# Unit 1: Review and Electromagnetism

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 electricity.

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:
• Ohm's law
• energy and power
• series circuits
• parallel circuits
• series-parallel circuits
• circuit theorems
• capacitors and inductors in D.C. circuits.
• Read on for a review of these topics.
##### Ohm's Law
• Unit 5 of EET 1150 covered the following topics:
• Ohm's law
• estimation
• series-parallel circuits.
• To see how well you remember these topics, take the following self-test:
• If you'd like to review this material, work through the entire e-Lesson for Unit 5 of EET 1150.
##### Energy and Power
• Unit 6 of EET 1150 covered the following topics:
• energy and power
• resistor power ratings
• power supplies and efficiency.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 6 of EET 1150
##### Series Circuits
• Unit 7 of EET 1150 covered the following topics:
• series connections and series paths
• series circuits
• voltage drops and voltage rises
• voltage sources in series
• Kirchhoff's Voltage Law (KVL)
• voltage dividers and the voltage-divider rule
• power in series circuits
• open circuits and short circuits.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 7 of EET 1150.
##### Parallel Circuits
• Unit 8 of EET 1150 covered the following topics:
• parallel connections and parallel circuits
• Kirchhoff's Current Law (KCL)
• total resistance of resistors in parallel
• parallel-connected sources
• current-divider rule
• power in parallel circuits
• shorts and opens in parallel circuits.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 8 of EET 1150.
##### Series-Parallel Circuits
• Unit 9 of EET 1150 covered the following topics:
• identifying series and parallel relationships
• analyzing series-parallel circuits
• power in series-parallel circuits
• ground symbol and bubble symbol.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 9 of EET 1150.
##### More Series-Parallel Circuits
• Unit 10 of EET 1150 covered the following topics:
• the Wheatstone bridge
• troubleshooting series-parallel circuits.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 10 of EET 1150.
##### Circuit Theorems
• Unit 11 of EET 1150 covered the following topics:
• practical voltage sources
• practical current sources
• source conversion
• the superposition theorem.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 11 of EET 1150.
##### More Circuit Theorems
• Unit 12 of EET 1150 covered the following topics:
• Thevenin's theorem
• the maximum power transfer theorem.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 12 of EET 1150.
##### Capacitors in DC Circuits
• Unit 13 of EET 1150 covered the following topics:
• capacitance
• charge and voltage on a capacitor
• capacitor specifications
• types of capacitors
• capacitors in series, in parallel, and in series-parallel
• DC RC circuits
• calculating initial values, steady-state values, and transient values in DC RC circuits
• time constant of a DC RC circuit.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 13 of EET 1150.
##### Inductors in DC Circuits
• Unit 14 of EET 1150 covered the following topics:
• inductance
• types of inductors
• energy stored in an inductor
• inductors in series, in parallel, and in series-parallel
• calculating initial values, steady-state values, and transient values in a DC RL circuit
• time constant of a DC RL circuit.
• To see how well you remember these topics, take the self-test by clicking the icon below:
• If you'd like to review this material, work through the entire e-Lesson for Unit 14 of EET 1150.
##### Table of Electrical Quantities
• 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.
 Quantity Abbreviation Unit Abbreviation for the Unit charge Q coulomb C current I ampere A voltage (or emf) V (or E) volt V resistance R ohm Ω conductance G siemens S energy (or work) W joule J power P watt W efficiency η capacitance C farad F time constant τ second s inductance L henry H
• 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 lab report.
• 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.
##### Electronics Games
• 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
• 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.
##### Magnetic Fields
• 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 magnetic fields.
• 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 them.
##### Electromagnetism
• 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.
##### Electromagnet
• 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 icon now.
##### 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.
##### Solenoid
• 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.
##### Relay
• 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 high currents.

##### Electromagnetic Induction
• 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 induction.
• 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
• electromagnetism
• electromagnetic induction.
• 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.