 # Unit 4. Electric Circuits

In the previous unit we looked at voltage, current, and resistance individually. Now it’s time to put them all together and look at the concept of an electric circuit. As we’ll see, most circuits contain a voltage source, resistance, one or more switches, and some wire. We’ll get an introduction to two common circuit types, which are called series circuits and parallel circuits. We’ll also look at how to measure voltage, current, and resistance in circuits. Then we’ll consider some important safety precautions that you should observe whenever you work on circuits. Finally, we’ll start to use a powerful computer program called Multisim, which lets you build virtual circuits on the computer and make virtual measurements on them.

##### Electric Circuits
• In general, an electric circuit consists of a voltage source, a load, and a path for current to flow between the voltage source and the load.
• As discussed in Unit 3, voltages sources provide the force needed to move electrons. Voltage sources include batteries, solar cells, generators, and dc power supplies.
• The load is the part of the circuit that actually does useful work. Some examples of loads include light bulbs (which produce light), loudspeakers (which produce sound), and heating elements (which produce heat). There are many other types of loads.
• The path for current to flow from the voltage source to the load (and then back again to the voltage source) is usually a wire or some other piece of conductive material.
• ##### A Simple Circuit
• A flashlight is a very simple example of an electric circuit.
• The flashlight’s voltage source is a battery (or set of batteries).
• The flashlight’s load is a light bulb.
• The flashlight’s path for current takes the form either of wires or of some other piece of metal, and usually also includes an on/off switch.
• When the switch is in the ON position, electrons are pushed out of the battery, through a conductor, through the light bulb (causing the bulb to light up), then through another conductor, and then back into the battery.
##### More Schematic Symbols
• In previous units we’ve seen the schematic symbols for a number of components, including resistors, capacitors, inductors, potentiometers, and rheostats.
• Here are the symbols for the components in a flashlight:
• battery (or other voltage source) • light bulb • on/off switch • ##### Closed Circuit and Open Circuit
• In the flashlight schematic drawing shown above, the switch is shown in the ON position, since it is completing the connection between the battery and the light bulb. In this case we have a closed circuit, meaning that there is a complete path for current to flow out of the voltage source, through the load, and back to the voltage source.
• If the switch were in its other position, the current path would be broken, resulting in an open circuit. In this case no current would flow through the bulb and therefore the bulb would not light up. The diagram shown below is the same flashlight circuit, but with the switch in its OFF position. • ##### Polarity of Voltage Source
• A voltage source has two terminals, or connection points. One of these is called the positive terminal, and the other is called the negative terminal. For example, you’ve probably noticed that one end of a flashlight battery has a small "nipple," while the other end does not. The end with the nipple is the battery’s positive terminal, and the other end is the battery’s negative terminal. When you insert a battery into a device or circuit, it’s important to insert it in the correct direction.
• In our schematic symbol for a voltage source, the end with the longer line represents the voltage source’s positive terminal, and the end with the shorter line represents the negative terminal. Here’s another look at the symbol, this time with the two terminals labeled positive or negative. • ##### Conventional Current Flow
• When people first started building and studying electric circuits, electrons and protons had not been discovered yet. People assumed that circuits worked as a result of positively charged particles flowing out of a voltage source’s positive terminal, through the rest of the circuit, and back into the voltage source’s negative terminal.
• Today, we know that this picture is wrong. We know that in reality, current is the motion of negatively charged particles (electrons), and that these particles flow out of a voltage source’s negative terminal, through the rest of the circuit, and back into the voltage source’s positive terminal.
• Even though we know this, for historical reasons most engineers and technicians still usually think of current as flowing out of a voltage source’s positive terminal and into its negative terminal. When we talk or think this way, we’re using what is called conventional current flow. Most textbooks, including our textbook for this course, use conventional current flow.
• Some textbooks adopt the opposite direction for current flow. Those textbooks use what is called electron current flow.
• Looking at our flashlight circuit again (repeated below), someone using conventional current flow would imagine current in this circuit flowing out of the top of the voltage source, through the switch, then through the bulb, then back into the bottom of the voltage source.
• On the other hand, someone using electron current flow would imagine current flowing in the opposite direction, out of the bottom of the voltage source, then through the bulb, then through the switch, then back into the top of the voltage source. • It doesn’t matter which way you think of current as flowing, but realize that when you’re talking with someone else, confusion may result if you’re thinking in terms of conventional current flow and the other person is thinking in terms of electron current flow.
• In these lessons I’ll use conventional current flow, since that’s what our textbook uses. But be aware that the Wisonsin learning objects use electron current flow: when they show an animated picture of current flowing through a circuit, they show it flowing out of the voltage source’s negative terminal and back into the voltage source’s positive terminal.
• ##### Protective Devices
• Too much current flowing through an electric circuit can damage the circuit and can create a safety hazard.
• How much current is "too much current"? That depends on the circuit and its components. For some circuits, 1 A would be too much current, while for other circuits 1 A would be perfectly acceptable. (Recall that A stands for ampere. You should read "1 A" as "one ampere.")
• Fuses and circuit breakers are protective devices used to ensure that too much current does not flow through a circuit. A fuse or circuit breaker is designed to create an open circuit if too much current flows through it. You can think of it as a switch that automatically turns itself off if the current through it exceeds a certain level.
• When a fuse is "blown" by having too much current pass through it, the fuse is ruined and must be replaced. On the other hand, when a circuit breaker is "tripped" by excessive current, the circuit breaker can be reset and used again, instead of being discarded.
• A flashlight does not need a fuse or circuit breaker because there is no way that a flashlight’s batteries can produce enough current to create a dangerous situation.

