1. Chapter 14 Magnetism
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2. Uses of Magnets
Magnets are used to make electric motors and generators. Without these electric motors and generators we would not have telephones, lights, electric heat, television, or computers.
Magnets are used in alarm systems and small electrical appliances.
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3. Magnets & Magnetic Fields 14.1
Lodestone:
naturally occurring magnet rocks.
Composed of iron called Magnetite.
Named after the region of discovery in Greece - Magnesia.
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4. Materials such as lodestone that are always magnetic are referred to as Permanent magnets.
Lodestone can be used to induce other materials to become permanent magnets.
These materials will lose their magnetism. Iron can lose magnetism by heating or hammering.
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6. Magnets are classified as
hard or soft
Hard magnets
Materials that are difficult to magnetize, yet they hold magnetism well.
Cobalt, Nickel
Soft Magnets
Material is easily magnetized, and loses magnetism easily.
Iron
Things such as heat and pounding with a hammer can cause a magnet to weaken.
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7. The magnetic force can be passed through objects.
Chain of nails or paper clips.
When the magnet is removed the force no longer acts between the nails or paper clips.
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8. Coulomb’s Law of Magnetic Force
Similar to electrical field forces which depend on distance and charge.
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9. Magnets have a pair of poles
Magnetic poles:
the location of the strongest magnetic force.
Magnets have a pair of poles
North and South (at opposite ends.)
Impossible to separate a north and south pole.
Opposites attract/ Like poles repel
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10. Magnetic Fields
Two like poles exert a force on each other that acts at a distance.
This force is due to the interaction of magnetic fields.
Magnetic force is a field force.
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11. All magnets exert a magnetic field.
The strength of the field depends and the material and strength of the magnet.
The field lines for magnets are similar to those for electrical charges.
Similar to electrical field forces which depend on distance and charge.
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12. Conpass & Magnetic Fields
A Compass is a magnet that floats on cork in water or is balanced on a point.
One end will point north and the other south.
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14. Earth is a giant bar magnet. Like poles repel each other.
Opposite poles attract.
Earths poles have reversed many times.
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15. The geographic south of earth is now the magnetic north.
The magnetic south is the geographic north.
This is done to confuse students.
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16. Magnetism from electric currents 14.2
An electric current through a wire will always produce a magnetic field.
Due to this field iron filings form a pattern around the wire.
When a compass is brought near a wire carrying a current the needle direction changes.
The needle points in a direction tangent to the circles of the iron filings.
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17. Now a compass placed above the wire points in the direction of the net magnetic field above the wire, while a compass placed below the wire points in a different direction, which is the direction of the net magnetic field below the wire
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18. No Current
Current
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20. Use right hand rule to determine direction of magnetic field produced
Use right hand rule to determine direction of magnetic field produced. Thumb points in the direction of the conventional(+) current.
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21. Magnetic field of a coiled wire resembles that of a bar magnet
The magnetic field around a wire is weak.
Increase by increasing the current through the wire (fire hazard) or wrapping wire into a coil.
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22. Solenoid Wire wrapped into a coil is a solenoid.
The strength of a magnetic field of a solenoid depends on the number of loops and the amount of current being carried.
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23. Electromagnets A coil of wire with current running through it.
The more turns of wire the more current and the stronger the magnet.
Reversing the current will reverse the polarity of the electromagnet.
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24. Magnetism is caused by moving charges
Magnetism of bar magnets is due to movement of charged particles.
The movement is electron spin, which creates a magnetic field around each electron.
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25. When electron spins cancel each other out the material is not magnetic.
Materials that are magnetic have a magnetic field around each atom.
Within a material magnetic fields of atoms align themselves with nearby atoms creating smaller regions called domains
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26. Domains in demagnetized materials are not aligned.
Magnetism can be induced by bringing a strong magnet in proximity with the material.
The domains realign to produce a magnet.
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27. Using a magnet to induce magnetism
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28. Electromagnetic Devices
Galvanometer
Devices used to measure current or voltage.
Ammeter or voltmeter
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29. Electric Motors
Electric motors turn electrical energy into mechanical energy
The coil of wire continues to spin in a motor; when the coil is attached to a shaft it can do useful work.
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30. Commutator
A device used to reverse current direction in the coil every half turn. This causes magnetic field of the coil to be reversed every half turn.
The coil is repelled by both N and S poles of magnetic field and thus keeps turning in one direction.
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31. Stereo Speakers
Speakers consist of a permanent magnet and a coil of wire is attached to a paper cone.
Current through the coil causes a magnetic field which interacts with the permanent magnet.
Changing current causes the paper cone to move creating sound waves (longitudinal wave).
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32. Faraday’s Law Electromagnetic Induction Electric Currents
& Magnetism
Electromagnetic Induction
Electromagnetic induction is considered a law of Physics, known as Faraday’s Law
Faraday’s Law
An electric current can be produced in a circuit by a changing magnetic field.
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33. Energy is produced in this process.
Moving a magnet in and out of a circuit will create a current in the wire.
Energy is produced in this process.
Does not violate conservation of energy.
Energy is put into the system by moving the magnet.
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34. Charged particles will experience a force when moving through a magnetic field.
The force experienced is zero when a wire moves parallel to the magnetic field lines.
The force is at a maximum when the particle moves perpendicular to the magnetic field lines.
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35. Generators AC current is produced
A generator uses electromagnetic induction to convert ME to electrical energy.
Generators are similar to motors. Only they convert ME to electrical energy.
In each ½ rotation of the loop the current reverses direction.
AC current is produced
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36. Generators
The magnitude and direction of the current depend on the orientation of the loop in the field.
When the loop is perpendicular to the field the current is zero.
As the loop turns the current increases until it reaches its maximum.
This is when the loop is parallel to the field. (see page 477)
Electrical Generator - AC or DC
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37. Electricity & Magnetism
Visible light travels in EM waves.
EM waves are made up of oscillating electric and magnetic fields.
Each field regenerates the other.
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38. Transformers
A device that can change an AC voltage to a different AC voltage.
Transformers may increase the voltage (step-up) or decrease the voltage (step-down).
This allows for a more efficient movement of the electricity from the power plant to the site of use.
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39. 10.0 volts Step up Transformer
The Primary coil has fewer turns of wire than the secondary coil.
10.0 volts
Primary receives 5.0V and has10 turns. Secondary has 20 turns. What is the resulting voltage?
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40. 2.5V Step Down Transformer
Primary coil has more turns than the secondary.
Primary receives 10.0V and has 20 turns; secondary has 5 turns.
How many volts leave the transformer?
2.5V
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