 at theright.)
Semiconductors are
intermediate in theirproperties between goodconductors and goodinsulators.
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Charging by induction
In Figure 21.7 below, the negative rod is able to charge the metalball without losing any of its own charge. This process is called
charging by induction.
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Electric forces on uncharged objects
The charge within an insulator can shift slightly. As a result, twoneutral objects can exert electric forces on each other, as shown in
Figure 21.8 below.
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Electrostatic painting
Induced positive charge on the metal object attracts thenegatively charged paint droplets.
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Coulombs law
Coulombs Law: Themagnitude of the electric
force between two pointcharges is directly
proportional to theproduct of their charges
and inversely proportionalto the square of thedistance between them.(See the figure at the
right.)
Mathematically:F = k|q1q2|/r
2 = (1/40)|q1q2|/r2
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Measuring the electric force between point charges
The figure at the upperright illustrates how
Coulomb used a torsionbalance to measure theelectric force between
point charges.
Example 21.1 comparesthe electric and
gravitational forces.Follow it using Figure21.11 at the lower right.
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Force between charges along a line
Read Problem-SolvingStrategy 21.1.
Follow Example 21.2 fortwo charges, using Figure21.12 at the right.
Follow Example 21.3 forthree charges, using Figure21.13 below.
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Vector addition of electric forces
Example 21.4 shows that we must use vector addition whenadding electric forces. Follow this example using Figure 21.14
below.
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Electric field
A charged body produces an electric fieldin the space around it(see Figure 21.15 at the lower left).
We use a small test charge q0 to find out if an electric field ispresent (see Figure 21.16 at the lower right).
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Definition of the electric field
Follow the definition in the text of the electric fieldusing Figure 21.17 below.
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Electric field of a point charge
Follow the discussion in the text of theelectric field of a point charge, using
Figure 21.18 at the right.
Follow Example 21.5 to calculate themagnitude of the electric field of asingle point charge.
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Electric-field vector of a point charge
Follow Example 21.6 to
see the vector nature of the
electric field. Use Figure
21.19 at the right.
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Electron in a uniform field
Example 21.7 requires us to find the force on a chargethat is in a known electric field. Follow this exampleusing Figure 21.20 below.
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Superposition of electric fields
The total electric field at a point is the vector sum of the fields dueto all the charges present. (See Figure 21.21 below right.)
Review Problem-Solving Strategy 21.2.
Follow Example 21.8 for an electric dipole. Use Figure 21.22 below.
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Field of a ring of charge
Follow Example 21.9 using Figure 21.23 below.
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Field of a charged line segment
Follow Example 21.10 and Figure 21.24 below.
Fi ld f if l h d di k
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Field of a uniformly charged disk
Follow Example 21.11 using Figure 21.25 below.
Fi ld f i l h d i fi i h
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Field of two oppositely charged infinite sheets
Follow Example 21.12 using Figure 21.26 below.
El t i fi ld li
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Electric field lines
An electric field line is an imaginary line or curve
whose tangent at any point is the direction of the electricfield vector at that point. (See Figure 21.27 below.)
El t i fi ld li f i t h
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Electric field lines of point charges
Figure 21.28 below shows the electric field lines of a single point
charge and for two charges of opposite sign and of equal sign.
El t i di l
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Electric dipoles
An electric dipole is a pairof point charges having
equal but opposite sign andseparated by a distance.
Figure 21.30 at the rightillustrates the water
molecule, which forms anelectric dipole.
F d t di l
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Force and torque on a dipole
Figure 21.31 below left shows the force on a dipole in an electric
field.
Follow the discussion of force, torque, and potential energy in the
text.
Follow Example 21.13 using Figure 21.32 below right.
El t i fi ld f di l
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Electric field of a dipole
Follow Example 21.14 using Figure 21.33.