Download chap5_electricityandmagnetism

Chapter 5
Electricity and Magnetism
http://www.youtube.com/watch?v=6mXrsMH_LYM&feature=related
http://homepages.ius.edu/DTRAUGHB/Units/unit%2014/unit14.pdf Coulombs Law
http://www.youtube.com/watch?v=QxZ6AWLpnUw&feature=related demos
http://www.youtube.com/watch?v=Dz_vvw_fsTo&feature=related fast demo
http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-andMagnetismSpring2002/VideoAndCaptions/detail/embed02.htm MIT Lecture
http://www.youtube.com/watch?v=pJ36EtABLAk&feature=related Electr Induction
http://www.youtube.com/watch?v=F1p3fgbDnkY&NR=1 Potential difference
http://www.youtube.com/watch?v=YGvu9iqjJq4&feature=related Resistance
http://www.youtube.com/watch?v=jIKnti0H_LA&feature=related Electricity and matter
6mXrsMH_LYM.3gp
1
Electricity and Magnetism
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
OUTLINE
Electric Charge
5.1 Positive and Negative Charge
5.2 What Is Charge?
5.3 Coulomb’s Law
5.4 Force on an Uncharged Object
Electricity and Matter
5.5 Matter in Bulk
5.6 Conductors and Insulators
5.7. Superconductivity
Electric Current
5.8 The Ampere
5.9 Potential Difference
5.10 Ohm’s Law
5.11 Electric Power
•
•
•
•
•
•
•
•
•
•
Magnetism
5.12 Magnets
5.13 Magnetic Field
5.14 Oersted’s Experiment
5.15 Electromagnets
Using Magnetism
5.16 Magnetic Force on a Current
5.17 Electric Motors
5.18 Electromagnetic Induction
5.19 Transformers
2
Goals
•
•
•
•
•
•
•
•
1. Discuss what is meant by electric
charge.
2. Describe the structure of an atom.
3. State Coulomb's law for electric force
and compare it with Newton's law of
gravity.
4. Account for the attraction between a
charged object and an uncharged one.
5. Distinguish among conductors,
semiconductors, and insulators.
6. Define ion and give several ways of
producing ionization.
7. Define superconductivity and discuss its
potential importance.
8. Describe electric current and potential
difference (voltage) by analogy with the
flow of water in a pipe.
•
•
•
•
•
•
•
•
•
•
9. Use Ohm's law to solve problems that involve the
current in a circuit, the resistance of the circuit, and the
voltage across the circuit.
10. Relate the power consumed by an electrical appliance
to the current in it and the voltage across it.
11. Describe what is meant by a magnetic field and discuss
how it can be pictured by field lines.
12. State the connection between electric charges and
magnetic fields.
13. Use the right-hand rule to find the direction of the
magnetic field around an electric current.
14. Explain how an electromagnet works.
15. Describe the force a magnetic field exerts on an
electric current.
16. Discuss the operation of an electric motor.
17. Describe electromagnetic induction and explain how a
generator makes use of it to produce an electri current.
18. Explain how a transformer changes the voltage of an
alternating current and why this is useful.
3
Amber with Insects
40 to 60 Million years old
4
What is Electricity??
• These are some of the questions we will
answer in this unit.
– Why can you feel a spark when you grab a
doorknob after scuffing your feet along carpet?
– What is the difference between current and
voltage-and which shocks us?
– How do circuits work?
– Is electricity related to magnetism- if so, how?
• Let’s start with the basic idea of CHARGE.
5
Electric Charge
• Two types of charges exist
 Positive
 Negative
• The terms positive and negative refer to electric
charge, the fundamental quantity underlying all
electric phenomena.
• Like charges repel one another
• Unlike charges attract one another
6
Force on an Uncharged Object
• One sign that an object has a charge is that it causes small,
uncharged objects such as dust particles, bits of paper, a
suspended plastic ball to move toward it.
• Where does the force come from????
– Electrons in solids have some freedom to move.
