Chapter 25
Electromagnetic
Induction
24-Mar-16
Physics 1 (Garcia) SJSU
WARNING
Strong magnets will be
passed around the room.
Keep these magnets away
from iPods and laptops
(especially my laptop)!
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Physics 1 (Garcia) SJSU
Electromagnetic Induction
Voltage is induced whether the magnetic field of a
magnet moves near a stationary conductor or
the conductor moves in a stationary magnetic
field.
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Physics 1 (Garcia) SJSU
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 magnet’s motion, the greater the
induced current.
24-Mar-16
Physics 1 (Garcia) SJSU
Note: Do this in lab too.
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).
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Physics 1 (Garcia) SJSU
Hard disk
Electric Generator
Electric generator moves a conductor in a
magnetic field to produce voltage via
electromagnetic induction
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Physics 1 (Garcia) SJSU
Demo: Electric Generator
Turn the shaft by
hand and as
the coils pass
the magnets a
voltage is
induced.
DC current is
generated.
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DC Output
S
H
A
F
T
Magnet
Magnet
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Coils
Note: Do this in lab too.
Induction: No Free Lunch
Takes work to turn
the generator crank
to produce electric
current.
The faster we turn
the crank to
produce more
current, the more
difficult it is to turn.
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Physics 1 (Garcia) SJSU
More difficult to
push the magnet
into a coil with
more loops
because the
magnetic field of
each current loop
resists the motion
of the magnet.
Faraday’s 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
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Medium
Voltage
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Large
Voltage
Demo: Electromagnetic Oscillations
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
Oscillating
Magnetic
Field
ElectroMagnet
Connect to
alternating
current (AC)
Demo: Electric Motor
Can create an electrical motor by passing a
current through a set of electro-magnets
mounted on a rotating shaft.
Current out
Current in
Electro-magnets
Electric Motor, Analyzed
Electromagnet
mounted on a
shaft with
opposing magnets
on each side.
Current direction
always such that
electromagnet is
repelled, causing
shaft to turn.
Current
N
S
Current
N
S
Demo: Generator Becomes Motor
Pass a current into the generator and it
becomes an electric motor.
DC Input
Battery
Magnet
Magnet
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Physics 1 (Garcia) SJSU
Rotor
Spins
Note: Do this in lab too.
Self-Induction
When a current is
induced by a
changing magnetic
field, that current
itself produces its
own magnetic field.
This effect is called
self-induction.
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Primary Magnetic Field
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Current
Self-Induced
Magnetic Field
Demo: Lenz’s Law
Induced current
produces a
secondary magnetic
field that is always
opposed to the
primary magnetic
field that induced it,
an effect called
Lenz’s law.
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Metal ring is
levitated by
self-induced
secondary
magnetic field
ElectroMagnet
Connect to
alternating
current (AC)
Physics 1 (Garcia) SJSU
Oscillating
Magnetic
Field
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?
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Physics 1 (Garcia) SJSU
ElectroMagnet
Check Yourself
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?
Oscillating
Magnetic
Field
ElectroMagnet
Connect to
alternating
current (AC)
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Physics 1 (Garcia) SJSU
Demo: Magnetic Brakes
Strong magnet dropped into
a copper pipe falls slowly
due to secondary magnetic
field induced by its motion.
Great America’s Drop Zone
has a 22 story freefall,
lasting four seconds,
decelerated by magnetic
braking.
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Physics 1 (Garcia) SJSU
Eddy Currents
Changing magnetic field
induces eddy currents
within any conductor.
These internal currents
produce self-induced
magnetic fields, which
by Lenz’s law are in
opposition of the
primary magnetic field.
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Primary Magnetic Field
Eddy
Currents
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Self-Induced
Magnetic Field
Metal Detectors
Can detect the presence of
metals by using a
transmitter coil to create
an oscillating primary
magnetic field.
This creates a secondary
magnetic field due to eddy
currents in the metal.
Can detect this secondary
magnetic field by using a
receiver coil.
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Physics 1 (Garcia) SJSU
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.
24-Mar-16
Physics 1 (Garcia) SJSU