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Vern J. Ostdiek
Donald J. Bord
Chapter 8
Electromagnetism and EM Waves
(Section 3)
8.3 Principles of Electromagnetism
• The interactions between electricity and
magnetism described in the previous section,
along with other similar observations, suggest the
following two general statements.
• We might call these the principles of
electromagnetism:
1. An electric current or a changing electric field
induces a magnetic field.
2. A changing magnetic field induces an electric
field.
8.3 Principles of Electromagnetism
• These two statements summarize the previous
observations and also emphasize the symmetry
that exists.
•
In both cases, a “changing” field means that the
strength or the direction (or both) of the field is
changing.
8.3 Principles of Electromagnetism
• The first principle can be used to explain the first
observation:
•
As a charge moves past a point in space, the
strength of the electric field increases and then
decreases.
• All the time the direction of the field is changing as
well.
•
The effect of this is to cause a magnetic field to be
produced.
• Similarly, the second principle
explains electromagnetic
induction.
8.3 Principles of Electromagnetism
• A transformer is a device used to step up or step
down AC voltages.
•
•
It represents one of the most elegant applications of
electromagnetism.
In essence, a transformer consists of two separate
coils of wire in close proximity.
8.3 Principles of Electromagnetism
• An AC voltage is applied to one of the coils called
the input or primary coil, and an AC voltage
appears at the other coil called the output or
secondary coil.
8.3 Principles of Electromagnetism
• The AC in the primary coil produces an oscillating
magnetic field through both coils.
•
Most transformers have both coils wrapped around
a single ferromagnetic core to intensify the
magnetic field and guide it from one coil to the
other.
• This oscillating (and therefore changing) magnetic
field induces an AC current in the output coil.
•
Note that a DC input would produce a steady
magnetic field that would not induce a current in the
output coil.
• Transformers do not work with DC.
8.3 Principles of Electromagnetism
• Now, how can the voltage of the output be
different from the voltage of the input?
• Each “loop” or “turn” of the output coil has the
same voltage induced in it.
•
•
The voltages in all of the turns add together so that
the more turns there are in the output coil, the
higher the total voltage.
The ratio of the number of turns in the two coils
determines the ratio of the input and output
voltages.
8.3 Principles of Electromagnetism
• In particular,
voltage of output number of turns in output coil
=
voltage of input
number of turns in input coil
Vo N o
=
Vi N i
8.3 Principles of Electromagnetism
• If there are twice as many turns in the output coil
as in the input coil, then the output voltage will be
twice the input voltage.
•
If there are one-third as many turns in the output
coil, then the output voltage will be one-third the
input voltage.
• Thus, the AC voltage can be stepped up or
stepped down by any desired amount by adjusting
the ratio of the number of turns in the two coils.
8.3 Principles of Electromagnetism
Example 8.1
• A transformer is being designed to have a 600-volt
output with a 120-volt input.
•
If there are to be 800 turns of wire in the input coil,
how many turns must there be in the output coil?
Vo N o
=
Vi N i
600 V N o
=
120 V 800
800 ´ 5 = N o
N o = 4,000 turns
8.3 Principles of Electromagnetism
• In addition to being used to change voltages in
electrical distribution systems, transformers are
found in a wide variety of electrical appliances.
•
Most electrical components used in radios,
calculators, and the like require voltages much
smaller than 120 volts.
• Appliances designed to operate on household AC
must include transformers to reduce the voltage
accordingly.
8.3 Principles of Electromagnetism
• High-intensity desk lamps also use transformers,
which is what makes their bases so heavy.
8.3 Principles of Electromagnetism
• The spark used to ignite gasoline in automobile
engines is generated using a type of transformer
called a “coil.”
•
The number of turns in the output coil is many times
the number of turns in the input coil.
• A spark is produced by first sending a brief current
into the input.
•
•
A magnetic field is produced that quickly
disappears.
This induces a very high voltage (around 25,000
volts) in the output, which is conducted to the spark
plugs to ignite the fuel.
8.3 Principles of Electromagnetism
• Understanding electromagnetism allows us to
better appreciate how the metal detectors
introduced at the start of the chapter work.
• The magnetic pulses are produced by sending an
electric current through a coil of wire for a short
period of time.
•
•
When the current stops, the magnetic field that was
created dies out quickly, and this decreasing field
induces an electric current in the coil.
This current is used to monitor how swiftly the
magnetic pulse dies out.
8.3 Principles of Electromagnetism
• Metals are detected because the rapidly changing
magnetic field of each pulse induces electrons in
the metal to move—as in the secondary coil in a
transformer—and this current produces an
opposite magnetic pulse.
•
•
This change in the total magnetic field affects the
current induced in the coil.
The electronics are designed to detect any such
change and signal an alarm.