To get started, we’ll review some
material that you studied in EET 1150 (D.C. Circuits).
After this review , we’ll discuss the close
relationship between electricity and magnetism. Many common and
useful devices, such as solenoids and loudspeakers, rely on the fact
that electricity can be used to produce magnetism.
Many other common devices, such as electrical generators,
rely on the fact that magnetism can be used to produce
For any topic below with a Self-Test icon (which looks like this ),
click the icon to test your understanding of that topic. Self-Tests are
for your practice only; no grades are recorded. The questions will appear
in a new window. You may wish to resize this window so that you can read
the questions more easily; then close the window when you’re finished
with the questions for that topic.
EET 1150 Review
Here is a list of the main topics that we’ll review from EET 1150:
energy and power
capacitors and inductors in D.C. circuits.
Read on for a review of these topics.
Unit 5 of EET 1150 covered the following topics:
To see how well you remember these topics, take the following self-test:
The table below summarizes the electrical quantities that you studied
in EET 1150. The table shows the abbreviation for each
quantity, along with the standard unit for measuring the quantity and
the abbreviation for the unit.
Abbreviation for the
voltage (or emf)
V (or E)
energy (or work)
If you look closely, you will notice that the abbreviations of the
quantities are written with italicized letters (such as Q and I).
But the abbreviations of the units are written with plain, non-italicized
letters (such as C and A). This is the standard, accepted way of doing
things. Our textbook (along with other books) follows this rule, and
you should too when you use these abbreviations in a typed paper or
In this course we’ll learn quite a few new quantities, and this table
will grow. Unit 2 will present an expanded
version of the table.
For further review of some of the skills you learned in EET 1150,
play the following games. Each game has a Study mode that reviews the
theory, a Practice mode that lets you practice with no time pressure,
and a Challenge mode that tests your skill while the clock is running.
If you’re fast, you may even get your name on the high-score board!
This completes our review of material from EET
1150. Let’s move on now to some new material.
Permanent magnets, like the kind you probably have hanging
on your refrigerator, keep their magnetic properties for a long time.
One end of a magnet is called its north (N) pole and the opposite
end is called its south (S) pole.
When two magnets are placed near each other, opposite poles attract,
and like poles repel.
If you stop and think about it, it’s surprising that one magnet can
push or pull another magnet when they’re not touching each other. How
can that happen?
To explain this fact, it’s useful to think of a magnet as being surrounded
by a magnetic field, which carries the force that it exerts
on other magnets. So even when two magnets aren’t directly touching
each other, they can still affect each other indirectly through their
We draw the magnetic field as lines leaving the magnet’s north pole
and entering the south pole.
These imaginary lines are called flux lines.
Scientists have developed a complete theory of magnetic fields.
This theory defines measurable quantities such as magnetic
flux and magnetic
flux density, and it derives mathematical equations that relate
these quantities to each other. For this course you won’t need to study
Today we know that electricity and magnetism are very closely related.
But nobody realized this until the 1800’s.
In 1820, Hans Oersted discovered that electrical current creates
a magnetic field. This phenomenon is called electromagnetism.
Again, scientists have developed a detailed theory of electromagnetism
that involves lots of new terms (such as permeability, reluctance,
and magnetomotive force) and equations. You won’t
need to know these concepts for this course.
Applications of Electromagnetism
Electromagnetism (using electricity to create magnetism)
has many, many practical applications.
A few devices that use electromagnetism are electric motors, loudspeakers,
relays, antennas, and solenoids.
Many of the devices just mentioned contain electromagnets. An electromagnet displays
magnetic properties only when current is passing through it. So you
can think of it as a magnet that you can turn on or off. This is different
from a permanent magnet, which you can’t "turn off."
Electromagnets are constructed simply by wrapping wire around a core,
which is just a rod or bar usually made of an iron alloy. When you
pass current through that wrapped-up wire, the whole thing behaves
like a magnet. When you stop passing current through the wire, it stops
behaving like a magnet.
An Animated Lesson from our Friends in Wisconsin
Instructors in the Wisconsin Technical College System have created
a library of short online animations and quizzes to help students learn
electronics. I’ll include links to some of these "learning objects." Whenever
you see the icon below, click it to see a learning object on the material
you’re studying. The Wisconin learning object will open in a new window;
close the winow when you’re finished and want to return to this lesson.
This first one will tell you more about electromagnets. Click the
Turns and Windings
In an electromagnet, each complete wrap of wire around the core is
called a turn, and all of the turns taken together are called
a coil or a winding.
A solenoid is an electromagnetic device in which
an electrical signal is used to control the position of a metal rod
called a plunger. Solenoids are widely used in valves
that control the flow of liquids and gases. They’re also used in other
applications, such as the locks in car doors.
A relay is an electromagnetic device in
which an electrical signal in one circuit is used to close or open
a switch, thus either completing or breaking another electric circuit.
Typically the first circuit (called the
control circuit) is a low-voltage, low-current circuit, often containing
a pushbutton that an operator can press. And in many cases the other
circuit is a high-voltage, high-power circuit. So we have a low-voltage
circuit being used to control the operation of a high-voltage
circuit. This arrangement provides safety benefits, since it lets the
human operator control the high-voltage circuit without having
to touch any switches that carry dangerously
As mentioned above, Oersted realized in 1820 that every electrical
current creates a magnetic field. (That’s called electromagnetism.)
A few years later, Michael Faraday realized that this works in the
opposite direction too: you can use magnetic fields to create electricity.
This is called electromagnetic
In particular, Faraday found that if you move a wire through a magnetic
field, you’ll create a voltage across the ends of the wire. We say
that a voltage is induced across the wire.
A voltage can also be induced by moving the
magnetic field past a wire that is at rest.
In both of these cases, the important thing is that the wire’s motion
or the field’s motion results in a changing magnetic field in
the vicinity of the wire. In fact, a voltage is induced whenever there’s
a change in the size of the magnetic field surrounding
a wire, even if the wire and the magnetic field are both at rest.
In all of these cases, the induced voltage will be greater if you
use a coil of wire rather than a straight piece of wire. So most
devices that rely on electromagnetism contain coils.
The basic law of electromagnetism is called Faraday’s law. It can
be written as an equation, or stated in words.
Stated in words, it says that 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.
Applications of Electromagnetic Induction
Electromagnetic induction has many practical applications. Perhaps
the most important practical application is the electrical generator,
which creates electricity by rotating a coil of wire between the two
ends of a magnet.
Take a look at the following lesson for a basic explanation
of how a generator works.
Another interesting application is the metal detector, which is
explained in this lesson:
Unit 1 Review
This e-Lesson has covered several important topics, including:
a review of material from EET 1150
To finish the e-Lesson, take this self-test to check your understanding
of these topics.
Congratulations! You’ve completed the e-Lesson for this unit.