##### Wires
• Wires are the most widely used conductors in electric circuits.
• Wires come in a wide range of sizes (diameters). We could say how thick a wire is by giving its diameter (such as "one-quarter inch in diameter"), but instead we normally use a special set of numbers called the American Wire Gauge (abbreviated AWG). The numbers in this system range from 0000 (for the thickest wire) to 40 (for the thinnest wire).
• To give you some idea how thick these wires are, AWG 0000 wire is about 11.7 millimeters in diameter (a little less than one-half inch thick).
• At the opposite end of the scale, AWG 40 is less than 0.1 millimeters in diameter (about as thick as a strand of hair from your head).
• This system can be confusing when you’re first learning about it, because small numbers stand for thick wires, and large numbers stand for thin wires.
• The wiring inside the walls of your home is usually AWG 10 or AWG 12 wire.
• The wires that you’ll use to build circuits on a breadboard in Sinclair’s labs is usually AWG 22 or AWG 24 wire.
• Table 2-4 on page 46 of the textbook gives you the size of each size of wire in the AWG. This information can be a bit confusing to understand, because the table gives the size of the wire in a unit called circular mils, which most people aren’t familiar with. From a practical standpoint, though, the more useful information in that table is in the column labeled "Resistance," which we’ll discuss in a minute.
• ##### Wire Resistance
• Wires are very good conductors, which means that they have very little resistance. In fact, their resistance is so small that often we consider it to be zero. (In other words, we often treat wires as perfect conductors.)
• But in some cases, we want to know exactly how much resistance a piece of wire has. Assuming you’re dealing with solid copper wire, the numbers in the "Resistance" column of the textbook’s Table 2-4 will let you figure this out, as long as you know how long the wire is and what AWG size it is. (If the wire is not solid wire made of copper, then you cannot use those resistance numbers from Table 2-4.)
• Example: According to the table, AWG 12 wire has a resistance of 1.588 Ω per 1000 feet of wire. (As the table notes, this is true at 20°C, which is approximately room temperature.) So if you have a 50-foot long piece of this wire, how much resistance does it have? The answer is 79.4 mΩ, since

50 ft × 1.588 Ω / 1000 ft = 79.4 mΩ

That’s much less than 1 Ω, so it’s not a very large resistance at all. In fact, that resistance is so small that in many cases we could consider it to be zero.
• • ##### Ground
• The term ground can be confusing because it has several different, but related, meanings. Also, there are several different schematic symbols used for ground, but not everyone uses these symbols in the same way. Shown below are three symbols for ground:   • In one usage, ground is used simply to identify a convenient reference point in a circuit, or a point to which several different components are connected. (When using the term in this way, people may use the term "common" instead of "ground.") This is done primarily to simplify schematic diagrams by reducing the number of lines that need to be drawn in the diagram.
• Example: Shown below is our flashlight circuit diagram, redrawn using a ground symbol. Notice that we no longer have a line running directly from the bulb to the battery’s negative terminal. But the ground symbols tell us that those two points are indeed connected to each other. This is a silly example, because the original diagram is not very difficult to read, and we’re not making it any easier to read by introducing a ground symbol. But it gives you the idea. • In another usage, the points represented by the ground symbol are connected not only to each other, but also to a metal chassis that is used as a return path to the circuit’s voltage source. (When using the term in this way, people may say "chassis ground" instead of just using the word "ground.") This is done to reduce the number of wires needed, by using a device’s metal chassis as a conductor.
• Example: In many automobiles, the battery’s negative terminal is connected directly to the car’s chassis. That way, a circuit that runs to the other end of the car doesn’t need to have return wires running all the way back to the battery. Instead, the circuit can simply be connected to the far end of the car’s chassis, so that current flows back to the battery through the chassis.
• In a third usage, the points represented by the ground symbol are connected not only to each other, but also to a metal stake or pipe that is driven into the earth. (When using the term in this way, people may say "earth ground" instead of just using the word "ground.") This is done for safety reasons, to reduce the risk of electric shock by providing a low-resistance path for current to flow into the earth instead of flowing through your body.
• Example: You’re probably aware that the power cords on some electric devices, such as kitchen blenders, have three prongs that plug into the wall outlet, while some other devices have just two prongs. The third prong serves to connect the chassis of the device to earth ground.
• The term ground has still other, slightly different meanings. When you study digital circuits, you’ll learn that many such circuits have a digital ground that is separate from the circuit’s analog ground.
• ##### Series versus Parallel
• Components can be connected to each other in different ways. For example, suppose we want to build a circuit containing a voltage source and two resistors. (For convenience, we’ll refer to the two resistors as R1 and R2.) The diagrams below show two fundamentally different ways of connecting those resistors to each other and to the voltage source.

