Transcript Electromagnetic Induction

Electromagnetic Induction
Magnetic Fields Can Move
Charges
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A moving charge or a group of moving charges (a
current) produce a magnetic field—flowing current
will deflect a magnet
Magnets can cause forces on moving charges—a
magnetic field can deflect a current carrying wire
This force is always at right angles to the current and
the magnetic field
Voltage in Wires Moving in Magnetic
Fields
Charges at ends of rod
exert electrostatic force
on any charge q in rod.
DV = vBL
Magnetic Force on Induced Current
F = IBL
Magnetic force to the left
resists push to the right by
the hand.
A Current Generator
The First Generator
Michael Faraday
(1791-1867)
Magnetic Flux
Φ = BA cos θ
Φ = BA
Changing Flux
Wire is pulled to the right by an unseen force.
Induced emf = vBL
Changing Flux and Faraday's Law
Faraday's Law:
The induced emf in a single
coil of wire is
∆Φ / ∆ t
Φ = BA
∆Φ = ∆ (BA)
= BDA
= B(L∆x)
(1)
(2)
(3)
(4)
∆Φ / ∆ t = BL ∆ x / ∆ t
(5)
= BLv
(6)
If there is a stack of N
loops of wire, then the
induced emf is N times
as great:
Induced emf = NDF/Dt
Faraday's Law Example
This time, DF changes because
the B-field changes.
DF = (DB)A
---------------------------------Example: B0 = 0.04 T
B = 0.07 T
DB = 0.03 T
A = 0.004 m2
Dt = 0.005 s
DF = (0.03)(0.004)
= 1.2 x 10-3 T-m2
Induced emf = DF/D t
F = BA
Induced emf = DF/Dt
= 1.2 x 10-3 /0.005
= 0.24 V
Faraday's Law Examples
Flux through coil
changes because bar
magnet is moved up
and down.
AC current in bottom coil
causes changing B-field
along iron core.
Flux Changing by Changing
Areas
F = BA
DF = B (DA)
(1)
(2)
Magnetic field doesn't
change; area changes.
Induced emf = N DF/Dt
The more quickly the
loop is stretched, the
smaller will be Dt and
the larger will be the
transient emf
Do Now (1/24/14):
1.
2.
What is Faraday’s Law?
A magnetic field increases by 2T over a
time of 5 s. The magnetic field is
enclosed by a circular loop of diameter 5
cm. What is the induced EMF in the wire?
Changing Magnetic Fields Cause
Changing Flux
DF changes in this case
because the magnet is
brought closer to the loop;
consequently, more B
arrows penetrate the plane
|of the loop.
Flux F increases.
This is the field of the
induced current.
Induced emf = N DF/Dt
Changing Magnetic Fields
Magnet is removed
from the loop;
consequently, fewer B
arrows penetrate
the plane|of the loop.
Flux F decreases.
Induced emf =
NDF/Dt
This is the field of the
induced current.
Practice:
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Work with your peers on the three AP
problems. We will finish working on them
on Monday.
Radio Antenna Uses Faraday's
Law
Radio waves are electromagnetic. The oscillating B-field of the EM
waves induce an emf in the coil.
The Induction Stove
• A changing flux through
the bottom of the metal
pot generates an emf
which causes current to
circulate around the
bottom of the pot.
• I2R heat is dissipated in
metal pot, but not in the
glass pot or the stove top
because they're
insulators.
Ground Faults
Current takes shorter path to
ground by going through
hand, arm and legs of the
person.
A properly grounded wire
will prevent this. Faulty
grounding can be overcome
by a "ground-fault circuit
interrupter"
(GFCI or GFI)
Ground Fault Circuit Interrupters
Explained
Net current through iron
ring is zero unless there's
a short in the circuit.
If the return current is less
than the entering current,
a magnetic field will be
suddenly be created
in the iron ring, and this
changing field will induce
an emf in the sensing
coil.
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Electric Guitar's Pick-Up Coil
Vibrating string changes flux through coil, inducing an
emf which oscillates in rhythm with the guitar string
Playback Read Head in a Tape
Player -- Moving Coil Microphone
Iron core becomes
magnetized, causing the
flux through the coil to
change
Fluctuating air pressure moves
diaphragm with coil attached
alternately toward and away
from magnet
Writing Data on Magnetic Disk
Lenz's Law
An induced emf always gives rise to a
current whose magnetic field opposes the
original change in flux
Cause: Magnet moving to the right
Effect: Coil becomes an electromagnet
to oppose movement of bar.
Rule: "see counterclockwise, see north"
Another way to look at it:
Cause: More B-arrows puncture plane
Magnet is brought closer,
increasing the number of Bfield penetrations of the plane
of the loop.
Effect: Induced electromagnet creates
its own B-field arrows pointing
in the opposite direction, partially
canceling the increase.
Practice:
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Work with your peers on the three AP
problems. We will finish working on them
on Monday.
Do Now (1/27/14):
Use last week’s Do Now sheet, please!!!
 In your own words, describe Lenz’s Law
DO NOT TOUCH MATERIALS UNTIL
INSTRUCTED TO DO SO!!!
Copper Tube Demo
1.
2.
3.
Theoretically, if you drop the pencil and the
magnet at the same time, which will hit the
ground first and why?
Predict what will happen if you drop your pencil
inside the tube.
