Invention of the first Transistor in Nov.17Nov.17-Dec.23 1947,
the most important invention of 20th century
Goals
Physics 213
„
„
Knowledge of physics
„
„
„ Critical thinking, idealization,
approximation, mathematical and
graphical representations of phenomena.
„
„
Electric Charge, Field, Energy, Currents
Magnetism
Electromagnetism
Light and Optics
Modern Physics including Special
Relativity, Quantum Mechanics and
Nuclear Physics
16_08a.jpg
Chapter 15
Electric Forces
Electric Fields
16_22.jpg
Outline for Today
„
„
What is electric charge?
Like rest mass, it is a fundamental
property of some of the elementary
particles of which all matter is composed
Electric Charge
Coulomb’s Law
Properties of Electric Charges
„
Two types of charges exist
„
„
Electric charge is the fourth quantity
we have learned (energy, linear
„
„
momentum, and angular momentum)
Like charges repel and unlike charges attract
one another
Nature’s basic carrier of positive charge is the
proton
„
More Properties of Charge
„
Nature’s basic carrier of negative
charge is the electron
„
„
Gaining or losing electrons is how an
object becomes charged
Electric charge is always conserved
„
„
Charge is not created, only exchanged
Objects become charged because negative
charge is transferred from one object to
another
They are called positive and negative
Named by Benjamin Franklin
Protons do not move from one material to another
because they are held firmly in the nucleus
Properties of Charge
„
Charge is quantized
„
All charge is a multiple of a fundamental
unit of charge, symbolized by e
„
„
„
„
Quarks are the exception
Electrons have a charge of –e
Protons have a charge of +e
The SI unit of charge is the Coulomb (C)
„
e = 1.6 x 10-19 C
Question: A negative electric charge
Question: Electric charge
(a)
(b)
(c)
(d)
is a continuous quantity that can be
subdivided indefinitely
is a continuous quantity but it cannot be
subdivided into smaller parcels than 1.6x1019 C
occurs only in separate parcels, each of
1.6x10-19 C
occurs only in separate parcels, each of 1 C
Answer: c
(b)
(c)
(d)
(b)
(c)
(d)
interacts only with positive charges
interacts only with negative charges
interacts with both positive and
negative charges
may interact with either positive and
negative charges, depending on
circumstances
Answer: c
Question: An object has a positive electric
(a)
(a)
charge whenever
it has an excess of electrons
it has a deficiency of electrons
the nuclei of its atoms are positively
charged
the electrons of its atoms are
positively charged
Conductors
„
Conductors are materials in which the
electric charges move freely
„
„
Answer: b
Copper, aluminum and silver are good
conductors
When a conductor is charged in a small
region, the charge readily distributes itself
over the entire surface of the material
Metal: dρ/dT>0
Insulators
0.5
CaRuO
ab-plane
3
0.4
„
SrRuO
0.3
3
Insulators are materials in which
electric charges do not move freely
„
0.2
Mott-Ioffe-Regel lim it
0.1
ρ ~T
RuO
2
„
2
0
0
200
400
600
T(K)
800
1000
Glass and rubber are examples of
insulators
When insulators are charged by rubbing,
only the rubbed area becomes charged
„
There is no tendency for the charge to move
into other regions of the material
Semiconductors
Insulator: dρ/dT<0
109
Ca RuO
2
4
„
10
7
105
„
1000
The characteristics of semiconductors
are between those of insulators and
conductors
Silicon and germanium are examples of
semiconductors
10
0.1
50
100
150
200
250
300
T (K)
Polarization
Charging by Induction
Charging by Rubbing
Coulomb’s Law
F21+q 2
+q 1 F
12
r12
qq
F12 = ke 1 22
r12
Coulomb’s Law, cont.
„
ke is called the Coulomb Constant
„
„
Typical charges can be in the µC range
„
„
ke = 8.99 x 109 N m2/C2
Remember, Coulombs must be used in the
equation
Remember that force is a vector
quantity
Example:
Coulomb force
QQ
F 12 = ke 1 22
r12
Gravitational force
F 12 = G
The hydrogen atom has the simplest structure of all atom, consisting
of a proton and an electron whose average separation is 5.3x10-11 m.
The mass of electron and proton is 9.1x10-31 kg, 1.67x10-27 kg,
respectively
m1m2
r122
Fe=kQeQp/r2=(9.0x109 N•m/C2)(1.6x10-19 C2)/(5.3x10-11 m)2
But electric forces may be either attractive or repulsive,
whereas gravitational forces are always attractive.
