1. Electromagnetic Induction
Created for CVCA Physics By
Dick Heckathorn
13 April 2K + 5

2. What physics principles does this picture illustrate?

3. Purpose for Chapter 19
To investigate how one can generate electrical energy (electricity)
2. Techniques for distribution

4. Only known source of continuous electric potential
19.1 Electromagnetic Age 738
Voltaic Cell
Only known source of continuous electric potential

5. Can a magnetic field cause electrons to move?
19.2 Faraday’s Discovery 738
Question?
Can a magnetic field cause electrons to move?

6. Large magnet – wire – Galvanometer
Demonstration #1
Move a wire
through the jaws
of a
horseshoe magnet.
Results?
Large magnet – wire – Galvanometer

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

8. when the conductor was moving through the magnetic field.
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.

9. Bar magnet – coil – wire – Galvanometer
Demonstration #2
Plunge
a bar magnet
into and out of
the core of a coil.
Bar magnet – coil – wire – Galvanometer

10. Demonstration #2 FMech I B Direction of blue arrows
Magnet’s field has direction of:
Due to mechanical force to right
Or coil moves to right
Magnet plunge into the coil.
B field of magnet at bottom of coil is what direction?
Thus induced current is in direction….

11. Demonstration #2 FMech I B Magnet’s field has direction of:
Direction of blue arrows
Magnet pulled out of the coil.
B field of magnet at bottom of coil is what direction?
Or coil moves to left
Due to mechanical force to left
Thus induced current is in direction….

12. Direction of the Induced Current is:B I FM B I FM B FM
None

13. the bar magnet is moving
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.

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

15. What happens to the meter?
Demonstration #3
Open the switch.
What happens to the meter?
Green or large power source – iron ring or my coils - wire – Galvanometer

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

17. Factors affecting magnitude of induced current
Number of turns of wire in the coil
Strength of magnetic
field of the magnet
3. Rate at which magnetic field changes relative to wire (relative speed)

18. 19.3 Magnitude of Induced Electric Potential 738

19. Ohm’s Law Says:

20. 19.4 Direction of Induced Current: Lenz’s 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

21. inducing field of the magnet.
Lenz Reasoned
The induced current
sets up an
induced magnet field.
This induced field
interacts with
inducing field of the magnet.

22. How do they interact?
Either one or the other.

23. an induced magnetic field in coil The current would produce
Lets assume S
an induced magnetic field in coil
R-hand rule says right end is:
The current would produce S
of bar magnet pulling them together.
attract the N-Pole
S-pole would
Lenz reasoned:
Impossible
Why?

24. Lets look at other option
an induced magnetic field in coil
R-hand rule says right end is:
The current would produce N
Must do work to bring them together
N-pole coil opposes N-pole magnet

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

26. 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

27. What is the pole of magnet?
Left end of coil must be: S
Why?
oppose S-pole of coil
Pole of bar magnet must
Must be: S

28. 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:

29. 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:

30. Polarity of Coil? Direction I ?
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)

31. 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)

32. 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

33. AC Generator B I F Look at segment X-W Force in direction?
Induced current in direction?
B in direction?

34. AC Generator B F Induced current in direction? Force in direction?
Look at segment X-W ½ turn later B F
Induced current in direction?
Force in direction?
B in direction?

35. AC Generator Summary I I

36. DC Generator B I F Force in direction? Induced current in direction?
Look at segment X-W B I F
Force in direction?
Induced current in direction?
B in direction?

37. DC Generator B F I Force in direction? B in direction?
Look at segment X-W ½ turn later B F I
Force in direction?
B in direction?
Induced current in direction?

38. DC Generator - Summary I I

39. Maximizing Output Increase # turns on coil
Winding coil on soft iron core
Increase speed of rotation
Increase strength of B-field

40. electrical generating
19.6 The Transformer
All large scale
electrical generating
systems
generate electricity
using
AC generators.

41. Energy Transmission
12,000 V
240,000 V
2400 V
240 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.

42. % of Electrical Power Wasted
480-kW Power transmitted
10-ohm Resistance
12-kV Electric Potential
P = I2 . R
16 kW
3.33% loss

43. % of Electrical Power Wasted
480-kW Power transmitted
10-ohm Resistance
240-kV Electric Potential
P = I2 . R
40 W
0.0083% loss

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

45. 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.

46. 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.

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

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

49. 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.

50. Green Power Source – Dissectible transformer – 2 multimeters
Demonstration
Repeat the potential difference and current measurements as done with coil within a coil.
Investigate the construction of a dissectible transformer.
Green Power Source – Dissectible transformer – 2 multimeters

51. Designing Transformers
Copper coils – have low R
to reduce power loss
Core – High Permeability
to reduce energy to ΔB in core
Core’s Shape
to maximize induction

53. That’s all folks!