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Magnetism
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Section 1 Magnets and Magnetic Fields
Section 2 Magnetism from Electricity
Section 3 Magnetic Force
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Section 1
Magnetism
TEKS
Section 1
The student is expected to:
5D identify examples of electric and magnetic
forces in everyday life
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Magnetism
What do you think?
Section 1
`
• An iron nail is attracted to an iron magnet but not
to another nail. Two magnets can attract each
other.
• Is either end of the nail attracted to either end of the
magnet?
• Is either end of one magnet attracted to either end of
the other magnet? Explain.
• Both are made of iron, but the magnet behaves
differently. Why?
• How does the nail change when near the magnet so
that it is attracted?
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Magnetism
Section 1
Properties of Magnets
• Magnets attract metals classified as
ferromagnetic.
– Iron, nickel, cobalt
• Magnets have two poles, north and south.
– Like poles repel each other.
– Opposite poles attract each other.
• When free to rotate, the north pole points toward
the north.
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Magnetism
Section 1
Magnetic Poles
Click below to watch the Visual Concept.
Visual Concept
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Magnetism
Magnetic Domains
• In ferromagnetic materials,
groups of atoms form
magnetic domains within the
material.
• In a paper clip or nail, the
domains are randomly
arranged.
• In a magnet, the domains are
more aligned.
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Section 1
Magnetism
Section 1
Magnetic Domains
• What would happen to the
domains?
– They would better align.
• How would the paper clip be
different afterward?
• Suppose you rubbed
a paper clip
repeatedly in one
direction with the
north pole of a
magnet.
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– It would behave as a magnet.
• Would it remain magnetized?
– The domains would gradually
become more randomly
oriented.
Magnetism
Section 1
Magnetic Fields
• What object is used to detect a gravitational field?
– Any mass - when released it falls in the direction of the
field
• What object was used to detect an electric field?
– A positively charged test particle - when released it
moves in the direction of the field
• What object would be used to detect a magnetic
field?
– A compass - the north pole points in the direction of the
magnetic field
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Magnetism
Section 1
Magnetic Fields
• Compass needles show
the direction of the field.
– Out of the north and into
the south
• The distance between
field lines indicates the
strength of the field.
– Stronger near the poles
• The field exists within the
magnet as well.
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Magnetism
Magnetic Flux
• Flux measures the number of
field lines passing
perpendicularly through a fixed
area.
– More flux near the poles
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Section 1
Magnetism
Section 1
Representing the Direction of a Magnetic Field
Click below to watch the Visual Concept.
Visual Concept
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Magnetism
Section 1
Earth’s Magnetic Field
• The north pole of a magnet
points toward the
geographic north pole or
Earth’s south magnetic
pole.
– Opposites attract
• The magnetic poles move
around.
• The magnetic and
geographic poles are about
1500 km apart.
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Magnetism
Section 1
Earth’s Magnetic Field
• Which way would a compass
needle point in the U.S.?
– Toward the north and slightly
downward into Earth
– Field lines go into Earth as seen
in the diagram; they are not
parallel to the surface.
• Earth’s poles have reversed
many times in the past, as
evidenced by core samples
showing differing magnetic
field directions.
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Magnetism
Section 1
Now what do you think?
• An iron nail is attracted to an iron magnet but not
to another nail. Two magnets can attract each
other.
• Is either end of the nail attracted to either end of the
magnet?
• Is either end of one magnet attracted to either end of
the other magnet? Explain.
• Both are made of iron but the magnet behaves
differently. Why?
• How does the nail change when near the magnet so
that it is attracted?
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Magnetism
Section 2
What do you think?
• Electromagnets are used every day to operate
doorbells and to lift heavy objects in scrap yards.
• Why is the prefix electro- used to describe these
magnets?
• Is electricity involved in their operation or do they create
electricity?
• Would such a magnet require the use of direct current
or alternating current?
• Why?
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Magnetism
Magnetism from Electricity
• A compass needle held near a
current carrying wire will be
deflected.
– Electric current must produce a
magnetic field.
– Discovered by Hans Christian
Oersted
• Many compasses placed
around a vertical current
carrying wire align in a circle
around the wire.
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Section 2
Magnetism
Section 2
Right-Hand Rule
• To find the direction of the
magnetic field (B) produced by a
current (I):
– Point your right thumb in the
direction of the current
– Curl your fingers and they will show
the direction of the circular field
around the wire.
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Magnetism
Section 2
Magnetic Fields
C
B
A
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• Use the right hand rule to
decide what direction the
magnetic field would be
at points A, B, and C.
• Since magnetic fields are
vectors, how would the
net field appear in the
center of the loop?
Magnetism
Section 2
Magnetic Field of a Current Loop
Click below to watch the Visual Concept.