That is why matter in the universe tends to come together to
form large bodies, these bodies are always found in groups,
such as galaxies of stars and families of planets.
On an atomic scale, electricity is much more important than gravity
=8.2x10-8
N
F
2
g=Gmemp/r
=(6.7x10-11 N•m/kg2)(9.11x10-31 kg)(1.67x10-27 kg)/(5.3x10-11 m)2
=3.7x10-47 N
me=9.11x10-31 kg, mp=1.673x10-27 kg
The electric force is over 1039 times greater than the gravitational force
16_11.jpg
Question:
Four point charges, each of the same magnitude,
with varying signs are arranged at the corners of a
square as shown. Which of the arrows labeled A, B,
C, and D gives the correct direction of the net force
that acts on the charge at the upper right corner?
a A
b. B
c. C
d. D
e. The net force on that charge is zero
Answer: b
QUICK QUIZ 15.2
Object A has a charge of +2 µC, and
object B has a charge of +6 µC.
Which statement is true:
(a) FAB = –3FBA, (b) FAB = –FBA, or
(c) 3FAB = –FBA
Answer (b)
Electric charge always occurs in multiples of e
Q = ne
(n =±0、1、2、3…)
e = 1.60
× 10
19
C
Question:
A conducting sphere has a net charge of −4.8x10−17 C.
What is the approximate number of excess electrons on
the sphere?
(a) 100
(c) 300
(b) 200
(d) 400
(e) 500
Answer: c (-4.8x10-17 C/-1.6x10-19 C=300 electrons)
Question:Two charges of +Q are 1 cm
apart. If one of the charges is replaced
by a charge of –Q, the magnitude of
the force between them is
(a) zero
(b) smaller
(c) the same
(d) larger
Answer: c
Question: A charge of +q is placed 2 cm from a
charge of –Q. A second charge of +q is then
placed next to the first. The force on the
charge of –Q
(a) decreases to half its former magnitude
(b) remains the same
(c) increases to twice its former magnitude
(d) increases to four times its former magnitude
Answer: c
Question: Two charges, one positive and the
other negative, are initially 2 cm apart and
are then pulled away from each other until
they are 6 cm apart. The force between
them is now smaller by a factor of
(a) 9
(b) 3
(c) 27
(d) √3
Answer: a
Question:
Two positive point charges Q and 2Q are separated by a
distance R. If the charge Q experiences a force of
magnitude F when the separation is R, what is the
magnitude of the force on the charge 2Q when the
separation is 2R ?
(a) F/4
(c) F
(b) F/2
(d) 2F
(e) 4F
Answer: a
Outline for Today
„
„
Electric Field
Electric Field Lines
Electric Field, cont.
Electric Field
„
„
Maxwell developed an approach to
discussing fields
An electric field is said to exist in the
region of space around a charged
object
„
„
„
When another charged object enters this
electric field, the field exerts a force on the
second charged object
Electric Field
„
„
„
„
Mathematically, E =
Direction of Electric Field
F k eQ
= 2
qo
r
Use this for the magnitude of the field
The electric field is a vector quantity
The direction of the field is defined to
be the direction of the electric force
that would be exerted on a small
positive test charge placed at that point
Direction of Electric Field, cont
„
The electric field
produced by a
positive charge is
directed away from
the charge
„
A positive test
charge would be
repelled from the
positive source
charge
A charged particle,
with charge Q,
produces an electric
field in the region of
space around it
A small test charge,
qo, placed in the
field, will experience
a force
„
The electric field
produced by a
negative charge is
directed toward the
charge
„
A positive test
charge would be
attracted to the
negative source
charge
Electric Field Lines
„
„
A convenient aid for visualizing electric
field patterns is to draw lines pointing in
the direction of the field vector at any
point
These are called electric field lines and
were introduced by Michael Faraday
Electric Field Lines, cont.