• In a metal this freedom is considerable
• In other substances the electrons can shift a little
• A charged object attracts an uncharged one by first causing a
separation of charge, or polarization, in the latter one.
• Only a small charge separation occurs so only very light things
can be picked up.
7
Electric Charge
The terms positive and negative refer to
electric charge, the fundamental quantity
underlying all electric phenomena.
Every atom is composed of a positively
charged nucleus surrounded by negatively
charged electrons.
When an atom loses an electron, it becomes a
positive ion (cation).
When an atom gains an electron, it becomes a
negative ion (anion).
8
What about those electrons?
• Innermost electrons in an atom are attracted
very strongly to the oppositely charged
nucleus.
• Outer most electrons are attracted more
loosely and can be dislodged.
ee-
e-
e9
Conservation of Charge
A basic fact of electricity – whenever
something is charged, no electrons are
created or destroyed, they simply transfer
from one material to another!
Charge is CONSERVED, just like energy and
momentum were.
NEVER HAS THERE BEEN A CASE WHERE THE
CREATION OR DESTRUCTION OF CHARGE HAS
BEEN FOUND.
10
Electric Charge
Figure 5-1
Charging by Friction
When the rubber
rod is touched
against the plastic
ball, some of the
negative charge
flows to the ball.
11
Charge Polarization
• Inflate a balloon, rub your hair (charging by friction), and then place the
balloon against a wall (charging by induction).
• It STICKS.
• Why?
– Because the charge on the balloon alters the charge distribution in the
atoms or molecules in the wall (effectively inducing an opposite
charge on the wall).
• How?
– The molecules do not move from their relatively stationary positions,
but their “centers of charge” move.
– The positive part of the atom or molecule is attracted toward the
balloon and the negative part is repelled.
– The atom or molecule is said to be “polarized.”
12
Charging by Induction
13
Charge Polarization
14
Electric Charge
• Benjamin Franklin (1706 – 1790) suggested
these positive and negative charges.
• He referred to a rubber rod, which had
been rubbed across a piece of fur as having
a negative charge.
• A glass rod rubbed with a piece of silk as
having a positive charge.
• An object whose positive and negative
charges exactly balance out is said to be
electrically neutral.
15
Electric Charge
• Whenever electric charge is produced by
contact between 2 objects of different
materials, one of them ends up with a positive
charge and the other a negative charge.
• Which is which depends on the particular
materials used.
• Electrons are neither gain nor lost, but
exchanged
16
The Triboelectric Series
Some materials create more static electricity
than others.
– Static electricity is the collection of electrically
charged particles on the surface of a material.
Various materials have a tendency to either
GIVE UP or ATTRACT electrons.
The Triboelectric Series is a list of materials
showing which have a greater tendency to
become (+) and which have a greater
tendency to become (-).
17
The Triboelectric Series
Donates Electrons
Material
Dry Human Skin
Increasing to decreasing tendency to GIVE UP electrons and
become positively (+) charged
Greatest tendency to giving up electrons and
becoming highly positive (+) in charge.
Leather
+
Rabbit fur
Fur is used to create static electricity
Glass
The glass on TV screens gets charged and
collects dust.
Human hair
“flyaway hair” is a good example of a substance
with moderate (+) charge.
Nylon
Wool
Lead
Unusual that lead would collect as much static
as cat fur.
Cat fur
Silk
Aluminum
Paper
Gives up some electrons
18
+
The Triboelectric Series
Receives Electrons
Material
Teflon
Increasing to decreasing tendency to ATTRACT
electrons and become NEGATIVELY (-) charged
Greatest tendency of gathering electrons
on its surface and becoming highly (-) in
charge.
-
Silicon
Vinyl (PVC)
Many electrons will collect on PVC surfaces
Polypropylene
Polyethylene (like Scotch Tape)
Pull tape off surface and it will become
charged
Polyurethane
Saran Wrap
Likes to stick to things
Styrene (styrofoam)
Packing peanuts stick to everything
Polyester
Static cling (remember the 70’s?)
