Electromagnetic Induction

1
Electromagnetic Induction

• Motional emf
• Faradays law
• Examples
• Generator

2
Reading Question
Currents circulate in a piece of metal that is
pulled through a magnetic field. What are these
currents called?
1. Induced currents 2. Displacement currents 3.
Faradays currents 4. Eddy currents 5. This topic
is not covered in Chapter 33.
3
Reading Question
Currents circulate in a piece of metal that is
pulled through a magnetic field. What are these
currents called?
1. Induced currents 2. Displacement currents 3.
Faradays currents 4. Eddy currents 5. This topic
is not covered in Chapter 33.
4
Reading Question
Electromagnetic induction was discovered by
1. Faraday. 2. Henry. 3. Maxwell. 4. Both Faraday
and Henry. 5. All three.
5
Reading Question
Electromagnetic induction was discovered by
1. Faraday. 2. Henry. 3. Maxwell. 4. Both Faraday
and Henry. 5. All three.
6
Reading Question
The direction that an induced current flows in a
circuit is given by
1. Faradays law. 2. Lenzs law. 3. Henrys
law. 4. Hertzs law. 5. Maxwells law.
7
Reading Question
The direction that an induced current flows in a
circuit is given by
1. Faradays law. 2. Lenzs law. 3. Henrys
law. 4. Hertzs law. 5. Maxwells law.
8
Electromagnetic Induction

• Motional emf

9
Student Workbook
10
Student Workbook
11
Student Workbook
12
Student Workbook
13
Student Workbook
14
Student Workbook
15
Electromagnetic Induction

• Magnetic Flux

16
Student Workbook
17
Student Workbook
18
Student Workbook
19
Electromagnetic Induction

• Faradays Law
• On the table you will find a bar magnet and a
  coil. The coil is connected to channel A on the
  Pasco Interface so that we can monitor the
  voltage across the coil. Turn on the interface
  and PC. Start DataStudio and configure the
  system by clicking and dragging the analog sensor
  plug icon to the channel A input. Choose the
  voltage sensor and connect to channel A. Next
  select the graph display icon and drag it to the
  channel A icon. Set the time scale maximum to 10
  sec. and the voltage scale maximum to 1.0 V.

20
Electromagnetic Induction

• Faradays Law
• On the graph below draw (dash line) your
  prediction for the emf due to thrusting a
  magnetic into the center of the coil with the
  north pole toward the coil. Do not withdraw the
  bar magnetic but stop the bar magnetic in the
  center of the coil after thrusting it in.

coil
21
Electromagnetic Induction

• Faradays Law
• Now lets do the experiment. Start the recording
  and thrust the bar magnet into the center of the
  coil with the north pole toward the coil. Do not
  withdraw the magnet and stop the recording.
  Observe the deflection of the graph, both the
  magnitude and the sign. How does your prediction
  compare with the measurements? Draw the results
  on the axis above with your prediction.

22
Electromagnetic Induction

• Faradays Law
• Now try different speeds. Draw the voltage for
  two different speeds on the same graph above.
  Describe what happens.

Get actual data
23
Electromagnetic Induction

• Faradays Law
• Now try to predict the curve if you reverse the
  magnetic so that the south pole is toward the
  coil. Draw the curve on the graph on the next
  page.
• Reverse the poles so that the south pole is
  toward the coil and thrust the bar magnetic to
  the center of the coil. Draw the trace below.

24
Electromagnetic Induction

• Faradays Law
• Now try this. Place the bar magnet so that the
  north pole is inside the coil. Start recording
  and pull the bar magnetic out of the coil (south
  pole first). Draw your prediction below.
• Now do the experiment.

25
Electromagnetic Induction

• Faradays Law
• This phenomena is called electromagnetic
  induction and is described by Faradays law.
  Write Faradays law.
• In order to understand Faradays law we need to
  understand the concept of magnetic flux. Write
  the definition of magnetic flux.

26
Electromagnetic Induction

• Faradays Law
• The magnetic flux is exactly like the electric
  flux we studied in Gausss law. The flux is
  defined in terms of a vector area dA. Describe
  the magnitude and direction of this vector.

27
Electromagnetic Induction

• Lenzs Law
• The direction of the emf and thus the current is
  given by Lenzs law. The statement in bold in
  the center of page 789 is a statement of Lenzs
  law. Use this to find the direction of the
  current. If you are looking down on the loop
  from above, is the current flowing clockwise or
  counter clockwise? Explain.

28
Electromagnetic Induction

• Lenzs Law

The magnetic is moving away from the coil so the
magnetic field is decreasing, thus the current is
in a direction to off-set the decrease.
The magnetic is moving toward the coil so the
magnetic field is increasing, thus the current is
in a direction to off-set the increase.
29
Electromagnetic Induction

• Faradays Law

Does it make a difference if it is the magnetic
moving or the coil? This was a major point in
Einsteins theory of relativity.
30
Electromagnetic Induction

• Faradays Law
• What about these two cases?