•  • In the first circuit, we’ve connected the components in series with each other. In the other circuit, we’ve connected them in parallel with each other. Read on for further discussion of these terms.
• Note: at this point in your studies, we don’t want to get bogged down in detailed definitions or in complicated equations. So this will be an informal discussion. Later you’ll learn official definitions, along with lots of useful equations.
##### Components in Series
• Two components are connected in series if they are connected to each other at exactly one point and there are no other components connected to that point.
• Example: In the circuit shown below (which is the same as the first circuit shown earlier), the voltage source is connected in series with R1, and R1 is connected in series with R2, and R2 is connected in series with the voltage source. • ##### Series Circuit
• A series circuit is a circuit in which all of the connections between components are series connections.
• Example: The circuit discussed just above is a series circuit.
• The photograph below shows this series circuit built on a breadboard. The red and black wires go to the positive and negative terminals of the power supply. • ##### Components in Parallel
• Two components are connected in parallel if they are connected to each other at two points.
• Example: In the circuit shown below, the voltage source is connected in parallel with R1, and R1 is connected in parallel with R2, and the voltage source is connected in parallel with R2. • ##### Parallel Circuit
• A parallel circuit is a circuit in which every component is connected in parallel with every other component.
• Example: the circuit discussed above is a parallel circuit.
• The photograph below shows this parallel circuit built on a breadboard. The red and black wires go to the positive and negative terminals of the power supply. • ##### Measuring Voltage
• A voltmeter is an instrument designed to measure voltage.
• Voltage measurements are always made across components: that is, it is not necessary to disconnect any components to connect a voltmeter and make a voltage measurement.
• Below is a schematic diagram showing how to connect a voltmeter (or multimeter) to measure the voltage drop across resistor R1: • Check out the following lesson from our friends in Wisconsin, and remember what I said above: these Wisconsin learning objects use electron current flow, so they show current flowing out of a battery’s negative terminal and back into its positive terminal.
• ##### Measuring Current
• Current is measured by an instrument called an ammeter.
• To measure the current flowing through a resistor, you must disconnect the resistor and insert an ammeter in such a way that all the current flowing through the resistor also flows through the ammeter.
• Below is a schematic diagram showing how to connect an ammeter (or multimeter) to measure the current through resistor R1: • ##### Measuring Resistance
• An ohmmeter is an instrument designed to measure resistance.
• Resistance should never be measured when there is a voltage source connected across it or when there is any other component connected to it.
• This is an important point: If you measure a resistor’s resistance while the resistor is connected to a voltage source, then you’ll definitely get a wrong value for the measurement and you may also damage the meter.
• ##### A Common Mistake
• Students often forget how to connect voltmeters and ammeters. In particular, they often connect an ammeter as if it were an voltmeter (see the diagram on the left below), or they connect a voltmeter as if it were an ammeter (diagram on the right).
• Connecting a meter in these ways will certainly give an incorrect reading and may also damage the meter. • ##### Digital Multimeter
• A digital multimeter (DMM) can measure either voltage, current, or resistance, depending on the setting of a selector switch. So it’s like having a voltmeter, an ammeter, and an ohmmeter combined into one piece of equipment.
• Shown below is a professional-quality bench-top DMM, the Fluke 8050. • Shown below is an inexpensive handheld DMM. • All multimeters have two test leads, one red and one black. You make a measurement by touching the metal tips of these test leads to the proper points in the circuit being measured. The picture below shows some typical test leads. For safety, you must never touch the metal part of a test lead with your fingers when you are making a measurement. • A multimeter must not be set to measure current when it is connected as a voltmeter, or set to measure voltage when it is connected as an ammeter. (This is the same point made earlier in the diagrams of incorrect meter connections.)
##### Multimeter Challenge Game
• You’ll need to become an expert at setting the controls on a digital multimeter.
• To work on this skill, be sure to play Multimeter Challenge. Like all of the games on the Games page, this game has a Study mode, a Practice mode, and a Challenge mode.

##### Electrical Safety
• Electricity is dangerous. It can shock you, burn you, kill you, and start fires that destroy buildings.
• A current of 10 mA passing through your body will result in a painful shock. A current of 100 mA or more though your body can be fatal.
• Some important safety precautions include:
• Avoid contact with any voltage source. Turn off power before you touch any circuit parts. (For example, if you build a circuit and then realize that you used the wrong resistor in building it, turn off the power before you replace that resistor.)
• Remove rings, watches, and other metallic jewelry when you work on circuits.
• Make sure power cords are in good condition and grounding pins are not missing or bent.
• Handle tools properly and maintain a neat work area.
• Wear safety glasses when soldering or when clipping wires.
• Never handle instruments when your hands are wet.
• Never assume that a circuit is off. Double check it with a meter to be sure.
• Ask your instructor if you have any questions about the proper way to do something in lab.

##### Unit 4 Review

Congratulations! You’ve completed the e-Lesson for this unit.