Electromagnetic Induction

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ElectromagneticInduction

• Created for CVCA Physics
• By
• Dick Heckathorn
• 3 May 2K4

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What physics principles does this picture
illustrate?
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Purpose for Chapter 19

• To investigate how one can generate electrical
  energy (electricity)
• 2. Techniques for distribution

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19.1 Electromagnetic Age 738

• Voltaic Cell
• Only known source of continuous electric potential

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19.2 Faradays Discovery 738

• Question?
• Can a magnetic field cause electrons to move?

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Demonstration 1

• Move a wire
• through the jaws
• of a
• horseshoe magnet.
• Results?

Large magnet wire Galvanometer
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A Mechanical Force is
but in the opposite direction.
to the mechanical force
that is equal in magnitude
exerted on a wire in a B field.
This induced current in a B field
gives rise to a magnetic force
An induced current is produced.
N
FMag
FMech
IInd
B
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Demonstration 1

• Electrons only flowed
• when the conductor was moving through the
  magnetic field.
• They were moving in a direction opposite to the
  induced current as the induced current was the
  movement of positive charges.

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Demonstration 2

• Plunge
• a bar magnet
• into and out of
• the core of a coil.

Bar magnet coil wire Galvanometer
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Demonstration 2
FMech
I
B
Or coil moves to right
Magnets field has direction of
Direction of blue arrows
B field of magnet at bottom of coil is what
direction?
Magnet plunge into the coil.
Due to mechanical force to right
Thus induced current is in direction.
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Demonstration 2
FMech
I
B
Thus induced current is in direction.
Magnets field has direction of
Due to mechanical force to left
B field of magnet at bottom of coil is what
direction?
Or coil moves to left
Magnet pulled out of the coil.
Direction of blue arrows
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Direction of the Induced Current is
B
I
FM
B
I
FM
B
FM
None
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Conclusion

• Charges flow only when
• the bar magnet is moving
• into or out of the coil
• or
• when the coil moves
• relative to the magnet.

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Demonstration 3

• Close the switch.

What happens to the meter?
The two wires are not connected.
Anything surprising?
Green or large power source iron ring or my
coils - wire Galvanometer
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Demonstration 3

• Open the switch.

What happens to the meter?
Green or large power source iron ring or my
coils - wire Galvanometer
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Conclusion

• There is an induced current
• only
• when the
• magnetic field
• is changing
• in the iron ring.

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Factors affecting magnitude of induced current

1. Number of turns of wire in the coil
2. Strength of magnetic

• field of the magnet
• 3. Rate at which magnetic field changes relative
  to wire (relative speed)

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Demonstration 3
I
?B
I
Results ?
Close Switch
Induced
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Demonstration 3
I
?B
I
Results ?
Open Switch
Induced
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19.3 Magnitude of Induced Electric Potential 738
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Ohms Law Says
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19.4 Direction of Induced Current Lenzs Law 741

• Know so far?
• S-Pole enters coil
• Current in one direction
• S-Pole removed from coil
• Current in opposite direction

Know so far? S-Pole enters coil Induced current
is in a direction opposite that when N-Pole was
involved
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Lenz Reasoned

• The induced current
• sets up an
• induced magnet field.
• This induced field
• interacts with
• inducing field of the magnet.

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How do they interact?

• Either one or the other.

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Lets assume
S

• The current would produce

an induced magnetic field in coil
R-hand rule says right end is
S
of bar magnet pulling them together.
S-pole would
attract the N-Pole
Impossible
Why?
Lenz reasoned
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Lets look at other option
N

• The current would produce

an induced magnetic field in coil
R-hand rule says right end is
N
N-pole coil opposes N-pole magnet
Must do work to bring them together
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Conclusion

• Add photo here?

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Conclusion

• An induced current
• flows in such a direction
• that the created
• induced field
• opposes the action of
• the inducing field.

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What is direction of Iinduced?

• Lower end of coil must be

N
Why?
Coil must oppose removal of S-pole
N
R-hand rules says current flows
across front of the coil.
to left
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What is the pole of magnet?
S
S

• Left end of coil must be

S
Why?
Pole of bar magnet must
oppose
S-pole of coil
Must be
S
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Polarity of Coil? Direction I ?