Gold, Platinum
A little surprising that these metals attract
electrons almost as much as polyester
Brass, Silver
Nickel, Copper
Hard rubber
Amber
Combs
-
19
The Triboelectric Series
“A handy little tool”
Material
Neutral
Cotton
Best for non-static
clothes.
Steel
Not useful for static
electricity.
20
The Triboelectric Series
“A handy little tool”
• The best combo of materials to create static
electricity would be one from the positive list
and one from the negative list.
21
Check your Understanding!
1. What happens to a material that collects
electrons on its surface?
a) It has a negative charge
b) It has a positive charge
22
Check your Understanding!
1. Rubbing which materials together would
produce the most static electricity?
a) Leather and Teflon
b) Dry skin and cat fur
c) Wood and paper
23
Check your Understanding!
1. If you combed your hair with a plastic comb,
which would give up its electrons?
a) Your hair
b) The comb
GOOD JOB!
24
Atoms
• Every substance is composed of tiny bits of
matter called atoms.
• Three elementary particles compose atoms:
– Proton
– Neutron
– Electron
25
Three Elementary Particles
• Proton – which has a mass of 1.673 x 10-27 kg
and is positively charged
• Neutron – which has a mass of 1.675 x 10-27 kg
and is uncharged
• Electron – which has a mass of 9.11 x 10-31 kg
and is negatively charged
26
Atoms
Every atom has a
small, central
nucleus comprised
of protons and
neutrons with its
electrons moving
about the nucleus
some distance
away.
Figure 5-5
27
Atoms
• Different types of atoms have different
combinations of protons and neutrons in their
nuclei.
• The electrons in an atom are normally equal in
number to the protons, so the atom is
electrically neutral unless disturbed in some
way.
28
Electric Charge
• The unit of electric charge is
the coulomb (C).
– The proton has a charge of
+1.6 x 10-19 C.
– The electron has a charge of
-1.6 x 10-19 C.
• This basic quantity of charge is
referred to as e and is equal to
1.6 x 10-19 C.
29
Coulomb’s Law
• So what affects the force between two
charged objects?
• According to Charles Coulomb (1736-1806),
the force between a charged rod and a
charged plastic ball depends on two things:
– How close the rod is to the ball
– How much charge each one has
30
Relationship between Distance and Amount of
Charge on the Force Between Two Objects
Figure 5-7
Tam5s6_6
31
Coulomb’s Law
The force between
two charges varies
inversely as the
square of their
separation;
increasing distance
reduces the force.
The force is also
proportional to the
product of the
charges.
Figure 5-8
32
Coulomb’s Law
• F = Electric Force
• K = 9 x 109 N m2/C2
(electric force constant)
• Q = magnitude of
charges
• R = distance between
charges
Q1Q2
FK 2
R
33
Electricity and Matter
• Coulomb’s law resembles Newton’s law of gravity with the
difference that gravitational forces are always attractive BUT
electric forces may be either attractive or repulsive.
• On a cosmic scale gravitational forces are significant and
electric ones are not.
• On an atomic scale the reverse is true.
– The masses of subatomic particles are too small for them to
interact gravitationally to any appreciable extent.
– Whereas their electric charges are enough for electric forces to
exert a large effect.
34
Hydrogen Atom
Gravity, what a wimp!
radius
The electric force for the hydrogen atom is 1039
times greater than the gravitational force.
35
Conductor
• A substance through which electric charge can
flow readily
– In metals, each atom gives up one or more
electrons to a “gas” of electrons that can move
relatively freely inside the metal.
36
Insulator
• A substance through which electric charge can
flow only with great difficulty
– The electrons in a nonmetallic solid are bound
to particular atoms or groups of atoms.
37
Semiconductors
• Between conductors and insulators in their
ability to let charge move through them
• Semiconductors have made possible the
devices called transistors, whose ability to
transmit charge can be changed at will.
– Phones, televisions, radios, computers
38
39
Ions
Conduction of electricity through gases and
liquids involving the movement of charged
atoms and molecules called ions.