Predict what will happen if you drop the magnet
inside the tube.
Copper Tube Demo
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Please DO NOT drop the magnets!!!
1.
Is the magnet attracted to the copper tube? Why or why
not?
Drop your pencil inside of the copper tube. Is this faster
or slower than you would expect from normal free fall?
Why?
Describe what happens as the magnet falls through the
copper tube. Make a guess as to why this occurs.
2.
3.
Review: Lenz’s Law
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When an emf is generated by a change in
magnetic flux according to Faraday's Law,
the polarity of the induced emf is such that
it produces a current whose magnetic field
opposes the change which produces it.
The induced magnetic field inside any loop
of wire always acts to keep the magnetic
flux in the loop constant.
Review: Lenz’s Law
Lenz’s Law: In the examples below, if the B field is increasing,
the induced field acts in opposition to it. If it is decreasing, the
induced field acts in the direction of the applied field to try to
keep it constant..
Lenz's Law
Cause: Magnet moving away, to the left
Effect: Coil becomes an electromagnet
to attract back the bar magnet.
Rule: "see clockwise, see south"
Another way to look at it:
Cause: Fewer B-arrows puncture plane
Magnet is taken away from
the loop, decreasing the
number of B-field
penetrations of the plane of
the loop.
Effect: Induced electromagnet creates
its own B-field arrows pointing in the
same direction as the bar magnet's field,
partially canceling the loss of B arrows.
A second way to look at it:
Lenz's Law
The induced current as viewed
from the left is clockwise, making
the left face of loop the south
pole, which is repelled by the
south pole of the electromagnet.
(Effect opposes cause.)
A third way:
Growth of counter-clockwise current
is opposed by growth of clockwise
current.
Cause: Increase in flux
Effect: Induced current in loop creates a magnetic
field (not shown) which partially cancels flux
Cause: Decrease in flux
Lenz's Law
Effect: Induced current in loop
creates a magnetic field
(not shown) which
partially restores flux
Lenz's Law
“See counterclockwise, see north"
Ring on left acts like a magnet
with a north face on top to repel
the falling magnet (effect
opposing cause)
As viewed from above is
current in ring clockwise, or
counter-clockwise?
What happens in the split ring?
Lenz's Law
Cause: bar magnet moving
away.
Effect: induced electromagnet's
polarity will be such that it will
try to attract the magnet back.
What will be the polarity, north,
or south, of the left face of the
induced electromagnet?
Lenz's Law
Current is suddenly established in wire at bottom. What is
the direction--clockwise, or counterclockwise--in the
loop?
Lenz's Law
What will be the direction of the current in the resistor when
the switch is closed?
Hint: what will be the polarity of the right face of the first
magnet?
Faraday's and Lenz's Laws
An emf is generated only
if the flux is changing.
Note that current is
zero while the loop is
completely inside the
magnetic field.
Practice:
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Continue working with your peers on the
AP practice problems. We will discuss
them and grade them as a class.
Do Now (1/28/14):
Consider a flat square coil with N = 5 loops.
The coil is 20 cm on each side, and has a
magnetic field of 0.3 T passing through it.
The plane of the coil is perpendicular to the
magnetic field: the field points out of the
page.
(a) If nothing is changed, what is the induced
emf?
(b) The magnetic field is increased uniformly
from 0.3 T to 0.8 T in 1.0 seconds. While the
change is taking place, what is the induced
emf in the coil?
Do Now (solutions)
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A.
Do Now (solutions)
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B. Probably the most straight-forward way
to approach this is to calculate the initial
and final magnetic flux through the coil.
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The induced emf is then:
Taking it Further:
(c) While the magnetic field is changing, the
emf induced in the coil causes a current to
flow. Does the current flow clockwise or
counter-clockwise around the coil?
Taking it Further: Apply Lenz's
law, as well as the right-hand rule.
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While the magnetic field is being changed, the
magnetic flux is being increased out of the page.
According to Lenz's law, the emf induced in the
loop by this changing flux produces a current that
sets up a field opposing the change.
The field set up by the current in the coil, then,
points into the page, opposite to the direction of the
increase in flux.
To produce a field into the page, the current must
flow clockwise around the loop. This can be found
from the right hand rule.
Lenz’s Law:
Taking it Further:
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One way to apply the rule is this: Point the
thumb on your right hand in the direction
of the required field, into the page in this
case. If you curl your fingers, they curl in
the direction the current flows around the
loop - clockwise.
AC Generator Explained
Electrons in opposite sides of loop are
moving in opposite directions, so the
forces on them are oppositely directed.
A half-cycle later, those electrons are
moving in the opposite direction.
Electric Generator
Counter Torque
Principles of Transformer Action
Iron Core Transformer
This is a "step-up" transformer because the voltage is increased
Power Transmission and
Transformers
Output power = IV
-------------------Why is output at low
current and high
voltage, and not
high current and
low voltage?
-------------------Answer: I2R losses
Automobile Ignition System
Spark plug gap is about 1/50 inch
(0.020 in), or about 1/20 cm.
Breakdown strength of air is
30,000 V/cm, so 1500 V is
required across the 1/20 cm
spark plug gap.
If there are 1000 turns on the
primary, how many turns must
the secondary have?
Transformer Station and
Telephone Pole Transformer
Steps down from 8000 V to 240 V
Steps down from 240,000 V to 8000 V