• Top of coil must be

N
Why?
Must oppose N-pole of magnet.
N
Current in coil must be (in wire near us)
in direction
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Polarity of Coil? Direction I ?

• Bottom of coil must be

S
Why?
Must oppose S-pole of magnet.
S
Current in coil must be (in wire near us)
in direction
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Polarity of Coil? Direction I ?
N

• R-end of coil must be

N
Why?
Must oppose S-pole of magnet.
Current in coil must be in direction (in wire
near us)
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Polarity of Coil? Direction I ?
S

• R-end of coil must be

S
Why?
Must oppose S-pole of magnet.
Current in coil must be in direction (in wire
near us)
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19.5 Electrical Generators AC and DC 745

• Ready to produce a device
• capable of producing
• a continuous electric current
• and electric potential difference
• by electromagnetic induction

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AC Generator

• Look at segment X-W ½ turn later

B
F
I
Force in direction?
Induced current in direction?
B in direction?
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AC Generator

• Look at segment X-W

B
I
F
Force in direction?
Induced current in direction?
B in direction?
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AC Generator
I
I
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AC Current vs Time
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AC Current vs Time
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AC Generator Direction of I?
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AC Generator Direction of I?
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AC Generator Direction of I?
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AC Generator Direction of I?
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DC Generator

• Look at segment X-W

B
F
I
Induced current in direction?
Force in direction?
B in direction?
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DC Generator

• Look at segment X-W ½ turn later

B
F
I
Force in direction?
Induced current in direction?
B in direction?
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DC Generator
I
I
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Maximizing Output

• Increase turns on coil
• Winding coil on soft iron core
• Increase speed of rotation
• Increase strength of B-field

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19.6 The Transformer

• All large scale
• electrical generating
• systems
• generate electricity
• using
• AC generators.

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Energy Transmission
2400 V
240 V
12,000 V
240,000 V
The voltage must then be reduced to a value that
is acceptable for home usage.
The voltage is increased to reduce the energy
lost as it is transferred through the wires over
a long distance.
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of Electrical Power Wasted

• 10-kW Power transmitted
• 1-ohm Resistance
• 200-V Electric Potential

P I2 . R 2.5 kW 25 loss
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of Electrical Power Wasted

• 10-kW Power transmitted
• 1-ohm Resistance
• 2000-V Electric Potential

P I2 . R 25 W 0.25 loss
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Demonstration

• Measure the potential difference
• of a coil within a coil.
• Have one coil connected to one coil and then the
  other as the measurements are made.
• First with DC and then AC
• Electrical Potential (Voltage)

Green Power Source - Double coil apparatus 2
voltmeters
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The Transformer

• changes the
• electric potential difference (V)
• by varying
• number of windings
• of two different coils
• around a common
• soft iron core.

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Conclusion

• If power source is connected
• to the coil with the
• smaller number of turns,
• the output potential difference
• is greater than the input potential difference.

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Conclusion

• If power source is connected
• to the coil with greater
• number of turns,
• the output potential difference
• is less than the input potential difference.

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Demonstration

• Measure the current
• of a coil within a coil.
• Have one coil connected to one coil and then the
  other as the measurements are made.

Green Power Source - Double coil apparatus 4
multimeters
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Conclusion

• If power source is connected
• to the coil with the
• smaller number of turns,
• the output current
• is less than the input current.

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Conclusion

• If power source is connected
• to the coil with greater
• number of turns,
• the output current
• is greater than the input current.

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Question

• How does the output power compare to the input
  power?
• Did Poutput Pinput ?
• If so, there is a
• Conservation of Energy as the potential
  difference is changed.

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Demonstration

• Investigate the construction of a dissectible
  transformer.

Repeat the potential difference and current
measurements as done with coil within a coil.
Green Power Source Dissectible transformer 2
multimeters
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Designing Transformers

• Copper coils have low R
• to reduce power loss
• Core High Permeability
• to reduce energy to ?B in core
• Cores Shape
• to maximize induction

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Thats all folks!
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