– An atom or molecule gains a positive charge when
it loses an electron (called a positive ion or cation).
– An atom or molecule gains a negative charge when
extra electrons beyond its normal number become
attached to it (called a negative ion or anion).
40
Ionization
• The process of forming ions is referred to as
ionization.
• Ionization of a gas such as ordinary air can
occur in a number of ways.
– Zapping air with x-rays, uv light or radiation from a
radioactive material pass thru it, when an electric
spark is produced, or when a flame burns in it
41
Ionization of Air
Figure 5-12
Tam5s6_10
42
Superconductivity
• Even the best conductors resist to some
degree the flow of charge thru them at
ordinary temperatures.
• At very low temperatures, a substance loses
all electrical resistance and produces the
phenomenon called superconductivity
(Kamerlingh Onnes, Netherlands, 1911).
• Aluminum is a superconductor at -272 C.
43
Superconductivity
The disadvantage to superconductivity is the
difficulty and expense associated with
reaching a very low temperature.
The advantage to superconductivity is that
during long distance or large current
transmission of electricity, energy can be lost
as heat (about 10%).
A room temperature superconductor would
revolutionize the world’s technology.
44
Electric Current
• The flow of charge from one place
to another
• The completion of a conducting
path is called a circuit.
• The unit of electric current is
called the ampere (A) (Andre
Ampere-French physicist).
• An Ampere (A) is equal to 1
coulomb/second (1A=1C/s).
Q
I
t
I = electric current (Amps)
Q = charge transferred
(Coulomb)
t = time (seconds)
45
The Ampere
Figure 5-13
Tam5s6_11
46
Potential Difference or Voltage
• Voltage between two points is the work required to take a
charge of 1 C from one point to another.
– A coulomb of negative charge on the (–) terminal of a battery is
repelled by the (–) terminal and attracted by the (+) terminal,
and has a certain amount of PE.
– When the Coulomb of charge has moved to the (+) terminal, the
PE is gone.
– The decrease in PE as the charge moves from the (-) to the (+)
terminal is the potential difference between the two terminals.
• The unit of potential difference is the volt (V) and is equal to
1J/C.
47
Potential Difference
Tam5s6_12
Figure 5-14
48
Voltage Examples
The normal potential
difference (voltage)
between the terminals of a
car’s storage battery is
about 12V and 1.5V in a
dry cell (flashlight
batteries).
Potential difference inside
a cloud can be millions of
volts resulting in electric
discharges
Figure 5-18
49
Ohm’s Law
• When different voltages are applied to the ends of the same
piece of wire, the current in the wire is proportional to the
potential difference (voltage).
• In other words, doubling voltage doubles the current.
• This is Ohm’s Law after Georg Ohm 1787-1854.
• Ohm’s law is not a basic physical principle and is obeyed only
by metallic conductors, not gaseous or liquid conductors or
by electronic devices such as transistors.
50
Ohm’s Law
V
I
R
• I = Current
• V = Voltage
• R = Resistance
51
Resistance
• Resistance is the property of a conductor that
opposes the flow of charge.
• Can be thought of as a kind of friction
• The unit of resistance is the Ohm, whose
abbreviation is .
• Substituting the SI units in Ohm’s Law (I=V/R)
– A = V/ and solving for ,  = V/A
52
Resistance
• The resistance of a wire or other metallic
conductor depends on:
– Material – what it is made of
• iron wire has 7 times the resistance than copper wire
– Length
• Longer the wire, the more its resistance
– Cross sectional area
• Greater area, less resistance
– Temperature
• Higher temperature, more resistance
53
Factors Affecting Resistance
Figure 5-20
Tam5s6_15
54
voltage, amperage, and resistance
55
Electric Power
• Electric energy is useful because it is carried
easily by wires and is easily converted into
other forms of energy.
