Transcript Chapter 23

Part 4 of Our Book: Electricity & Magnetism
The “Transrapid Maglev” Train, Shanghai, China.
“Maglev”  Magnetic Levitation.
It makes no contact with the rails! It’s weight is
100% supported by electromagnetic forces!!
Section 23.2
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Chapter 23: Electric Fields
The comb & the pieces of
paper have opposite static
electric charge, so they
attract each other.
Section 23.2
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Some Fun with Static Electricity!
Mother & daughter
are both charged with
static electricity. Each
hair on their heads is
charged & exerts a
repulsive force on
all other hairs.
Section 23.2
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More Fun with Static Electricity!
This woman is electrically charging her body.
Each hair becomes charged & exerts a repulsive
force on the other hairs, resulting in this
“stand-up” hairdo.
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Some Topics in Chapter 23
• Static Electricity; Electric Charge & Its Conservation
• Electric Charge in the Atom; Insulators & Conductors
• Induced Charge; The Electroscope
Coulomb’s Force Law
The Electric Field
• Electric Field Calculations for Continuous Charge Distributions
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Electric Field Lines
Electric Fields & Conductors
Motion of a Charged Particle in an Electric Field
Electric Dipoles
Electric Forces in Molecular Biology: DNA
Some Applications:
Photocopy Machines & Computer Printers Use Electrostatics
Section 23.2
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Electricity and Magnetism
The Laws of Electricity &
Magnetism are Very Important:
• In everyday life, they play a central role
in the operation of many modern
electronic devices.
• In basic materials physics, the
interatomic and intermolecular forces
responsible for the formation of solids
and liquids are electric in nature.
Section 23.2
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Brief History of Electricity & Magnetism
Ancient Chinese
•Some documents suggest that
magnetism was observed as early
as 2000 BC in China.
Ancient Greeks
• Electrical and magnetic phenomena
were known as early as 700 BC.
Experiments with amber & magnetite
Section 23.2
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1600: William Gilbert
• Gilbert showed electrification effects
were not confined to just amber. The
electrification effects were a general
phenomena.
1785: Charles Coulomb
Coulomb confirmed the
inverse square law form
for electric forces.
Section 23.2
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1819: Hans Oersted
• Oersted found that a compass needle
deflected when near a wire carrying an
electric current.
1831: Michael Faraday &
Joseph Henry
• Faraday & Henry showed that when a
wire is moved near a magnet, an electric
current is produced in the wire.
Section 23.2
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1873: James Clerk Maxwell
• Maxwell used observations & other
experimental facts as a basis for
formulating the laws of
electromagnetism. He achieved the
Unification of Electricity &
Magnetism!!!!!
(Maxwell’s Equations!!!)
(The “Theme” of Physics 2401!)
Section 23.2
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Electricity & Magnetism: Forces
• The concept of Force came originally from
Isaac Newton. It connects the study of
electromagnetism to one of the main topics of
Physics I: Newton’s Laws of Motion!
•As we said earlier,
The Electromagnetic Force
between charged particles is one of the
Fundamental Forces of Nature.
Section 23.2
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From Physics I: Newton’s Laws of Motion
Newton’s 2nd Law:
Based on experiment!
Not derivable
1 mathematically!!
∑F = ma
A VECTOR equation!! Holds component by component.
∑Fx = max, ∑Fy = may, ∑Fz = maz
This is one of the
Most Fundamental & Important
Laws of Classical Physics!!!
Section 23.1: Properties of Electric Charge
• Experiments (first done by Coulomb!) show
that there are
Two kinds of electric charges.
They are called positive & negative
• Negative Charges
Are the type possessed by electrons.
• Positive Charges
Are the type possessed by protons.
• Experiments (Coulomb) also show that:
Charges of the same sign repel one another &
charges with opposite signs attract one another.
Section 23.2
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Static Electricity Conservation of Electric Charge
Experimental Fact
Objects can be charged by rubbing.
Section 23.2
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Experimental Facts
As we already said
Charge comes in 2 types:
Positive (+) & Negative (-).
Like charges repel, opposite
charges attract. Also
Electric Charge
is Conserved:
The arithmetic sum of the
total charge cannot
change in any interaction
Section 23.2
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•In the figure, the
rubber rod is
negatively charged.
•The glass rod is
positively charged.
•So,
The two rods will
attract each other.
Section 23.2
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More About Electric Charges
Experimental Fact:
Electric charge is always
conserved in an isolated system.
• For example, charge is not created in the
process of rubbing two objects together.
• The electrification is due to a
Transfer of charge from
one object to another.
Section 23.2
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Conservation of Electric Charge
Example
• A glass rod is rubbed
with silk.
