Magnetism

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First found May 22, 2018

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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 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?
A. The charge on the car body is doubled.
B. 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.
The tendency of a stationary object to resist
being put into motion is known as
A. acceleration.
B. inertia.
C. weight.
D. velocity.
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
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 becomes
zero.
D. The force of attraction stays the
same.
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.
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 vthe 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
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?
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.
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.
FOCUS
Select your
answer a or b and
explain why.
What can one do to strengthen
the force from a bar magnet?
What would happen if you
broke a bar magnet in half?
Learning Goals
• SW describe the domain theory of
magnetism.
• SW recognize the relationship between
electricity and magnetism
Magnetic Poles
• North–seeking 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.
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 directionscreating opposite magnets. The
magnetic poles therefore cancel each
other. Unpaired electrons create
stronger magnets. (ferromagnetism)
• Magnetic domainsclusters of aligned
atoms
Magnetic Saturation –
all the domains are
aligned
Dropping a magnet can
cause the domains to
become unaligned.
Concept check 1: REVIEW
1. Must every magnet have a north and south pole? Explain.
2. How can a magnet attract a piece of iron that is not
magnetized?
3. Why will a magnet not pick up a penny or a piece of wood?
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
_______.
Phet simulations
magnetic fields
Learning Goals
• SW recognize the relationship between
electricity and magnetism
Magnetism and Current
• What causes all magnetism?
• What is current?
• Will current cause magnetism?
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.
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.
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).
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.
Magnetic field
of a wire
Because 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.
Focus (copy and complete)
1. A moving electron has a ___________ field and an
___________field around it.
2. Magnetic field lines show the ________ and the
__________ of the field.
3. A magnetic field goes from _____ to ____.
4. Paramagnetism is due to _________.
Diamagnetism is due to ____________.
Ferromagnetism is due to _____________.
5. The items needed to build an electromagnet include
_____, ______, & _______.
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)
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.
Parts of an electric motor
An 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.
Electromagnet in a
horseshoe magnet
You 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.
.
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.
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.
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.
Brushes and commutator
The "flipping the electric field" part of
an 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
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 earth’s magnetic field for a
science fair project. How would you place the
magnet inside the ball of clay (what direction?)?
3.
Generator
Mechanical energy
electrical energy
Motor
Electrical energy
Mechanical energy
Faraday’s 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.
Lenz’s Law
The current induced in a wire
is such as to oppose the force
that induced it.
11/7
Focus (label, copy, and
complete)
1. A moving electron has a ___________ field and
an ___________field around it.
2. Magnetic field lines show the ________ and the
__________ of the field.
3. A magnetic field goes from _____ to ____.
4. Paramagnetism is due to _________.
Diamagnetism is due to ____________.
Ferromagnetism is due to _____________.
What is the advantage of using a horseshoe
magnet over a bar magnet –assuming the two
are identical as bar magnets?
What are the energy conversions for
a generator?
Mechanical energy
electrical energy
What are the energy conversions for a motor?
Electrical energy
Mechanical energy
• Magnetic domainsclusters of aligned
atoms
Magnetic Saturation –
all the domains are
aligned
Dropping a magnet can
cause the domains to
become unaligned.
S
N
N
S
Earth
Compass
N
S
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?
WiTricity?
• Wireless Electricity
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).
Learning Goals
• SW describe how to make electricity from
magnetism (electromagnetic induction)
• SW understand Lenz’s Law and Faraday’s
Law
Phet Simulations
•
•
•
•
Review bar magnet and electromagnet
Discovery activities pickup coil
transformer
Faraday’s Law and Lenz’s Law
Faraday’s 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.
Lenz’s Law
The current induced in a wire
is such as to oppose the force
that induced it.
WiTricity?
• Wireless Electricity
SW apply understanding of
magnetism to motors
In WiTricity…
Why does the current have to be
AC and have to be a higher
frequency?
Whose law?
Faraday’s 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.
Lenz’s Law
The current induced in a wire
is such as to oppose the force
that induced it.
Magnetism vs. electricity
Both due to electrons
Both force at a distance
Both like repel unlike attract
BUT magnetic poles cannot
be separated
Magnetism
Domain Theory
Paramagnetism- weak attraction –
due to spin of electrons
Diamagnetism – weak repulsiondue to revolving electrons
Ferromagnetism – strong-due to
unpaired electrons
How can a magnet attract a piece of
iron that is not magnetized?
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.
Learning Goal
SW understand the generator and
the motor effect
Electromagnet in a
horseshoe magnet
You 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.
.
Generator
Mechanical energy
electrical energy
Motor
Electrical energy
Mechanical energy
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