An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive charge and the paint droplets are given a negative charge. The paint droplets experience an attractive - PowerPoint PPT Presentation

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An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive charge and the paint droplets are given a negative charge. The paint droplets experience an attractive

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Title: An electrostatic paint sprayer is used to spray paint evenly onto the surface of a car. Before the paint is sprayed, the car body is given a positive charge and the paint droplets are given a negative charge. The paint droplets experience an attractive


1
  • An electrostatic paint sprayer is used to spray
    paint evenly onto the surface of a car. Before
    the paint is sprayed, the car body is given a
    positive charge and the paint droplets are given
    a negative charge. The paint droplets experience
    an attractive force as soon as they are released
    from the sprayer. The paint droplets are
    originally sprayed at a distance of 30 cm from
    the car body, as shown below.
  • Which of the following changes will cause the
    largest increase in the attractive force on the
    paint droplets?
  • The charge on the car body is doubled.  
  • The charge on the paint droplets is doubled.  
  • C. The distance between the paint droplets
    and the car body is halved.  
  • D. The distance between the paint droplets
    and the car body is doubled. 

2
  • The tendency of a stationary object to resist
    being put into motion is known as
  •   A. acceleration.
  •   B. inertia.  
  • C. weight.
  •   D. velocity.

3
In a copper wire, a temperature increase is the
result of which of the following?  
  • In a copper wire, a temperature increase is the
    result of which of the following?
  • A. an increase in the size of the copper
    particles  
  • B. a decrease in the mass of the copper particles
     
  • C. an increase in the motion of the copper
    particles  
  • D. a decrease in the distance between the copper
    particles 

4
  • Two oppositely charged objects are separated by a
    small distance. The objects are then moved three
    times farther apart from each other.
  • Which of the following statements best describes
    what happens to the electrical force between the
    objects?
  •   A. The force of attraction increases.
  •   B. The force of attraction decreases.  
  • C. The force of attraction becomeszero.  
  • D. The force of attraction stays thesame. 

5
Draw a circuit with a battery, a switch and a
bulb and resistor in series.Draw a circuit with
a battery, a switch and a bulb and resistor in
parallel.
6
A circuit is wired with a power supply, a
resistor and an ammeter (for measuring current).
The ammeter reads a current of 24 mA
(milliAmps). Determine the new current if the
voltage of the power supply was ... a. ...
increased by a factor of 2 and the resistance was
held constant. b. ... increased by a factor of 3
and the resistance was held constant. c. ...
decreased by a factor of 2 and the resistance was
held constant. d. ... held constant and the
resistance was increased by a factor of 2. e. ...
held constant and the resistance was increased by
a factor of 4. f. ... held constant and the
resistance was decreased by a factor of 2. g. ...
increased by a factor of 2 and the resistance was
increased by a factor of 2. h. ... increased by a
factor of 3 and the resistance was decreased by a
factor of 2. i. ... decreased by a factor of 2
and the resistance was increased by a factor of 2
v
7
Focus
  1. How is a positive ion made?
  2. What type of current is made with a generator?
    Battery?
  3. What happens to the current if the voltage is
    doubled?
  4. What happens to the current if the resistance is
    doubled?
  5. What law is used to answer questions 3 and 4?
  6. What is a short circuit?
  7. Why would it not be a good idea to replace a
    circuit breaker which is rated for 20 amps with
    one rated for 30amps?

8
Magnetism
9
Magnetic Forces
  • Magnetic forces, like electrical forces, are
    forces that act at a distance. (Objects do not
    have to be in contact.)
  • Like poles repel each other opposite poles
    attract.

10
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13
Magnetic field Region where a magnetic
influence (force) can be felt. Lines are drawn
to represent the strength and direction of the
field. Field is represented from N to S.
14
FOCUS Select your answer a or b and explain why.
15
What can one do to strengthen the force from a
bar magnet? What would happen if you broke a
bar magnet in half?
16
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17
Learning Goals
  • SW describe the domain theory of magnetism.
  • SW recognize the relationship between electricity
    and magnetism

18
Magnetic Poles
  • Northseeking poles- point northward.
  • South-seeking poles point southward.
  • Important difference between electric charges and
    magnetic poles is that electric charges can be
    isolated, but magnetic poles cannot.

19
Domain Theory
  • All magnetism is due to the motion of electrons.
  • Electrons spin, like the earth, setting up
    magnetic poles. (paramagnetism)
  • Electrons revolve around the nucleus, like the
    earth around the sun. (diamagnetism)
  • Electrons are paired in orbitals. Paired
    electrons spin opposite directions- creating
    opposite magnets. The magnetic poles therefore
    cancel each other. Unpaired electrons create
    stronger magnets. (ferromagnetism)

20
  • Magnetic domains- clusters of aligned atoms

Magnetic Saturation all the domains are
aligned Dropping a magnet can cause the domains
to become unaligned.
21
  • Concept check 1 REVIEW
  • Must every magnet have a north and south pole?
    Explain.
  • How can a magnet attract a piece of iron that is
    not magnetized?
  • Why will a magnet not pick up a penny or a piece
    of wood?

