unit 13 magnetism add a slide like fig 36.13 from the applied book. bend a wire into a loop, the...
TRANSCRIPT
Unit 13 Magnetism
Add a slide like fig 36.13 from the applied book. bend a wire into a loop, the field lines are concentrated
in the center. Add more loops and its even stronger (solinoid)
Unit 13 Magnetism
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 non-
magnetic.When these domains are aligned, the bar is a magnetic.
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Magnetic PolesMagnets have poles (ends).One pole is named the North pole.The other pole is named the South pole.
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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.
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Metal or Magnet. What is the difference?
What happens if a magnet breaks?
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Magnetic Field Lines Around every magnet there are many invisible lines known as
magnetic field lines. These lines influence other magnets and charge particles. Watch what happens to the compass and the blue charged
particle as they get near to the magnet.
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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.
A charged particle is a tiny magnet with a north and south pole.
Charged particles obeys the fundamental law of magnetism.
Like poles repel, and unlike poles attract.
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Interactions Between Electric and Magnetic Fields. A wire that carries a current (flowing electrons) will bend when it passes through 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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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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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.
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These small magnetic fields are deflected by the magnetic field of the magnet.
As a result, the beam of electrons bends.
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).
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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
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 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 themagnetic dipoles to align.
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Magnetic Induction(Mutual Induction)
• The process by which a body having electric or magnetic properties produces magnetism, an electric charge, or an electromotive force in a neighboring body without contact.
The Electromagnetic An electromagnetic is a magnet that you can turn off and on.
Moving electrons (current) generate electric fields which in turn generate magnetic fields.
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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.
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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The SolenoidLook 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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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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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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Other Magnetic DevicesSo far we have discussed the solenoid, the electromagnet,
speakers, and microphones.Each of the devices function on Direct Current (DC).
We now want to discuss transformers, motors, and generators.
All of these devices need Alternating Current (AC) to function.
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Alternating Current in Electromagnets When the current flows
through the electro-magnetic, 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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Alternating Current in Electromagnets When the current flows
through the electro-magnetic, 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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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
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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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. 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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TransformersNotice 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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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 the brushes and the commutators. Before we look at the operation of a DC Motor, lets look at its
parts.
Field MagnetArmature (Rotor)AxelPower SupplyBrushesCommutators
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When it does, the brushes switch commutators reversing the direction of current flow through the rotor.
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.
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
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.
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DC Generator (Alternator) A DC generator works in reverse of a DC motor. A windmill, turbine, or paddle wheel 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. This system is in every car in the form of an alternator.
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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?
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