1. Part 4 of Our Book: Electricity & Magnetism
Chapter 21 opener. This comb has acquired a static electric charge, either from passing through hair, or being rubbed by a cloth or paper towel. The electrical charge on the comb induces a polarization (separation of charge) in scraps of paper, and thus attracts them. Our introduction to electricity in this Chapter covers conductors and insulators, and Coulomb’s law which relates the force between two point charges as a function of their distance apart. We also introduce the powerful concept of electric field.
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

2. Chapter 23: Electric Fields
Chapter 21 opener. This comb has acquired a static electric charge, either from passing through hair, or being rubbed by a cloth or paper towel. The electrical charge on the comb induces a polarization (separation of charge) in scraps of paper, and thus attracts them. Our introduction to electricity in this Chapter covers conductors and insulators, and Coulomb’s law which relates the force between two point charges as a function of their distance apart. We also introduce the powerful concept of electric field.
The comb & the pieces of
paper have opposite static
electric charge, so they
attract each other.
Section 23.2

3. 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.
Mother and daughter are both enjoying the effects of electrically charging their bodies. Each individual hair on their heads becomes charged and exerts a repulsive force on the other hairs, resulting in the “stand-up” hairdos seen here.
Section 23.2

4. 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.
This young woman is enjoying the effects of electrically charging her body. Each individual hair on her head becomes charged and exerts a repulsive force on the other hairs, resulting in the “stand-up” hairdo seen here.
p690

5. Coulomb’s Force Law Some Topics in Chapter 23 The Electric Field
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
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

6. 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

7. 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

8. inverse square law form
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

9. 1831: Michael Faraday & Joseph Henry
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

10. (Maxwell’s Equations!!!)
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

11. 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

12. Most Fundamental & Important Laws of Classical Physics!!!
From Physics I: Newton’s Laws of Motion
Newton’s 2nd Law:
∑F = ma 1
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!!!
Based on experiment!
Not derivable
mathematically!!

13. 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.
Figure (a) Rub a plastic ruler and (b) bring it close to some tiny pieces of paper.
Section 23.2

14. Static Electricity - Conservation of Electric Charge
Experimental Fact
Objects can be charged by rubbing.
Figure (a) Rub a plastic ruler and (b) bring it close to some tiny pieces of paper.
Section 23.2

15. Electric Charge is Conserved:
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
Figure Like charges repel one another; unlike charges attract. (Note color coding: positive and negative charged objects are colored rose-pink and blue-green, respectively, in this book.)
Section 23.2

16. The two rods will attract each other.
In the figure, the
rubber rod is
negatively charged.
The glass rod is
positively charged.
So,
The two rods will
attract each other.
Figure Like charges repel one another; unlike charges attract. (Note color coding: positive and negative charged objects are colored rose-pink and blue-green, respectively, in this book.)
Section 23.2

17. 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.
Figure Like charges repel one another; unlike charges attract. (Note color coding: positive and negative charged objects are colored rose-pink and blue-green, respectively, in this book.)
Section 23.2

18. 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.
Figure Like charges repel one another; unlike charges attract. (Note color coding: positive and negative charged objects are colored rose-pink and blue-green, respectively, in this book.)
Section 23.2

19. 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
|e|  1.6  C
Electron: q = -e, Proton: q = +e
Figure Like charges repel one another; unlike charges attract. (Note color coding: positive and negative charged objects are colored rose-pink and blue-green, respectively, in this book.)
Section 23.2

20. Insulators and Conductors
A Conductor is a material in which
charge flows freely. The most common
types of conductors are metals.
Figure (a) A charged metal sphere and a neutral metal sphere. (b) The two spheres connected by a conductor (a metal nail), which conducts charge from one sphere to the other. (c) The two spheres connected by an insulator (wood); almost no charge is conducted.
Section 23.2

21. Insulators and Conductors
An Insulator is a material in which
almost no charge flows. Most non
metallic materials are insulators.
Figure (a) A charged metal sphere and a neutral metal sphere. (b) The two spheres connected by a conductor (a metal nail), which conducts charge from one sphere to the other. (c) The two spheres connected by an insulator (wood); almost no charge is conducted.
Section 23.2

22. 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!
Figure (a) A charged metal sphere and a neutral metal sphere. (b) The two spheres connected by a conductor (a metal nail), which conducts charge from one sphere to the other. (c) The two spheres connected by an insulator (wood); almost no charge is conducted.
Section 23.2

23. More on Conductors Experimental Fact
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

24. More on Insulators Experimental Fact
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

25. 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

26. 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

27. 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

28. Experimental Fact: A Similar Example
As we’ve just said, metal objects can be
charged by induction.
Figure A neutral metal rod in (a) will acquire a positive charge if placed in contact (b) with a positively charged metal object. (Electrons move as shown by the orange arrow.) This is called charging by conduction.
Section 23.2

29. It does not touch the sphere.
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

30. Grounding a Conductor 
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

31. 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.
Section 23.2

32. Experimental Fact A Similar Example
As we’ve just said, metal objects can be charged by induction, either while connected to ground or not:
Figure Charging by induction.
Figure Inducing a charge on an object connected to ground.
Section 23.2

33. A positive charge will be induced on the sphere.
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

34. will redistribute themselves.
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

35. 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

36. Experimental Fact
As we just said, nonconductors won’t become charged by conduction or induction, but will experience charge separation:
Figure A charged object brought near an insulator causes a charge separation within the insulator’s molecules.
Section 23.2

37. is an instrument used for detecting charge.
An Electroscope
is an instrument used for detecting charge.
Figure Electroscope.
Section 23.2

38. An Electroscope can be charged either by conduction or by induction.
Figure Electroscope charged (a) by induction, (b) by conduction.
Section 23.2

39. A charged Electroscope can be used to determine the sign of an unknown charge.
Figure A previously charged electroscope can be used to determine the sign of a charged object.
Section 23.2