22
Concept Check 2 Copy and Complete
  1. An iron nail is strongly attracted to the north
    end, the south end or both ends of a magnet
    equally strong.
  2. Magnetism is due to the motion of the electron as
    they __________ and ______________.
  3. Several nails dangle from the north pole of a
    magnet. The induced pole on the top of the first
    nail is______. The induced pole on the bottom of
    the lowest most nail is _______.

23
Phet simulations magnetic fields
24
Learning Goals
  • SW recognize the relationship between electricity
    and magnetism

25
Magnetism and Current
  • What causes all magnetism?
  • What is current?
  • Will current cause magnetism?

26
The basic idea behind an electromagnet is
extremely simple By running electric current
through a wire, you can create a magnetic field.
By using this simple principle, you can create
all sorts of things, including motors, solenoids,
read/write heads for hard drives, and speakers,
and so on.
27
Put the compass on the table and, with the wire
near the compass, connect the wire between the
positive and negative ends of the battery for a
few seconds. What you will notice is that the
compass needle swings. Initially, the compass
will be pointing toward the Earth's north pole
(whatever direction that is for you), as shown in
the figure.
When you connect the wire to the battery the
compass, needle swings because the needle is
itself a small magnet with a north and south end.
Being small, it is sensitive to small magnetic
fields. Therefore, the compass is affected by the
magnetic field created in the wire by the flow of
electrons.
28
The figure shows the shape of the magnetic field
around the wire. In this figure, imagine that
you have cut the wire and are looking at it
end-on. The green circle in the figure is the
cross-section of the wire itself. A circular
magnetic field develops around a current carrying
wire, as shown by the circular lines. The field
weakens as you move away from the wire (so the
lines are farther apart as they get farther from
the wire).
29
You can see that the field is perpendicular to
the wire and that the field's direction depends
on which direction the current is flowing in the
wire. The compass needle aligns itself with this
field (perpendicular to the wire). If you flip
the battery around and repeat the experiment, you
will see that the compass needle aligns itself in
the opposite direction.
30
Magnetic field of a wireBecause the magnetic
field around a wire is circular and perpendicular
to the wire, an easy way to amplify the wire's
magnetic field is to coil the wire.
31
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32
Focus (copy and complete)
  • A moving electron has a ___________ field and an
    ___________field around it.
  • Magnetic field lines show the ________ and the
    __________ of the field.
  • A magnetic field goes from _____ to ____.
  • Paramagnetism is due to _________.
    Diamagnetism is due to ____________.
  • Ferromagnetism is due to _____________.
  • 5. The items needed to build an electromagnet
    include _____, ______, _______.

33
Learning Goals
  • SW recognize the relationship between electricity
    and magnetism
  • SW describe how to make an electromagnet
  • SW describe how to make electricity from
    magnetism (electromagnetic induction)

34
Electromagnets
Materials list
wire
power supply (current)
core (anything from air to metal)
  • The strength of an electromagnet can be increased
    by
  • increasing the current in the wire,
  • increasing the number of turns of wire, or
  • by using a metal core.

35
Parts of an electric motorAn electric motor is
all about magnets and magnetism A motor uses
magnets to create motion. If you have ever played
with magnets you know about the fundamental law
of all magnets Opposites attract and likes
repel. So if you have two bar magnets with their
ends marked "north" and "south," then the north
end of one magnet will attract the south end of
the other. On the other hand, the north end of
one magnet will repel the north end of the other
(and similarly, south will repel south). Inside
an electric motor, these attracting and repelling
forces create rotational motion.
36
 Electromagnet in a horseshoe magnetYou can
see that this half-turn of motion is simply due
to the way magnets naturally attract and repel
one another. The key to an electric motor is to
then go one step further so that, at the moment
that this half-turn of motion completes, the
field of the electromagnet flips. .
37
The flip causes the electromagnet to complete
another half-turn of motion. You flip the
magnetic field just by changing the direction of
the electrons flowing in the wire (you do that by
flipping the battery over). If the field of the
electromagnet were flipped at precisely the right
moment at the end of each half-turn of motion,
the electric motor would spin freely.
38
Armature The armature takes the place of the
nail in an electric motor. The armature is an
electromagnet made by coiling thin wire around
two or more poles of a metal core.
39
The armature has an axle, and the commutator is
attached to the axle. In the diagram you can see
three different views of the same armature
front, side and end-on. In the end-on view, the
winding is eliminated to make the commutator
more obvious. You can see that the commutator is
simply a pair of plates attached to the axle.
These plates provide the two connections for the
coil of the electromagnet.
40
Brushes and commutator The "flipping the
electric field" part ofan electric motor is
accomplished by two parts the commutator and
the brushes. The diagram at the right shows how
the commutator and brushes work together to let
current flow to the electromagnet, and also to
flip the direction that the electrons are flowing
at just the right moment. The contacts of the
commutator are attached to the axle of the
electromagnet, so they spin with the magnet. The
brushes are just two pieces of springy metal or
carbon that make contact with the contacts of the
commutator
41
Focus
  1. What can be done to strengthen an electromagnet?
  2. What are the advantages of using an
    electromagnet?
  3. Your task is to build a model of the earth using
    a ball of clay and then place a magnet inside the
    clay to represent the earths magnetic field for
    a science fair project. How would you place the
    magnet inside the ball of clay (what direction?)?

