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Chapter
14
Electromagnetism
Topics Covered in Chapter 14:
14-1: Ampere-turns of Magnetomotive Force (mmf)
14-2: Field Intensity (H)
14-3: B-H Magnetization Curve
14-4: Magnetic Hysteresis
14-5: Magnetic Field around an Electric Current
Topics Covered in Chapter 14
 14-6: Magnetic Polarity of a Coil
 14-7: Motor Action between Two Magnetic Fields
 14-8: Induced Current
 14-9: Generating an Induced Voltage
 14-10: Relays
McGraw-Hill
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
14-1: Ampere-turns of
Magnetomotive Force (mmf)
 The strength of a coil’s magnetic field is proportional to
the amount of current flowing through the coil and the
number of turns per given length of coil.
 Ampere-turns = I × N = mmf
 I is the amount of current flowing through N turns of
wire.
 This formula specifies the amount of magnetizing force
or magnetic potential (mmf).
14-1: Ampere-turns of
Magnetomotive Force (mmf)
 The SI abbreviation for
ampere-turn is A · t.
Fig. 14-1: Two examples of equal ampereturns for the same mmf. (a) IN is 2 × 5 = 10.
(b) IN is 1 × 10 = 10.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-2: Field Intensity (H)
 The length of a coil influences the intensity of a
magnetic field. Intensity is different from mmf.
 Equation: H = mmf/length
 Units: A·t/m
ampere-turns per meter
 H is the mks unit
 Shorter magnetic circuits produce a greater field
intensity
14-2: Field Intensity (H)
Conductance (G)
V
I
How well current can go through a wire?
14-2: Field Intensity (H)
H
B
Permeability ():
How easily magnetic flux can be set up inside a piece
of material?
14-2: Field Intensity (H)
 Permeability () is a measure of the ability to
concentrate magnetic fields.
 Materials with high permeability can concentrate flux,
and produce large values of flux density B for a
specified H.
 The amount of flux produced by H depends on the
material in the field.
 These factors are reflected in the formulas:
 B=×H
 μ=B/H
T
 The unit is teslas per ampere-turn per meter: A · t/m
14-2: Field Intensity (H)
 Ferromagnetic materials have high values of
permeability (as high as 10,000).
 Paramagnetic materials The permeability is slightly
more than 1.
 Diamagnetic materials The permeability is less
than 1.
 Non-magnetic materials The permeability is
approximately 1.
14-3: B-H Magnetization Curve
 The B-H magnetization
curve shows how
much flux density B
results from increasing
field intensity H.
 Saturation is the effect
of little change in flux
density when the field
intensity increases.
Fig. 14-3: B-H magnetization curve for
soft iron. No values are shown near
zero, where μ may vary with previous
magnetization.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-2: Field Intensity (H)
Permeability drops drastically
at saturation.
+B
+ Bmax
Slope of
B/H is large
+H
Slope of
B/H is small
-H
- Bmax
-B
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-4: Magnetic Hysteresis
 Hysteresis Loop
 BR is due to retentivity
(memory), which is the
flux density remaining
after the magnetizing force
is reduced to zero.
 Note that H = 0 but B > 0.
 HC is the coercive force
(needed to make B = 0)
Fig. 14-4: Hysteresis loop for magnetic materials.
This graph is a B-H curve like Fig. 14-3, but H
alternates in polarity with alternating current.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-2: Field Intensity (H)
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html
14-4: Magnetic Hysteresis
 Hysteresis refers to a situation where the magnetic flux
lags the increases or decreases in magnetizing force.
 Hysteresis loss is energy wasted in the form of heat
when alternating current reverses rapidly and molecular
dipoles lag the magnetizing force.
 For steel and other hard magnetic materials, hysteresis
losses are much higher than in soft magnetic materials
like iron.
14-4: Magnetic Hysteresis
 Demagnetization: To demagnetize a magnetic material
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completely, the retentivity BR must be reduced to zero.
To demagnetize a magnetic material completely, the
retentivity BR must be reduced to zero.