• Electric energy in the form of electric current
becomes:
– Radiant energy in a lightbulb
– Chemical energy when a storage battery is charged
– Kinetic energy in an electric motor
– Heat in an electric oven
56
Electric Current
• In each of the previous examples, current
performs work on the device it passes
through, and the device then turns this work
into another kind of energy.
57
Electric Power
• Electric current is the rate at which a
current is DOING WORK---in other words,
the POWER of the current.
• P = IV Electric Power
– P = power
– I = current
– V = voltage
58
Problem
• A thick copper wire is connected to a Voltage
and current flows.
• The Wire is replaced with a longer wire of the
same thickness
• What happens to the current?
• What happens to the heat in the wire?
59
Problem
• The resistance of the longer wire is higher
• Consequently, the current flow is less
according the the % increase of resistance
• The heat in the wire is the Power loss
• P = I x I x R so lets say R is 10% higher
• Then I is 10% lower so P= .9x.9x1.1 =89% of
the shorter wire
60
61
Magnetism
• Electricity and magnetism were once
considered completely separate phenomena.
– One of the great achievements of the 19th century
was the realization that they were closely related.
– This realization led to the discovery of the
electromagnetic nature of light which allowed for
the invention of the electric motor and generator.
62
Magnetism
• Like magnetic poles
repel one another, and
unlike poles attract
one another
– Poles always come in
pairs.
– There is no such thing
as a single free
magnetic pole (blue
box, page 175).
Figure 5-26
63
There is no such thing as a single pole;
Cutting a magnet in half produces 2 other magnets
Tam5s6_19
Figure 5-27
64
Magnetism
We can conclude
that since a magnet
can be cut
indefinitely into
smaller and smaller
pieces, magnetism
is a property of the
IRON ATOMS
themselves.
Unmagnetized
Magnetized
65
Magnetism
A permanent magnet can be demagnetized in
one of 2 ways:
– Heating it strongly
– Hammering it
Iron, nickel, cobalt, and certain alloys can all
be magnetized.
All substances are affected by magnetism to
varying degrees (generally only very slightly).
– Some are attracted; most are repelled.
66
Magnetic Field
• Gravitational, electric and magnetic forces are unique and
remarkable in the sense that these forces act WITHOUT THE
OBJECTS INVOLVED TOUCHING EACH OTHER!
• The properties of space around a magnet are altered by the
magnet’s presence just as is the space around a mass or
electric charge are, although in different ways.
• The altered space around a mass, an electric charge, or a
magnet is called a Force Field.
• Physicists describe a force field in terms of what it does; which
is exert a force on appropriate objects.
67
Magnetic Field
• We cannot see a force field, but we can detect
its presence by its effects.
• It is traditional and convenient to think of a
magnetic field in terms of field lines.
– Field lines are imaginary lines that correspond to
the patterns formed by iron filings.
68
Direction of Magnetic Field
S
S
69
Oersted’s Experiment
• Hans Christian Oersted (1777-1851) discovered that electric
currents have magnetic fields around them.
• Magnetism and electricity are related only through moving
charges.
– An electric charge at rest has no magnetic properties.
• The direction of the magnetic field around a wire can be
found by encircling the wire with the fingers of the right hand
so that the extended thumb points along the wire in the
direction of the current.
– This is known as the right hand rule and says the current and
field directions are perpendicular to each other.
70
Oersted’s Experiment –
A Magnetic Field Surrounds
Every Electric Current
Figure 5-31
The field direction above the wire is opposite to that below the wire.
Tam5s6_22
71
The Right Hand Rule
Figure 5-32
Tam5s7_23
72
The Electromagnetic Field
• The proper way to regard the separate electric
and magnetic fields is that they are both
aspects of a single electromagnetic field that
surrounds every electric charge.
– The electric field is always present, but the
magnetic field appears only when relative motion
is present.
73
Electromagnets
How to Create a Strong Magnetic Field
• Several wires carrying current in the same
direction are placed side by side.
• Their magnetic fields ADD together to give a
stronger total magnetic field.
– A coil with 50 turns produces a field 50 times
greater than a coil with one turn.
