1. Principles & Applications
Electricity
Principles & Applications
Sixth Edition
Richard J. Fowler
Chapter 7
Magnetism and
Electromagnetism
© Glencoe/McGraw-Hill

2. INTRODUCTION Magnetic Forces Electromagnetism Induced Voltage
Magnetic Domains
Magnetic Forces
Electromagnetism
Induced Voltage
Reluctance
Magnetic Units

3. Facts About Magnetism Magnetism is an invisible force field.
Most magnets have a north pole and a south pole.
Flux leaves the north pole and enters the south pole.
Like poles repel; unlike poles attract.
Magnetic materials have magnetic domains.
Current-carrying conductors produce magnetic fields.
Magnetic flux follows the path of least reluctance.

4. Magnetic Domains in a Magnetic Field
(case 1) S N S N S N
Domains are randomly arranged in this
unmagnetized temporary magnetic material.
When subjected to a magnetic force field,
the domains are aligned with the field.
When the magnetic force field is removed,
the domains return to a random arrangement.

5. Magnetic Domains in a Magnetic Field
(case 2) S N S N S N
Domains are randomly arranged in this
unmagnetized permanent magnetic material.
When subjected to a magnetic force field,
the domains are aligned with the field.
When the magnetic force field is removed,
the domains remain aligned.

6. Reaction of Like Poles S N
Like poles produce a repelling force on these held-in-place magnets. The flux loops are distorted.

7. Reaction of Like Poles S N S N Like poles produce a repelling force
on these held-in-place magnets.
When freed, the magnets move apart
and the distortion decreases.

8. Reaction of Like Poles S N S N Like poles produce a repelling force.
The magnets move apart and the distortion decreases.

9. Reaction of Like Poles S N S N Like poles produce a repelling force.
The magnets move apart and the distortion decreases.

10. Magnetism Quiz In non-magnetized magnetic materials
the_____ are randomly arranged.
domains
Most magnets have ____ poles.
two
Flux leaves the ____ pole of a magnet.
north
Like magnetic poles ____ each other.
repel
Magnetic materials have magnetic ____.
domains
A magnetic force field contains ____.
flux

11. Electromagnetism A larger current produces more flux.
Current in a conductor
produces flux around the conductor.
A larger current
produces more flux.
Conductor
Direction
of current
Use the left-hand rule to determine
the direction of the flux.

12. Flux Around Parallel Conductors
The conductors repel each other when
the currents are in opposite directions.

13. Flux Around Parallel Conductors
The conductors repel each other when
the currents are in opposite directions.

14. Flux Around Parallel Conductors
The conductors repel each other when
the currents are in opposite directions.

15. Flux Around Parallel Conductors
The conductors repel each other when
the currents are in opposite directions.

16. Flux Around Parallel Conductors
The conductors attract each other when
the currents are in the same direction.

17. Flux Around Parallel Conductors
The conductors attract each other when
the currents are in the same direction.

18. Flux Around Parallel Conductors
The conductors attract each other when
the currents are in the same direction.

19. Repulsion of Conductors When Current is in Opposite Directions
= direction of current for
first ½ of each cycle
= direction of current for
alternate ½ cycles
As shown in the next slide, the conductors
repel each other when power is applied

20. Click on the image to run the display.
To rerun the display, click again on the image.

21. Attraction of Conductors When Current is in the Same Direction
Two-conductor cables
= direction of instantaneous current
As shown in the next slide, the two conductors in each
cable will be pulled together when power is applied.

22. Click on the image to run the display.
To rerun the display, click again on the image.

23. Electromagnetism Quiz
Current-carrying conductors produce
magnetic ____.
flux
The flux around a conductor ____
as the current increases.
increases
The ____ rule is used to determine
the flux direction around a conductor.
left-hand
Parallel conductors ____ each other when
the currents are in the same direction.
attract

24. Low Reluctance Flux Path
Iron
Flux
Flux bends to follow a low reluctance path.

25. Electromagnet With Extended Center Leg
The next slide shows the
results of dropping three
different rings on the rod.
The iron ring is both magnetic
and a conductor. Notice it is
hard to remove from the center
leg where the field is strongest.
This ring also has an induced
current that creates a repelling
field that is too weak to over-
come the magnetic attraction.
The aluminum ring produces a “floating action”. It is non-magnetic and its induced current creates an opposing
field that counteracts gravity.
The plastic ring produces
no reaction because it is
both nonmagnetic and
an insulator.
Iron rod setting on the
center leg of the core
Iron core with I
laminations removed
Coil connected to an ac source

26. Click on the image to run the display.
To rerun the display, click again on the image.
The next slide shows the behavior of a larger aluminum ring.

27. Click on the image to run the display.
To rerun the display, click again on the image.

28. Adding a Low-Reluctance Path in a Magnetic FieldS
Opposite magnetic poles produce a flux in the air between the poles.
When a low-reluctance path is provided, the flux increases and is concentrated in the low-reluctance path.

29. Induced Voltage X
Voltage is induced when conductors cut lines of flux.
Green arrow shows direction of conductor movement.
Dot and X show direction of current caused by voltage.
No voltage is induced when conductors move parallel to the to the lines of flux.

30. Magnetic Quantities and Units
magnetomotive force (mmf)
ampere-turn (A×t)
magnetic field strength (H)
ampere-turn/meter (A×t/m)
flux (f)
weber (Wb)
flux density (B)
tesla (T)
permeability (m)
Wb/(A×t×m)
relative permeability
unitless

31. Magnetic Field Strength
mmf = 3 A x 8 t = 24 A· t
8 t
3 A
Core length = 0.4 m
mmf = 3 A x 4 t = 12 A· t
24 A· t
H = =
60 A· t / m
Core length = 0.2 m
0.4 m
12 A· t
H = =
60 A· t / m
3 A
0.2 m
The magnetic field strength is the same for the two circuits.

32. A B Flux Density Material B carries twice as much flux as material A.
However, the flux density (B) is the same in both materials because the cross-sectional area of B is two times larger than that of A.

33. Quantities and Units Quiz
The opposition to flux is called ____.
reluctance
The unit of flux is the ____.
weber
Flux density is specified in ____.
tesla
Wb/(A×t×m) is the unit for ____.
permeability
The symbol for magnetic field strength is ____. H
The symbol for flux density is ____. B
The value of H is ____ when I = 4A ,
N = 12t , and l = 0.2m.
240 A×t/m

34. REVIEW Magnetic Domains Magnetic Forces Electromagnetism
Induced Voltage
Reluctance
Magnetic Units