A practical way to do so is to magnetize and demagnetize the
material with a decreasing hysteresis loop.
This method of demagnetization is called degaussing.
Applications of degaussing include:
 Metal electrodes in a color picture tube
 Erasing the recorded signal on magnetic tape.
14-4: Magnetic Hysteresis
 A magnetic field is produced by alternating current.
 The magnetic field and the magnetic material are moved
away from each other, or the current amplitude is reduced.
 The hysteresis loop then becomes smaller and smaller until it
effectively collapses.
14-5: Magnetic Field around an
Electric Current
 The left-hand rule for conductors: Grasp the
conductor with your left hand so the thumb points in
the direction of electron flow. Your fingers will encircle
the conductor in the same direction as the circular
magnetic field lines.
14-6: Magnetic Polarity of a Coil
 Bending a straight conductor into the form of a loop
produces two effects:
 The magnetic field lines are more dense inside the
loop.
 All the lines inside the loop aid in the same direction.
 This makes the loop field the same as a bar magnet,
with opposite poles at opposite faces of the loop.
14-6: Magnetic Polarity of a Coil
Fig. 14-10: Left-hand rule for north pole of a coil with current I. The I is electron flow.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-7: Motor Action between Two
Magnetic Fields
Fig. 14-13: Motor action of current in a straight conductor when it is in an external magnetic
field. The HI is the circular field of the current The HM indicates field lines between the north and
south poles of the external magnet.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-8: Induced Current
 Generator Action
 When a moving conductor cuts across flux lines, a
voltage is induced.
 The amount of induced voltage is proportional to:
 The conductor velocity
 The amount of flux
 The number of turns of wire
14-8: Induced Current
Fig. 14-15: Induced current produced by magnetic flux cutting across turns of wire in a coil.
Direction of I here is for electron flow.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-9: Generating an
Induced Voltage
 The motion of flux across a conductor in an open circuit
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
forces free electrons to move.
Since the ends are open, electrons accumulate at them,
creating a potential difference.
The potential difference is an electromotive force
(emf), generated by the work of cutting across the flux.
Induced emf increases with the number of turns in a
coil.
The polarity of the induced voltage follows from the
direction of induced current.
14-9: Generating an
Induced Voltage
 Faraday’s Law of Induced Voltage
 The amount of induced voltage can be calculated by
Faraday’s law:
dΦ (webers)
vind = N
dt (seconds)
 N = number of turns
 dΦ/dt = how fast the flux cuts across the conductor.
14-9: Generating an
Induced Voltage
 Consider a magnetic flux cutting a
conductor that is not in a closed
circuit, as shown in Figure 14-16.
 The motion of flux across the
conductor forces free electrons to
move.
 The potential difference is an
electromotive force (emf) generated
and only present while the motion of
flux is cutting across the conductor.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-9: Generating an
Induced Voltage
 Faraday’s Law of Induced Voltage
 The amount of induced voltage is calculated by
Faraday’s law:
d(Wb)
vind = N
dt(seconds)
 The induced voltage is directly proportional to the
number of turns times d/dt.
 To generate more voltage:
 Increase the number of turns
 Increase the flux (stronger magnet)
 Decrease the time (faster movement of the coil)
14-9: Generating an
Induced Voltage
 The polarity of induced voltage is determined by Lenz’s law.
 The induced voltage has the polarity that opposes the
change causing the induction?????????
 Movement of a wire/coil in the presence of magnetic field
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induces current/voltage.
Induced current/voltage interacts with the magnetic field to
produce a force.
The direction of the force depends on polarity of the induces
current/voltage.
The direction of the force is opposite to the direction of the
movement of the wire/coil.
It will try to slow down the wire/coil.
14-10: Relays
 A relay is an electromechanical device that operates on
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the basis of electromagnetic induction.
It uses an electromagnet to open or close one or more
sets of contacts.
Relays, like switches, have poles and throws.
Relays can switch or control high power loads with a
low amount of input power.
In remote-control applications, relays can control high
power loads long distances away more efficiently than
can mechanical switches.