Transcript Magnetic Field Lines

Unit 13 Magnetism
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Metal or Magnet. What is the difference?
 Which of the metal bars below is a magnet?
 Now, let us take a closer, microscopic look.
 Each metal bar has microscopic magnetic domains.
 When these domains are randomly aligned, the bar is nonmagnetic.
 When these domains are aligned, the bar is a magnetic.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Magnetic Poles
We say that a magnet has poles (ends).
One pole is named the North pole.
The other pole is named the South pole.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Fundamental Law of Magnetism
 Like poles repel, and unlike poles attract.
 This law is “fundamental” in understanding the operation of
many devices that use magnets or electromagnets.
 Some of these devices include speakers, microphones,
transformers, electromagnets, motors, generators, and
solenoids.
 We will look at these devices in detail in the coming slides.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Magnetic Field Lines
 Around every magnet there are many invisible lines known as
magnetic field lines.
 These lines influence other magnets and charged particles that
may be in the vicinity of the magnet.
 Watch what happens to the compass and the blue charged
particle as they get near to the magnet.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Magnetic Field Lines – Electric Charges
 In the previous slide we saw that there is a
magnetic field around a magnet.
 We also observed that the path of a moving
charged particle is deflected when it comes
close to a magnet.
 This deflection occurs because a charged
particle in motion also has a magnetic
field.
 Indeed, we can think of a charged particle
as being a tiny magnet with a north and
south pole.
 As a result, a charged particle obeys the
fundamental law of magnetism.
 Like poles repel, and unlike poles attract.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Interactions Between Electric and Magnetic Fields.
 A wire that carries a current (flowing electrons) will bend when it passes through
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a magnetic field.
This image is a picture of a magnetic field traveling into the board.
Right now the current in the wire is off.
Lets turn off the magnetic field and turn on the current.
Now turn the magnetic field back on and watch what happens.
The wire bent. Why?
The magnetic field produced by moving electric charges interfered with the large
magnetic field causing the wire to bend.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Motional Electromotive Force
 In the picture below, the “x” represent a uniform magnetic field.
 The gold rods are wires. The horizontal ones are electrically connected to
a LED (light emitting diode).
 Watch what happens as the vertical wire rod rolls through the magnetic
field.
 The LED lit up. Why do you think it did?
 The magnetic field exerted a force on the electrons (which are “tiny
magnets”) in the conductors causing them to move through the wires
thereby lighting the LED.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Motional Electromotive Force
 In the same way, we can bend a beam of electrons by moving a
magnet into close proximity to the beam.
 Here is what we really want to understand: a magnetic field
produced by a magnet causes electrons to move.
 The fact is that moving electrons (or other charges) generate
their own magnetic fields.
 These small magnetic fields are
deflected by the magnetic field of the
magnet.
 As a result, the beam of electrons
bends.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Interactions Between Electric and Magnetic Fields.
 The fact that moving electrons (or other charges) generate their
own magnetic fields can be observed by viewing the device
below.
 Note that all the compasses are pointed in the same direction
(towards the North).
 When electrons flow
through the wire, they
create a magnetic field.
 This magnetic field
interacts with the compass
needles (small magnets)
causing them to change
directions.
Return
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Magnetic Induction
 A coil of wire is wrapped around a nail as shown in the top picture below.
 This coil of wire is attached to a power supply in which the current direction may
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be reversed.
The bottom nail shows the alignment of the magnetic domains in the top nail.
Watch what happens as the electrons flow through the coil of wire around the nail.
The domains slowly realigned until they were all pointed in the same direction.
The nail/wire coil combination has become and electromagnet (see next slide).
What will happen when we reverse the current in the coil of wire?
In this animation, we saw that
moving electrons induced the
magnetic dipoles to align.
This animation provides an
example of magnetic induction.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
The Electromagnetic
 An electromagnetic is a magnet that you can turn off and on.
 When the switch is closed, the current (moving electrons) flows
through the wire.
 Moving electrons (current) generate electric fields which in turn
generate magnetic fields.
 These magnetic fields cause the
magnetic domains in the metal to
align creating a magnet.
 When you open the switch, the
domains realign themselves
turning the magnet back into
an ordinary piece of metal.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
The Solenoid
 A solenoid is simply a coil of wire.
 Even though there is not a metal core (like an electromagnet),
the electrons flowing through the wire still generate a magnetic
field.
 This field not only diverts the
compasses below, but also
it will attract metal as you
will see on the next slide.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
The Solenoid
 Look at the setup below.
The solenoid is the device
in the red rectangle.
 Watch what happens when
the switch is energized.
 The metal plunger was
drawn into the solenoid,
and the weights were lifted.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Speakers and Microphones
 Speakers and microphones are also
electromagnetic devices. They rely on
solenoids in order to emit and to capture
sound.
 They both have a cloth-like covering
(diaphragm) impregnated with
microscopic bits of metal.
 Lets take a closer look at the operation
of a speaker.
 When an electron moves through a
speaker, it sets up a magnetic field
which attracts the metal bits in the
diaphragm causing it to move in and to
deform. When it relaxes, it emits a
sound wave (longitudinal wave).
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Speakers and Microphones
 Again, when an electron moves through a speaker, it sets up a magnetic
field which attracts the metal bits in the diaphragm causing it to move and
to emit a sound wave (longitudinal wave).
 A microphone works in reverse.
 When the sound wave hits the microphone diaphragm, it pushes it in
making the electrons in the metal bits move through the magnetic field of
the microphone. This action causes a signal in the microphone which can
be captured and recorded.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Other Magnetic Devices
 So far we have discussed the solenoid, the electromagnet,
speakers, and microphones.
 Each of the devices function on Direct Current (DC).
