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Magnetism Physical Science
Bill Nye-Magnetism Video
18.1 Magnets and Magnetic Fields • What happens when the poles of two magnets are thought close together? • What causes a magnet to attract or repel another magnet? • How is Earth’s magnetic field oriented? What is an example of how you use magnets?
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Magnets around the House: Headphones Computer speakers Telephone receivers Phone ringers Microwave tubes Seal around refrigerator door Credit card magnetic strip TV deflection coil Computer monitor Computer hard drive Shower curtain weights / attach to tub Power supply transformers
Magnets inside of motors: CD and DVD spinner and head positioner Audio and VHS tape transport VHS tape loader Microwave stirring fans Kitchen exhaust fans Garbage disposal motor Sump pump Furnace blower and exhaust Garage door opener Bathroom exhaust fan Electric toothbrush Ceiling fan Pager or cell phone vibrator Clocks (not the wind-up type or LCD type) Computers
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Magnets in your Car: Starter motor A/C clutch Interior fan motor Electric door locks Windshield wiper motor Electric window motor Side-view mirror adjuster motor CD/tape player motor and playback Engine speed sensors Alternator Starter relay Windshield washer pump motor
Magnets More than 3,000 years ago Greeks discovered deposits of a mineral that was a natural magnet. The mineral is now called magnetite. Ex: lodestone These magnets were used by the ancient peoples as compasses to guide sailing vessels.
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Magnets • Today, the word magnetism refers to the properties and interactions of magnets. magnet – any material that attracts iron and materials that contain iron magnetism – the attraction or repulsion of magnetic materials Magnets attract or repel other magnets.
Common metals affected by magnetism are iron, nickel, and cobalt
IRON
NICKEL
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Magnets • Nonmagnetic materials, such as plastic, glass and wood, have electrons that flow in different directions.
Magnets
Magnetic Poles • Magnetic poles are where the magnetic force exerted by the magnet is strongest. • the two ends on any magnet, no matter what its shape
• All magnets have a north pole and a south pole. • magnetic force – the attraction or repulsion between magnetic poles
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Magnetic poles that are alike repel each other.
Magnetic poles that are unlike attract each other.
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Magnetic Poles
Poles- two ends of a magnet
every magnet has two poles north (N) pole south (S) pole
even if you break a magnet in half, each half will have a north pole and a south pole
No Monopoles Allowed It is not possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).
S
N
• Recall that even individual atoms of magnetic materials act as tiny magnets.
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Making a Magnet
materials that are hard to magnetize will also stay magnetized for a long time
a magnet that is hard to magnetize but tends to keep its magnetism is called a permanent magnet Ex: Cobalt, Nickel, Aluminum
Making a Magnet
a material that is easily magnetized tends to lose its magnetism quickly
a magnet made of this kind of material is called a temporary magnet Ex: Iron nails
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Where are the Poles on different shaped magnets? • The two ends of a horseshoe-shaped magnet are the north and south poles. • A magnet shaped like a disk has opposite poles on the top and bottom of the disk. • Magnetic field lines always connect the north pole and the south pole of a magnet.
Magnetic Fields Michael Faraday realized that ...
A magnet has a ‘magnetic field’ distributed throughout the surrounding space
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• A magnet is surrounded by a magnetic field. • A magnetic field exerts a force on other magnets and objects made of magnetic materials. • The magnetic field is strongest close to the magnet and weaker far away.
A magnetic field also has a direction & illustrated by arrows.
magnetic field lines – the lines that map out the magnetic field around a magnet
Magnetic field lines spread out from one pole, curve around the magnet, and return to the other pole forming a closed loop.
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The density of field lines indicates the strength of the field Weak Field
Strong Field Magnetic Field Video
Field Lines Around Magnets Bar
Repelling
Sphere
Attracting
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magnetic domain – a grouping of billions of atoms that have their magnetic fields lined up in the same direction
Atom’s electrons spin around when placed near a magnetic field.
An unmagnetized substance looks like this…
While a magnetized substance looks like this…
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Earth as a Magnet • Earth has a magnetic field with two main poles. • This magnetic field enables us to use compasses • Our planet's rotation causes molten ironnickel in its outer core to circulate • creating electrical currents and a magnetic field.
Earth as a Magnet
William Gilbert a British scientist Showed that the Earth behaves as a magnet Earth has north and south poles like a bar magnet
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The Earth’s Magnetic Field
The Earth is surrounded by a magnetic field which extends far into space. Called the magnetosphere
Earth as a Magnet Magnetic poles of the Earth are different from the geographic poles of the Earth
the North Magnetic Pole is located near the geographic South Pole
the South Magnetic Pole is located near the geographic North Pole
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Earth’s Magnetic Poles • Earth’s magnetic poles move slowly with time. • Sometimes Earth’s magnetic poles switch places so that Earth’s south magnetic pole is the southern hemisphere near the geographic south pole.
Video
Compasses Compasses are used to determine direction Compass needles are magnetized and respond to the magnetic field of the Earth We use the Earth’s magnetic field to find direction. Migration of birds
The needle of a compass always points toward the magnetic south pole. The compass needle points toward the geographic “North”
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Magnetic Effects The most visible effect of the earths magnetic field is a colorful light display, called an aurora Collisions between the charged particles and other particles in the upper atmosphere create glowing lights
Video
18.2 Magnetism from Electric Currents • What happens to a compass near a wire that is carrying a current? • Why are electric motors useful?