• Electrons are transferred
from the glass to the silk.
• Each electron adds a
negative charge to the silk.
• An equal positive charge
is left on the rod.
Section 23.2
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More About Electric Charges
Experimental Fact:
Electric charge is Quantized
• That is, an electric charge q is ALWAYS an
integer multiple of the charge on an electron e.
• Or, electric charge q exists only as discrete packets:
q  Ne N  Huge integer!
e  Fundamental Unit of Charge
-19
|e|  1.6  10 C
Electron: q = -e, Proton: q = +e
Section 23.2
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Insulators and Conductors
Conductor
A Conductor is a material in which
charge flows freely. The most common
types of conductors are metals.
Section 23.2
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Insulators and Conductors
Insulator
An Insulator is a material in which
almost no charge flows. Most non
metallic materials are insulators.
Section 23.2
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Insulators and Conductors
Semiconductor
A Semiconductor is a material
with special properties,
somewhere in between conductors
& insulators. Without
semiconductors (especially silicon, Si),
much of our technology would not
exist!
Section 23.2
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More on Conductors
• Electrical Conductors are materials in
which some of the electrons are “free electrons”.
“Free electrons” are not bound to the atoms.
“Free electrons” can move relatively freely.
through the material.
• Examples of good conductors include copper,
aluminum and silver.
Experimental Fact
• When a good conductor is charged in a small
region, the charge readily distributes itself over
the entire surface of the material.
Section 23.2
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More on Insulators
• Electrical Insulators are materials in
which all of the electrons are bound to atoms.
• These electrons cannot move relatively
freely through the material.
• Examples of good insulators include glass,
rubber and wood.
Experimental Fact
• When a good insulator is charged in a small
region, the charge is unable to move to other
regions of the material.
Section 23.2
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More on Semiconductors
• The electrical properties of Semiconductors
are somewhere between those of insulators
& conductors.
• Examples of semiconductor materials include
silicon & germanium.
• These materials are commonly used in making
electronic chips.
Experimental Fact
• The electrical properties of semiconductors
can be changed by the addition of controlled
amounts of certain atoms to the material.
Section 23.2
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Section 23.2
Charging Objects by Induction
Experimental Fact
• When a charged object is brought near enough
to an uncharged object, the uncharged object
can become charged. This process is called
Charging by Induction
• It is important to note that
Charging by Induction requires
contact with the object
inducing the charge!
Section 23.2
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Example
•Assume that we start with a neutral
metallic sphere.
See Figure a.
• Since it is neutral, the
sphere has the same
number of positive &
negative charges.
Section 23.2
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A Similar Example
Experimental Fact:
• As we’ve just said, metal objects can be
charged by induction.
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• Experiment I: Now, place a charged rubber
rod near the sphere. See Figure b.
It does not touch the sphere.
• The electrons in the neutral sphere are
redistributed due to interaction with the rod.
See Figure b.
Section 23.2
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Definition
Grounding a Conductor 
The process of placing a
conducting wire between the
conductor & the earth such that
the wire touches both the
conductor & the earth.
Section 23.2
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• Experiment II: Ground the charged sphere,
while leaving the charged rubber rod near it.
See Figure c.
• This allows some
electrons to leave
the sphere through
the ground wire,
as is shown in
Figure c.
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A Similar Example
Experimental Fact
As we’ve just said, metal objects can be charged by
induction, either while connected to ground or not:
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• Experiment III: Now, remove the
ground wire, as is shown in Figure d.
 There will now be more positive charges
than negative charges on the sphere.
• So, obviously, the charges
will no longer be uniformly
distributed on the sphere.
• That is,
A positive charge
will be induced
on the sphere.
Section 23.2
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• Experiment IV: Now, remove the rod, as
is shown in Figure e.
 The electrons remaining on the sphere
will redistribute themselves.
• There will still be a net positive
charge on the sphere.
• The charge on the sphere will
again be uniformly distributed.
Note: The rod will
have lost none of
its negative charge
during this process.
Section 23.2
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Charge Rearrangement in Insulators
A process similar to
induction can happen
in insulators.
• The charges within the molecules
of the material are rearranged.
• The proximity of the positive
charges on the surface of the
object and the negative charges
on the surface of the insulator
results in an attractive force
between the object and the
insulator. See the figure.
Section 23.2
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Experimental Fact
As we just said, nonconductors won’t become
charged by conduction or induction, but will
experience charge separation:
Section 23.2
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An Electroscope
is an instrument
used for detecting
charge.
Section 23.2
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An Electroscope can be charged either
by conduction or by induction.
Section 23.2
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A charged Electroscope can be used to
determine the sign of an unknown charge.
Section 23.2
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