31
Electromagnetic Induction

• Faradays Law
• Discuss the following experiment in your group.
  What will happen if you drop the bar magnet
  through the coil with the north pole toward the
  coil. Use a dash line to draw what you expect to
  see.

32
Electromagnetic Induction

• Faradays Law
• Now do the experiment. Do not let the bar magnet
  hit the floor. The bar magnet will lose its
  magnetism if it hits the floor. Draw the results
  on the axis above. Use a solid line. How did
  you do? If your prediction was different
  discuss the results to make sure you all
  understand.

33
Electromagnetic Induction

• Faradays Law
• Show that when you integrate the emf, e with
  respect to time you get the average change in
  flux in time ?t.

Average value
34
Electromagnetic Induction

• Faradays Law
• Now drop the magnet through the coil again and
  use Data Studio to integrate the voltage curve
  for the two peaks. How do the two compare?
  Write the answer here.
• Why is the maximum for the second peak larger in
  magnitude than the first?

35
Electromagnetic Induction

• Problem
• A circular wire loop with a radius of 20 cm. is
  in a constant magnetic field of 0.5 T .
• What is the flux through
• the loop if the normal to
• the loop makes an angle
• of 300 with the magnetic
• field?

36
Electromagnetic Induction

• Problem
• The magnetic field increases from 0.5 T to 2.5 T
  in 0.8 seconds. What is the average emf, e(t)
  induced in the loop.

37
Student Workbook
38
Student Workbook
39
Student Workbook
40
Student Workbook
41
Student Workbook
42
Student Workbook
43
Class Questions
Is there an induced current in this circuit? If
so, what is its direction?
1. Yes, clockwise 2. Yes, counterclockwise 3. No
44
Class Questions
1. F2 F4 gt F1 F3 2. F3 gt F2 F4 gt F1 3.
F3 gt F4 gt F2 gt F1 4. F4 gt F2 gt F1 F3 5. F4 gt
F3 gt F2 gt F1
45
Class Questions
A current-carrying wire is pulled away from a
conducting loop in the direction shown. As the
wire is moving, is there a cw current around the
loop, a ccw current or no current?
1. There is a clockwise current around the
loop. 2. There is a counterclockwise current
around the loop. 3. There is no current around
the loop.
46
Class Questions
A conducting loop is halfway into a magnetic
field. Suppose the magnetic field begins to
increase rapidly in strength. What happens to the
loop?
1. The loop is pushed upward, toward the top of
the page. 2. The loop is pushed downward, toward
the bottom of the page. 3. The loop is pulled to
the left, into the magnetic field. 4. The loop is
pushed to the right, out of the magnetic
field. 5. The tension is the wires increases but
the loop does not move.
47
Electromagnetic Induction

• Example Induction stove
• The pan on the stove is heated by eddy currents
  produced by induction.

• Would this stove work with a ceramic bowl?
• Does the surface of the stove get hot?

48
Electromagnetic Induction

• Faradays Law
• Now lets do another example of Faradays law.
  What happens when you pull a coil out of a
  magnetic field? Use the power supply and connect
  it to the large coil on the table. Turn the
  voltage knob to zero (counter clockwise) and turn
  on the supply. Make sure the current limit
  switch is on high and the current knob all the
  way clockwise to full range. Now increase the
  voltage to 30 V and the current should be about
  0.5 A. We are using this large coil to create a
  magnetic field. Turn the supply off.
• What direction is the field pointing when the
  supply is on? Use the current direction to find
  the field direction.

49
Electromagnetic Induction

• Faradays Law picture would help
• Get the small coil and place the coil above the
  large coil and in the center. Pull the coil
  horizontally out of the field. Discuss what you
  expect the voltage in the small coil to look
  like. Draw what you expect below. Assume
  positive voltage is when the emf and current is
  clockwise looking down on the coil.

50
Electromagnetic Induction

• Faradays Law
• Connect the Pasco interface voltage leads to the
  small coil and place the coil in the center and
  over the large coil. Turn on the supply, start
  DataStudio recording and pull the coil out of the
  field. Stop recording. Draw the measured
  voltage on the axis above. Does the measured
  voltage agree with your prediction?

51
Electromagnetic Induction

• Faradays Law
• What will happen if you flip the small coil by
  180 degrees and repeat the experiment? Draw what
  you expect below.
• Now turn on the supply and do it again. Draw the
  measured results. Does it agree with your guess?

52
Electromagnetic Induction

• Faradays Law
• You want to try one more. This time place the
  small coil on top of the large coil and increase
  the voltage from zero to 30 V in 1 or 2 seconds.
  Do it again for 5 seconds and 10 seconds. Record
  all three on the same graph below.
• Can you explain the change in the curves?
  Discuss this in your group and explain it below.

53
Electromagnetic Induction

• Application of Faradays Law
• Generator

54
Electromagnetic Induction

• Application of Faradays Law
• Magnetic Recording

55
Electromagnetic Induction

• The Most Important Point of Faradays Law
• A changing magnetic field produces
• or creates an electric field.

Two types of electric fields. One is created by
charge and the other is created by a changing
magnetic field.