Chapter 7
Magnetism and
Electromagnetism
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C.Y. Lee
Objectives
Explain the principles of the magnetic field
Explain the principles of electromagnetism
Describe the principle of operation for several
types of electromagnetic devices
Discuss the principle of electromagnetic induction
Describe some applications of electromagnetic
induction
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The Magnetic Field
A permanent magnet has a magnetic field
surrounding it
A magnetic field is envisioned to consist of flux lines
that radiate from the north pole to the south pole and
back to the north pole through the magnetic material
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Attraction and Repulsion
Unlike magnetic poles have an attractive force
between them
Two like poles repel each other
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Altering a Magnetic Field
When nonmagnetic materials such as paper, glass,
wood or plastic are placed in a magnetic field, the
lines of force are unaltered
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Altering a Magnetic Field
When a magnetic material such as iron is placed
in a magnetic field, the flux lines tend to be altered
to pass through the magnetic material
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Magnetic Flux
The force lines going from the north pole to the
south pole of a magnet are called magnetic flux
(φ); units: weber (Wb)
The magnetic flux density (B) is the amount of
flux per unit area perpendicular to the magnetic
field; units: tesla (T)
1 T = 10000 G (Gauss)
B=
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φ
A
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Magnetic Flux
B=
φ
A
Example: Find the flux density in a magnetic field in
which the flux in 0.1 m2 is 800 μWb
φ
800 μWb
B= =
= 8000 μT
2
A
0.1m
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Magnetizing Materials
Ferromagnetic materials such as iron, nickel and
cobalt have randomly oriented magnetic domains,
which become aligned when placed in a magnetic
field, thus they effectively become magnets
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Application (Magnetic Switch)
Operation of a
magnetic switch
Connection of a
typical perimeter
alarm system
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Electromagnetism
Electromagnetism
is the production
of a magnetic
field by current in
a conductor
The right-hand rule is used to determine the
direction of the lines of force
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Electromagnetic Field
When current passes through a conductor, an
electromagnetic field is created around the conductor
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Electromagnetic Devices
Electromagnets are used in devices such as magnetic
disk, electric motors, speakers, solenoids, and relays
Read/write function on a magnetic surface
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Electromagnetic Devices
The solenoid is used for applications such as opening
and closing valves and automobile door locks
Basic solenoid operation:
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Electromagnetic Devices
Relays differ from solenoids in that the electromagnetic
action is used to open or close electrical contacts rather
than to provide mechanical movement
Basic structure of a relay:
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Electromagnetic Devices
Permanent-magnet speakers are commonly used in
stereos, radios, and TVs, and their operation is based on
the principle of electromagnetism
Basic speaker operation with movement:
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Electromagnetic Induction
When a conductor is moved through a magnetic field,
a voltage is induced across the conductor
This principle is known as electromagnetic induction
The faster the relative motion, the greater the induced
voltage
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Relative Motion
When a magnetic field is moved past a stationary
wire, there is also relative motion
The induce voltage depends on the rate of relative
motion between the wire and the magnetic field
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Polarity of Induced Voltage
The direction of movement of a wire through a
magnetic field will affect the polarity of the induced
voltage as indicated below:
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Induced Current
The voltage induced by the relative motion of a wire
through a magnetic field will cause a current in a load
connected to the wire
This current is called an induced current
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Motor Action
When a current flows through a wire that is in a
magnetic field, the direction of current flow will result
in the magnetic field being weakened on one side, and
strengthened on the other
The result of the weakened and strengthened field is a
force on the conductor
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Faraday’s Law
The voltage induced across a coil of wire equals the
number of turns in the coil times the rate of change of
the magnetic flux
dΦ B
ε = −N
dt
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Lenz’s Law
Lenz’ Law states the polarity of the induced voltage in
a loop is such that it produces a current whose magnetic
field opposes the change in magnetic flux through the
loop
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DC Generator
Basic operation of a dc generator
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Summary
Unlike magnetic poles attract each other, and like
poles repel each other
Materials that can be magnetized are called
ferromagnetic
When there is current through a conductor, it produces
an electromagnetic field around the conductor
The right-hand rule can be used to establish the
direction of the electromagnetic lines of force around a
conductor
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Summary
When a conductor moves within a magnetic field,
or when a magnetic field moves relative to a
conductor, a voltage is induced across the conductor
The faster the relative motion between a conductor
and a magnetic field, the greater is the induced
voltage
The polarity of the induced voltage in a loop is such
that it produces a current whose magnetic field
opposes the change in magnetic flux through the
loop
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