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Electromagnetism • In 1820, Han Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related.
•
When a compass was brought near electric current, the compass needle no longer pointed north
• Oersted hypothesized: • Electric current must produce a magnetic field around the wire, and the direction of the field changes with the direction of the current.
Electromagnetism • When an electric current passes through a wire a magnetic field is formed. • The magnetic field depends on the direction of the current in the wire.
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Electromagnetism • The direction of the magnetic field around the wire reverses when the direction of the current in the wire reverses. • As the current in the wire increases the strength of the magnetic field increases.
Electromagnetism • Electromagnetism – relationship between electricity and magnetism • Oersted’s discovery is responsible for the invention of new tools based on the principles of electromagnetism.
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Electromagnetism When an electric current is passed through a coil of wire wrapped around a metal core, a very strong magnetic field is produced. This is called an electromagnet. The strength depends on the number of turns in the coil, the amount of current, and the size of the iron core.
Electromagnetism
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YOU CAN MAKE AN ELECTROMAGNET Electromagnets can be turned on and off You can make an electromagnet by wrapping a wire around a piece of iron and sending a current through the wire with a battery.
Classroom electromagnet vs. commercial electromagnet
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Electromagnetism • A single wire wrapped into a cylindrical wire coil is called a solenoid. • A type of electromagnet • The solenoid’s magnetic field magnetizes the iron core.
Electromagnetism • The magnetic field inside a solenoid is stronger than the field in a single loop. • The field inside the solenoid with the iron core can be more than 1,000 times greater than the field inside the solenoid without the iron core.
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Electromagnetism Properties of Electromagnets • Electromagnets are temporary magnets because the magnetic field is present only when current is flowing in the solenoid. • The strength of the magnetic field increases: • by adding more turns of wire to the solenoid • by increasing the current passing through the wire.
Properties of Electromagnets • Electromagnet has a north pole and a south pole. • If placed in a magnetic field, an electromagnet will align itself along the magnetic field lines, just as a compass needle will. • An electromagnet also will attract magnetic materials and be attracted or repelled by other magnets.
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Electromagnetic Devices Galvanometer A galvanometer gives a measurement of the current. an electromagnet that interacts with a permanent magnet. The greater the current passing through the wires, the stronger the galvanometer interacts with the permanent magnet.
Use of a Galvanometer Galvanometers are used as gauges in cars and many other applications. •
Functions as an ammeter to measure current, when other circuit elements are used.
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Functions as a voltmeter to measure voltage.
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Electromagnetic Devices Electric Motors • Electric Motor – device that changes electrical energy into mechanical energy • An electric motor is made up of an electromagnet and a permanent magnet
Electric Motors • A fan uses an electric motor • The motor in a fan turns the fan blades, moving air past your skin to make you feel cooler.
• Almost every appliance in which something moves contains an electric motor.
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Electric Motor • A simple electric motor also includes components called brushes and a commutator. • The brushes carry current to coil using the commutator. • The commutator is used to change the currents direction. • Form a closed electric circuit between the battery and the coil. Video
18.3 Electric Currents from Magnetism What happens when a magnet is moved into or out of a coil of wire? How are electricity and magnetism related? What are the basic components of a transformer?
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Electromagnetic Induction The process of inducing a current by moving a magnetic field through a wire coil without touching it. Causes charges to move within the wire.
Electromagnetic Induction • Working independently in 1831, Michael Faraday in Britain and Joseph Henry in the United States both found that: • Moving a loop of wire through a magnetic field caused an electric current to flow in the wire. • They also found that moving a magnet through a loop of wire produces a current.
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Electromagnetic Induction
Generators • electric generator – a device that converts mechanical energy into electrical energy • A generator uses motion in a magnetic field to produce an electric current
How a Generator Works – a current can be induced in a loop of wire by spinning the loop inside a magnetic field – due to the changing direction of the wire, an alternating current is produced
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Electromagnetic Induction Using Electric Generators • The type of generator in a car, is called an alternator. • The alternator provides electrical energy to operate lights and other accessories.
Induced current in a Generator
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Electromagnetic Induction • Electrical energy comes from a power plant with huge generators. • Produces the electrical energy that you use in your home. • The coils in these generators have many coils of wire wrapped around huge iron cores.
Electromagnetic Induction • The rotating magnets are connected to a turbine (TUR bine)a large wheel that rotates when pushed by water, wind, or steam. • Thermal energy is then converted to mechanical energy as the steam pushes the turbine blades.
Video
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Electromagnetic Force • Electromagnetic force: Electricity and magnetism are two aspects of a single force. • Electromagnetic energy results – Light is an example of electromagnetic energy. EM Waves are made of both electric and magnetic fields.
Electric field
Magnetic field
Transformers • A transformer is a device that increases or decreases voltage alternating current.
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Transformers • a transformer consists of two coils of wire around a magnet – the first coil, called the primary coil, is connected to the power source – the second coil, called the secondary coil, is connected to the load • a load is something that uses electricity, such as a light bulb or a motor
Transformers Two types of transformers: Step-up transformer a transformer that increases voltage
Step-down transformer a transformer that decreases voltage
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Transformers
Transmitting Alternating Current • This figure shows how step-up and step-down transformers are used in transmitting electrical energy from power plants to your home.
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