Visual Concept
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Magnetism
Section 2
Magnetic Field Around a Current Loop
• Magnets and loops of wire
have magnetic fields that
are similar.
• Solenoids are coils of wire
similar to the single loop.
– More loops strengthens the
field
– Placing an iron rod in the
center strengthens the field
as well
• Called an electromagnet
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Magnetism
Section 2
Now what do you think?
• Electromagnets are used every day to operate
doorbells and to lift heavy objects in scrap yards.
– Why is the prefix electro- used to describe these
magnets?
• Is electricity involved in their operation or do they create
electricity?
– Would such a magnet require the use of direct current
or alternating current?
• Why?
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Magnetism
TEKS
Section 3
The student is expected to:
5D identify examples of electric and magnetic
forces in everyday life
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Magnetism
Section 3
What do you think?
• When watching a television with a CRT, an image
is created on the screen by beams of electrons
striking red, green, and blue phosphors on the
screen.
• How are these beams aimed at the right phosphors?
• Why does holding a magnet near the screen alter the
image and sometimes permanently damage the
screen?
• How often does the TV produce a new still image for
you to see?
• How do these still images create movement?
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Magnetism
Section 3
Charged Particles in a Magnetic Field
• Magnetic fields exert a magnetic force on moving
charged particles.
– Force is greatest when the movement is perpendicular to the
magnetic field
– Force is zero when the particle moves along the field lines
– Force is in between these values for other directions
• When the movement is perpendicular, the magnetic
force is:
Fmagnetic = qvB
– where q is the charge, v is the velocity, and B is the magnetic
field strength.
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Magnetism
Section 3
Charged Particles in a Magnetic Field
• So, the magnetic field (B) can be determined from the
force on moving charged particles as follows:
• SI unit: Tesla (T)
– where T = N/(C•(m/s)) = N/(A•m) = (V•s)/m2
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Magnetism
Section 3
Charged Particles in a Magnetic Field
• The right-hand rule for the force
on a moving charged particle
– Thumb in the direction a positive
particle is moving
– Fingers in the direction of the
magnetic field
– The force will be in the direction of
your palm
• For negative particles, the force
is out the back of your hand.
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Magnetism
Section 3
Force on a Charge Moving in a Magnetic Field
Click below to watch the Visual Concept.
Visual Concept
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Magnetism
Section 3
Classroom Practice Problems
• An electron moving north at 4.5  104 m/s enters
a 1.0 mT magnetic field pointed upward.
– What is the magnitude and direction of the force on
the electron?
– What would the force be if the particle was a proton?
– What would the force be if the particle was a neutron?
• Answers:
– 7.2  10-18 N west
– 7.2  10-18 N east
– 0.0 N
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Magnetism
Section 3
Magnetic Force as Centripetal Force
• Use the right-hand rule to
determine the direction of
the force.
• Which direction would the
force be when the charge
is at the top? the left side?
the bottom?
– Always directed toward the
center
– Because of this magnetic
force, the charge moves in a
circle.
– The force is centripetal.
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Magnetism
Current-Carrying Wires
• Magnetic forces also exist on the
moving charges in current-carrying
wires.
– The right-hand rule to is used to determine
the direction, as shown in the diagram.
– The magnitude of the force is as follows:
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Section 3
Magnetism
Section 3
Parallel Current-Carrying Wires
• Current carrying wires create a magnetic
field which interacts with the moving
electrons in the nearby wire.
– Currents in the same direction produce
attraction.
– Currents in opposite directions cause the wires
to repel.
• Use the-right hand rule to verify the
direction of the force for each of the four
wires shown.
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Magnetism
Section 3
Classroom Practice Problem
• A 4.5 m wire carries a current of 12.5 A from
north to south. If the magnetic force on the wire
due to a uniform magnetic field is 1.1  103 N
downward, what is the magnitude and direction
of the magnetic field?
• Answer: 2.0  101 T to the west
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Magnetism
Section 3
Applications - Cathode Ray Tube
• Televisions and computer
monitors use CRTs.
• A magnetic field deflects a
beam of electrons back
and forth across the
screen to create an
image.
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Magnetism
Section 3
Applications - Speakers
• The forces on electrons as
they move back and forth
in the coil of wire cause
the coil to vibrate.
• The coil is attached to the
paper cone, so sound
waves are produced by
the vibration.
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Magnetism
Section 3
Galvanometer
Click below to watch the Visual Concept.
Visual Concept
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Magnetism
Section 3
Now what do you think?
• When watching a television with a CRT, an
image is created on the screen by beams of
electrons striking red, green, and blue
phosphors on the screen.
– How are these beams aimed at the right phosphors?
– Why does holding a magnet near the screen alter the
image and sometimes permanently damage the
screen?
– How often does the TV produce a new still image for
you to see?
• How do these still images create movement?
© Houghton Mifflin Harcourt Publishing Company