c25 induction
TRANSCRIPT
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Chapter 25
ElectromagneticInduction
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WARNING
Strong magnets will be
passed around the room.Keep these magnets away
from iPods and laptops(especially my laptop)!
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Electromagnetic Induction
Voltage is induced whether the magnetic field of a
magnet moves near a stationary conductoror
the conductor moves in a stationary magnetic
field.
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Demo: Magnet Induces Current
Voltage is induced when a magnet moves towards
or away from a coil, inducing a current in the coil.
Faster the magnets motion, the greater the
induced current.
Note: Do this in lab too.
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Magnetic Recording
Write data by magnetizing
recording media (e.g.,
video tape, hard disk)using electromagnets.
Data is read back using the
induced current produced
when magnetized media
moves past receiver coils
(reverse of writing data).
Hard disk
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Electric Generator
Electric generator moves a conductor in a
magnetic field to produce voltage via
electromagnetic induction
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Demo: Electric Generator
Turn the shaft byhand and as
the coils passthe magnets avoltage is
induced.DC current isgenerated.
Magnet
Magnet
Coils
DC Output
S
HA
F
T
Note: Do this in lab too.
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Induction: No Free Lunch
Takes work to turn
the generator crank
to produce electric
current.
The faster we turn
the crank to
produce morecurrent, the more
difficult it is to turn.
More difficult topush the magnetinto a coil with
more loopsbecause themagnetic field ofeach current loopresists the motionof the magnet.
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Faradays Law
The induced voltage in a coil is proportional to the
product of the number of loops and rate at which
the magnetic field changes within the loops.
Small
Voltage
Medium
Voltage
LargeVoltage
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Demo: Electromagnetic Oscillations
Connect to
alternating
current (AC)
Oscillating
Magnetic
Field
Electro-
Magnet
Put alternating current into an
electromagnet and you create
an oscillating magnetic field.
This oscillating magnetic field
induces electrical currents by
inducing electrical field
oscillations.
This is, effectively, a very low
frequency electromagnetic
antenna.
Coil with bulb
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Demo: Electric Motor
Can create an electrical motor by passing a
current through a set of electro-magnets
mounted on a rotating shaft.
Current inCurrent out
Electro-magnets
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Electric Motor, Analyzed
Electromagnetmounted on ashaft with
opposing magnetson each side.
Current direction
always such thatelectromagnet isrepelled, causingshaft to turn.
N S
N S
Current
Current
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Demo: Generator Becomes Motor
Pass a current into the generator and it
becomes an electric motor.
Rotor
Spins
Magnet
Magnet
DC Input
Battery
Note: Do this in lab too.
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Self-Induction
When a current is
induced by a
changing magnetic
field, that current
itself produces its
own magnetic field.
This effect is calledself-induction.
Current
Primary Magnetic Field
Self-Induced
Magnetic Field
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Demo: Lenzs Law
Connect to
alternating
current (AC)
Oscillating
MagneticField
Electro-
Magnet
Metal ring is
levitated by
self-induced
secondary
magnetic field
Induced current
produces a
secondary magnetic
field that is alwaysopposed to the
primary magnetic
field that induced it,an effect called
Lenzs law.
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Check Yourself
Does the ring levitate if we connect
the electromagnet to direct
current (e.g., to a battery)?
Does the ring levitate if it has a
gap?Electro-
Magnet
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Check Yourself
Connect to
alternating
current (AC)
Oscillating
Magnetic
Field
Electro-
Magnet
Hold the ring down on the shaft and it
starts to get hot. Why?
What if we cool the ring in liquid
nitrogen and repeat the demo?
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Demo: Magnetic Brakes
Strong magnet dropped into
a copper pipe falls slowly
due to secondary magneticfield induced by its motion.
Great AmericasD
rop Zonehas a 22 story freefall,
lasting four seconds,
decelerated by magnetic
braking.
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Eddy Currents
Changing magnetic field
induces eddy currents
within any conductor.
These internal currentsproduce self-induced
magnetic fields, which
by Lenzs law are in
opposition of the
primary magnetic field.
Eddy
Currents
Primary Magnetic Field
Self-Induced
Magnetic Field
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Metal Detectors
Can detect the presence ofmetals by using atransmitter coil to create
an oscillating primarymagnetic field.
This creates a secondarymagnetic field due to eddycurrents in the metal.
Can detect this secondarymagnetic field by using areceiver coil.
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Demo: Back Electromotive Force
Changing magnetic field in an electromagnet
induces a voltage in the electromagnet
itself.
When we disconnect a large
electromagnet the magnetic field
rapidly collapses.
This changing magnetic field
self-induces a large voltage in the
coil of the electromagnet.
Large spark occurs at the switch.