 Direct current exists when the moving electrons move in one
direction all of the time (like from a battery).
 We now want to discuss transformers, motors, and generators.
 All of these devices need Alternating Current (AC) to function.
 Alternating current exists when the moving electrons
periodically change directions while traveling through a
conductor (wire).
 The following slide will illustrate what happens in an
electromagnetic as a result of AC.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Direct Current v. Alternating Current
In a direct current circuit (one
with a battery) the electrons
flow in one and only one
direction.
In an alternating current
circuit (like the electricity from
your wall outlet), the electrons
repetitively change directions.
Either way, the light remains
lit because the electrons still
move through it.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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Alternating Current in Electromagnets
 When the current flows
through the electromagnetic, it aligns the
diploes in a certain
direction.
 When the current direction
is alternated, the diploes
immediately switch
directions.
 As a result, the magnetic
poles of the
electromagnet are
switched.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Alternating Current in Electromagnets
 When the current flows
through the electromagnetic, it aligns the
diploes in a certain
direction.
 When the current direction
is alternated, the diploes
immediately switch
directions.
 As a result, the magnetic
poles of the
electromagnet are
switched.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Magnetic Induction
 Realigning magnetic dipoles can also cause electrons to move.
 Flowing electrons from a power supply cause the domains in the vicinity of the
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first coil to align.
These domains in turn induce the other domains in the nail to realign.
Watch what happens to the electrons in the second coil and watch the light bulb as
the magnetic domains in the vicinity of the second coil realign.
As long as these domains were moving, electrons in the second coil moved
causing the light to go on.
Once the domains stop moving, the electrons stop moving, and the light goes out
even though the electrons are still moving in the first coil.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Transformers
 Transformers are used to step-up (increase) or to step-down
(decrease) AC voltages.
 In order to do so, they rely on changing magnetic fields.
 Transformers have primary and secondary coils of wire.
 If there are more turns of wire around the primary side, then the
transformer is a step-down transformer.
 If there are more turns of wire around the secondary side, then the
transformer is a step-up transformer.
 As the magnetic field changes on the primary
side, electrons will flow through the secondary
side.
 Once the field stabilizes, secondary side
current stops.
 For this reason transformers need AC.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Transformers
 Notice how the magnetic domains realign as the current
changes direction.
 As long as the current alternates, the light bulb will stay
on.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Transformer Operation Steps
 Turn on the power.
 Electrons flow through the
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primary coil.
Domains in the vicinity of
the primary coil align.
All of the domains in the
metal transformer core
align.
Aligning magnetic domains
cause the electrons in the
secondary coil to move.
The light turns on.
Current alternates and the
light remains on.
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
DC Electric Motor
 A DC electric motor (one powered by a battery) must also have
AC current in order to work.
 In a DC motor, the DC from the battery is converted into AC
by a combination of devices acting in cooperation as one
electric switch: the brushes and the commutators.
 Before we look at the operation of a DC Motor, lets look at its
parts.
Field Magnet
Armature (Rotor)
Axle
Power Supply
Brushes
Commutators
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
DC Electric Motor
 Current flows through the wire turning the rotor into an electromagnet.
 The South pole of the electromagnet is attracted to the North end of the field
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magnet. The same is true for the other poles.
The rotor rotates to the point where the opposite poles are aligned and “happy.”
However, rotational energy carries the rotor past its “happy” point.
When it does, the brushes switch
commutators reversing the
direction of current flow through
the rotor.
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Immediately, the magnetic poles
of the rotor reverse.
As a result, the poles of the two
magnets repel each other.
The cycle continues as long as
power is on.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
DC Electric Motor Operation Steps
 Turn on power.
 Electrons flow through the
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Rotor.
Rotor turns into an
electromagnet.
Rotor rotates.
Rotor passes its “happy”
point.
Brush/commutater pairs flip.
Current flow through rotor
changes direction.
Electromagnet poles reverse.
The cycle repeats until
power is turned off.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
DC Motor Functioning
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Electric motors are used to drive electrical devices.
The motor below is hooked to and drives a saw blade.
In this setup the battery provides the electrons that cause the motor to rotate.
The motor in turn causes the saw blade to rotate using a system of pulleys and a
belt.
 The current (moving electrons) flows from the negative side of the battery to the
positive side thereby discharging the battery.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
DC Generator (Alternator)
 A DC generator works in reverse of a DC motor.
 A windmill, turbine, or paddle wheel (below) turns the rotor (electromagnet) through the
field magnet.
 The electrons in the wire around the rotor interact with the permanent magnet’s magnetic
field and are moved through the circuit from the positive terminal to the negative terminal
of the battery thereby recharging the battery.
 A similar system is in every car in the form of an alternator.
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© 2001-2005 Shannon W. Helzer. All Rights Reserved.
Practical Application
 On a recent deer hunting trip to Camel Back
Mountain, Dr. Physics found himself unexpectedly
lost. The map to the left shows where the cars were
parked (5 point star). From this area, there is a dirt
path around part of the mountain. Dr. Physics
followed the path and eventually left the path to go
into the woods. Before leaving the path, he
determined using the position of the Sun and the
Moon that north was in the direction indicated. He
also knew that he would come across the path if he
went North. Unfortunately, when Dr. Physics was
very deep into the woods, a major snowstorm blew
in and obscured his view of the Sun. As it was
snowing very badly, Dr. Physics pulled out his
compass (4 point star) and began to walk North
towards the path as indicated by the compass. After
nearly two hours of walking, he stepped out of the
woods underneath some power lines (at the arrow
head). What went wrong with the compass reading?
Answer
© 2001-2005 Shannon W. Helzer. All Rights Reserved.
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