„
The field lines are related to the field by
„
„
The electric field vector, E, is tangent to
the electric field lines at each point
The number of lines per unit area through
a surface perpendicular to the lines is
proportional to the strength of the electric
field in a given region
Electric Field Line Patterns
„
For a negative
source charge, the
lines will point
inward
Electric Field Line Patterns
„
„
„
Electric Field Line Patterns
„
„
Electric Field Line Patterns
„
„
„
„
Two equal but like point
charges
At a great distance from the
charges, the field would be
approximately that of a
single charge of 2q
The bulging out of the field
lines between the charges
indicates the repulsion
between the charges
The low field lines between
the charges indicates a weak
field in this region
Point charge
The lines radiate
equally in all
directions
For a positive source
charge, the lines will
radiate outward
An electric dipole
consists of two
equal and opposite
charges
The high density of
lines between the
charges indicates
the strong electric
field in this region
Electric Field Patterns
„
„
Unequal and unlike
charges
Note that two lines
leave the +2q
charge for each line
that terminates on
-q
16_22.jpg
Question
B
A
+
C
+
If a positive test charge is placed at each of the three points,
which point experiences the strongest electric field?
And which point experience the weakest electric field?
Answer: Strongest at A, weaker at B and zero at C.
Question:
16_26.jpg
Two particles of the same mass carry charges +3Q and –2Q,
respectively. They are shot into a region that contains a uniform
electric field as shown. The particles have the same initial velocities
in the positive x direction. The lines, numbered 1 through 5, indicate
possible paths for the particles. If the electric field points in the
negative y direction, what will be the resulting paths for these
particles?
(a) path 1 for +3Q and path 4 for –2Q
Answer: e
(b) path 3 for +3Q and path 2 for –2Q
(c) path 4 for +3Q and path 3 for –2Q
(d) path 2 for +3Q and path 5 for –2Q
(e) path 5 for +3Q and path 2 for –2Q
Question:
An electron traveling horizontally enters a region where a uniform
electric field is directed upward.What is the direction of the force
exerted on the electron once it has entered the field?
QUICK QUIZ 15.5
(a) to the left
(b) to the right
(c) upward
(d) Downward
(e) out of the page, toward the reader
Answer: d
A circular ring of radius b has a total
charge q uniformly distributed around
it. The magnitude of the electric field
at the center of the ring is
(a) 0
(b) keq/b2 (c) keq2/b2
(d) keq2/b
(e) none of these.
QUICK QUIZ 15.5 ANSWER
(a). If a test charge is at the center of
the ring, the force exerted on the test
charge by charge on any small segment
of the ring will be balanced by the force
exerted by charge on the diametrically
opposite segment of the ring. The net
force on the test charge, and hence the
electric field at this location, must then
be zero.
QUICK QUIZ 15.6
A "free" electron and "free" proton are placed in an identical
electric field. Which of the following statements are true? (a)
Each particle experiences the same electric force and the
same acceleration. (b) The electric force on the proton is
greater in magnitude than the force on the electron but in the
opposite direction. (c) The electric force on the proton is
equal in magnitude to the force on the electron, but in the
opposite direction. (d) The magnitude of the acceleration of
the electron is greater than that of the proton. (e) Both
particles experience the same acceleration.
QUICK QUIZ 15.6 ANSWER
(c) and (d). The electron and the proton have
equal magnitude charges of opposite signs.
The forces exerted on these particles by the
electric field have equal magnitude and
opposite directions. The electron experiences
an acceleration of greater magnitude than
does the proton because the electron’s mass
is much smaller than that of the proton.
The electric field inside a good
conductor is zero in the static
situation
Any excess charge resides on the
surface of the conductor
Conductors in Electric Field
In a conductor electrons are free to move. If a conductor is placed into E, a
force F = -eE acts on each free electron. Soon electrons will pile up on the
surface on one side of the conductor, while the surface on the other side will
be depleted of electrons and have a net positive charge. These separated
negative and positive charges on opposing sides of the conductor produce
their own electric field, which opposes the external field inside the conductor
and modifies the field outside.
Why?
Electrons inside the conductor experience no force.
A cavity inside a conductor also is free of electric fields
A conductor shields its interior from any outside electric fields.
Even if there are holes in the surface, the electric field does not penetrate
very far. A rule of thumb is that the electric field falls to zero over a
distance approximately equal to the diameter of the hole.
Question
Why are you safest inside
your car during a
thunderstorm?
Answer: E=0 inside the car
In the static situation a conductor has the
following properties:
¾Any excess charge resides on the surface of the conductor
¾The electric field is zero within the solid part of the conductor
¾The electric field at the surface of the conductor is
perpendicular to the surface
¾ Charge accumulates and the field is strongest on pointy parts
of the conductor
Using Static Electricity
Photocopiers,
Paint Sprayers,
Air ionizers