Download Magnetic Field and Induction

MAGNETIC FIELD
Magnetic Force
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For centuries, humans observed “strange” force
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Force couldn't be gravity or electric
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Between iron and special stones called “lodestones”
Not enough mass or electric charge to explain force
Iron and lodestones must have something
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Other than mass and charge
Humans called these “North” and “South” (like + and –)
Magnetic Poles
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Like poles repel; opposite poles attract
S
N
S
N
S
N
N
S
Repulsive force
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Attractive force
Curious fact: “North” and “South” always come
in pairs (unlike + and –)
Magnetic Field
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Magnetic Force → Action-at-a-distance
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Just like gravity and the electric force
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Solution: The “magnetic field”
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Magnetic poles affect the space around them
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If another pole comes into this space, it feels a force
This still doesn't explain the origin of the magnetic field...
Magnetic Materials
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Experiments → possible to “magnetize” iron
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Most other metals don't show this behavior
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By touching an unmagnetized piece of iron to a lodestone
The iron holds its magnetism permanently
Iron can be “de-magnetized” by heating it
Iron must be magnetically “special”
For centuries, iron was used to make:
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Compasses
Bar magnets
Magnets and Electric Current
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Experiments on magnets
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Found that electric current can deflect a compass needle
Must be a link between electricity and magnetism!
creates
Electric
Magnetic Field
Current exerts force
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exerts force
creates
Electric
Current
Currents create magnetic fields of many sizes
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Atoms (due to current produced by orbiting electron)
Medium size (e.g. coil of wire → electromagnet)
Huge (Sun is made of rotating plasma → magnetic field)
Atoms and Magnetism
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Individual atoms act like tiny bar magnets
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Usually pointing in random directions
When many atomic magnets “line up”:
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An object is “magnetized”
Requires special conditions
Only iron and a few other materials are easily magnetized
Earth's Magnetic Field
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Earth itself acts like a huge “bar magnet”
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Lodestones
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Humans have observed the magnetic field for centuries
Naturally-formed magnetic rocks
Made as iron-rich lava cooled
Earth magnetized the iron inside the rocks
Origin of magnetic field not fully understood
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Can NOT be due to Earth's iron core (too HOT!)
Must be due to electric currents in molten core
Cosmic Radiation
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Dangerous high-energy particles hit Earth
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From the Sun and from deep space
Charged particles are deflected by the magnetic field
The deflected charged
particles produce the
aurora borealis
Uncharged particles
must be stopped by
the atmosphere
Electromagnetic Induction
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Magnetic fields resist changes to their state
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Similar to a large mass resisting changes in velocity
Result: New method for creating electric current
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Use a changing magnetic field
This is called electromagnetic induction
And it creates electric current without a battery!
This is called “alternating current”
(AC) because the current switches
direction...
Batteries produce direct current (DC)
Generators and Motors
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Generator
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Motor
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Heat energy or KE → electrical energy
Electrical energy → KE
Both devices are made up of:
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Magnet (to produce a magnetic field)
Rotating coil of wire (to carry electric current)
Both use electromagnetic induction