42
3.
43
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44
Generator
Mechanical energy electrical energy
Motor
Electrical energy Mechanical
energy
45
Faradays Law
The induced voltage in a coil is proportional to
the number of loops multiplied by the rate at
which the magnetic field changes within those
loops.
46
Lenzs Law
The current induced in a wire is such as to
oppose the force that induced it.
47
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50
11/7 Focus (label, copy, and complete)
  • A moving electron has a ___________ field and an
    ___________field around it.
  • Magnetic field lines show the ________ and the
    __________ of the field.
  • A magnetic field goes from _____ to ____.
  • Paramagnetism is due to _________.
    Diamagnetism is due to ____________.
  • Ferromagnetism is due to _____________.

51
What is the advantage of using a horseshoe magnet
over a bar magnet assuming the two are identical
as bar magnets?
52
What are the energy conversions for a generator?
Mechanical energy electrical energy
What are the energy conversions for a motor?
Electrical energy Mechanical
energy
53
  • Magnetic domains- clusters of aligned atoms

Magnetic Saturation all the domains are
aligned Dropping a magnet can cause the domains
to become unaligned.
54

S
N S
Earth
N
Compass
55
N
  • S

56
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57
Focus
  • 1. Why would someone want to use an
    electromagnet?
  • 2. How can an electromagnet be strengthened?
  • 3. What happens to a magnet if it is dropped and
    not handled or stored properly?

58
WiTricity?
  • Wireless Electricity

59
WiTricity Corp. was founded in 2007 to
commercialize an exciting new technology for
wireless electricity invented two years earlier
at the Massachusetts Institute of Technology
(MIT). 
60
Learning Goals
  • SW describe how to make electricity from
    magnetism (electromagnetic induction)
  • SW understand Lenzs Law and Faradays Law

61
Phet Simulations
  • Review bar magnet and electromagnet
  • Discovery activities pickup coil
  • transformer
  • Faradays Law and Lenzs Law

62
Faradays Law
The induced voltage in a coil is proportional to
the number of loops multiplied by the rate at
which the magnetic field changes within those
loops.
63
Lenzs Law
The current induced in a wire is such as to
oppose the force that induced it.
64
WiTricity?
  • Wireless Electricity

65
SW apply understanding of magnetism to motors
66
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67
In WiTricityWhy does the current have to be AC
and have to be a higher frequency?Whose law?
68
Faradays Law
The induced voltage in a coil is proportional to
the number of loops multiplied by the rate at
which the magnetic field changes within those
loops.
69
Lenzs Law
The current induced in a wire is such as to
oppose the force that induced it.
70
Domain Theory Paramagnetism- weak attraction
due to spin of electrons Diamagnetism weak
repulsion- due to revolving electrons Ferromagnet
ism strong-due to unpaired electrons
Magnetism vs. electricity Both due to
electrons Both force at a distance Both like
repel unlike attract BUT magnetic poles cannot
be separated
Magnetism
71
How can a magnet attract a piece of iron that is
not magnetized?
72
Electromagnets
Materials list
wire
power supply (current)
core (anything from air to metal)
  • The strength of an electromagnet can be increased
    by
  • increasing the current in the wire,
  • increasing the number of turns of wire, or
  • by using a metal core.

73
Learning GoalSW understand the generator and
the motor effect
74
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75
 Electromagnet in a horseshoe magnetYou can
see that this half-turn of motion is simply due
to the way magnets naturally attract and repel
one another. The key to an electric motor is to
then go one step further so that, at the moment
that this half-turn of motion completes, the
field of the electromagnet flips. .
76
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77
Generator
Mechanical energy electrical energy
Motor
Electrical energy Mechanical
energy
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