• An electromagnet is created when a ROD of
IRON is placed inside a coil of wire.
– The iron rod greatly increases the magnetic field
of the coil.
74
The magnetic field of a coil is like
that of a single loop but stronger.
Tam5s7_25
Figure 5-34
75
An electromagnet consists of a coil with IRON CORE
which enhances the magnetic field produced.
Tam5s7_26
Figure 5-35
76
Using Magnetism
• An electric motor uses magnetic fields to turn
electric energy into mechanical energy.
• A generator uses magnetic fields to turn
mechanical energy into electric energy.
77
Magnetic Force on a Current
Oersted’s Experiment In Reverse
• What if a horizontal wire connected to a battery is suspended as in Figure
5-37, so it is free to move from side to side?
• Then place the N pole of a bar magnet directly under it.
• Based on Oersted’s results and Newton’s third law of motion, the wire
should move.
– The direction of the wire’s motion is perpendicular to the bar magnet’s
field.
– Thus the force a magnetic field exerts on an electric current is not a
simple attraction or repulsion but a sidewise push; the maximum push
occurs when the current is perpendicular to the magnetic field
– At other angles the push is less.
– The push disappears when the current is parallel to the magnetic field.
78
Oersted’s Experiment in Reverse
The Sidewise Push
Tam5s7_27
Figure 5-37
79
Electric Motors
• Mechanical energy from electric energy
– The sidewise push of a magnetic field on a
current-carrying wire can be used to produce
continuous motion.
– A magnet gives rise to a magnetic field inside
which a wire loop is free to turn.
– To produce a continuous movement, the direction
of the current in the loop must be reversed when
the loop is vertical.
• A device that automatically changes the current
direction is called a commutator
80
Simple DC Motor
(electromagnet)
(permanent magnet)
81
Electromagnetic Induction
• The electric energy our homes and industries use
comes from generators driven by turbines
powered by running water or steam.
• The principle of the generator was discovered by
the 19th century English physicist Michael Faraday
(1791-1867).
• Faraday reasoned that if a current can produce a
magnetic field, then a magnet should be able to
generate an electric current.
82
Electromagnetic Induction
• What Faraday found instead was a current is
produced in a wire when there is relative
motion between the wire and a magnetic
field.
• Because current is produced by motion
through a magnetic field, this sort of current is
called induced current and the entire effect is
known as electromagnetic induction.
83
Electromagnetic Induction
The direction of induced current is
perpendicular to both the magnetic lines of
force and to direction in which the wire is
moving.
No current is induced when the wire is at rest.
84
Alternating and Direct Currents
• An alternating current (ac) occurs when the
induced current flows first one way and then
the other.
• This is how a generator works.
Alternating Current Generator
Figure 5-44
85
Alternating and Direct Currents
• A direct current (dc) comes from only one way
and can be reversed only by changing the
connections.
• Batteries and photoelectric cells are examples.
86
Transformers
Transformers step up or step down voltage.
A transformer works when an alternating
current in one coil of wire induces an
alternating current in another nearby coil.
Depending on the ratio of turns of the coils,
the induced current can have a voltage that is
larger, smaller, or the same as that of the
primary current.
87
A Simple Transformer
Tam5s7_13
Figure 5-48
88
Transformers
• Transformers are useful because the voltage of
the induced current can be raised or lowered
by adjusting the windings of the coils.
• Transformers are also important because they
permit the efficient long-distance transmission
of power - P = IV.
89
The winding with the greater
number of turns has the higher voltage across it and
carries the lower current.
Power is the same in both windings.
P2=IV
P1=IV
Higher voltage
because the
winding has a
greater number
of turns.
P1=P2
Lower voltage
because the
Winding has a
fewer number
of turns.
Lower current.
Higher current.
Tam5s7_14
Figure 5-49
90
Transformer Equation
Primary Turns
Primary Voltage
Secondary Current


Secondary Turns Secondary Voltage
Primary Current
N1 V1 I 2


N 2 V2 I1
91
The End
92