KEY CONCEPT Charge needs a continuous path to flow. Sunshine State STANDARDS SC.B.1.3.1: The student identifies forms of energy and explains that they can be measured and compared. SC.H.1.3.3: The student knows that science disciplines differ from one another in topic, techniques, and outcomes but that they share a common purpose, philosophy, and enterprise. BEFORE, you learned NOW, you will learn • Current is the flow of charge • Voltage is a measure of electric potential • Materials affect the movement of charge • About the parts of a circuit • How a circuit functions • How safety devices stop current EXPLORE Circuits How can you light the bulb? MATERIALS PROCEDURE 1 Tape one end of a strip of foil to the negative FCAT VOCABULARY circuit p. 375 VOCABULARY resistor p. 376 short circuit p. 378 terminal, or the flat end, of the battery. Tape the other end of the foil to the tip at the base of the light bulb, as shown. • 2 strips of aluminum foil • electrical tape • D cell (battery) • light bulb 2 Tape the second strip of foil to the positive ter- minal, or the raised end, of the battery. 3 Find a way to make the bulb light. WHAT DO YOU THINK? • How did you make the bulb light? • Can you find other arrangements that make the bulb light? Electric charge flows in a loop. VOCABULARY Use a frame game diagram to record the term circuit in your notebook. In the last chapter, you read that current is electric charge that flows from one place to another. Charge does not flow continuously through a material unless the material forms a closed path, or loop. A circuit is a closed path through which a continuous charge can flow. The path is provided by a low-resistance material, or conductor, usually wire. Circuits are designed to do specific jobs, such as light a bulb. Circuits can be found all around you and serve many different purposes. In this chapter, you will read about simple circuits, such as the ones in flashlights, and more complex circuits, such as the ones that run toys, cameras, computers, and more. check your reading How are circuits related to current? Chapter 11: Circuits and Electronics 375 The Parts of a Circuit The illustration below shows a simple circuit. Circuits typically contain the following parts. Some circuits contain many of each part. reminder 1 The voltage source in a circuit provides the electric potential for charge to flow through the circuit. Batteries are often the voltage sources in a circuit. A power plant may also be a voltage source. When you plug an appliance into an outlet, a circuit is formed that goes all the way to a power plant and back. 2 A circuit must be a closed path in order for charge to flow. That means that there must be a conductor, such as wire, that forms a connection from the voltage source to the electrical device and back. 3 A switch is a part of a circuit designed to break the closed path of charge. When a switch is open, it produces a gap in the circuit so that the charge cannot flow. 4 Electrical Device An electrical device is any part of the circuit that changes electrical energy into another form of energy. A resistor is an electrical device that slows the flow of charge in a circuit. When the charge is slowed, some energy is converted to light or heat. A light bulb is an example of a resistor. Remember, a battery consists of two or more cells. Voltage Source Conductor Switch Circuit Parts The parts of a basic circuit include a voltage source, conductor, switch, and one or more electrical devices. 3 A switch is used to open and close the circuit. 2 The conductor provides a path through which charge can flow. 1 The voltage source supplies electrical energy to the circuit. The resistor is an electrical device that converts electrical energy into another form of energy. Would the light bulb be lit if there were no switch in this circuit? Why or why not? 376 Unit 3: Electricity and Magnetism 4 Open and Closed Circuits Current in a circuit is similar to water running through a hose. The flow of charge differs from the flow of water in an important way, however. The water does not require a closed path to flow. If you cut the hose, the water continues to flow. If you cut a wire, the charge stops flowing. Batteries have connections at both ends so that charge can follow a closed path to and from the battery. The cords that you see on appliances might look like single cords but actually contain at least two wires. The wires connect the device to a power plant and back to make a closed path. Switches work by opening and closing the circuit. A switch that is on closes the circuit and allows charge to flow through the electrical devices. A switch that is off opens the circuit and stops the current. check your reading reminder Current requires a closed loop. How are switches used to control the flow of charge through a circuit? Standard symbols are used to represent the parts of a circuit. Some common symbols are shown in the circuit diagrams below. The diagrams represent the circuit shown on page 376 with the switch in both open and closed positions. Electricians and architects use diagrams such as these to plan the wiring of a building. Circuit Diagrams Symbols are used to represent the parts of a circuit. The circuit diagrams below show the circuit from page 376 in both an open and closed position. open switch = off Key closed switch = on cell 2-cell battery 4-cell battery open switch light bulb Would charge flow through the circuit diagrammed on the left? Why or why not? Chapter 11: Circuits and Electronics 377 Current follows the path of least resistance. OUTLINE Add this heading to your outline, along with supporting ideas. I. Main idea A. Supporting idea 1. Detail 2. Detail B. Supporting idea Since current can follow only a closed path, why are damaged cords so dangerous? And why are people warned to stay away from fallen power lines? Although current follows a closed path, the path does not have to be made of wire. A person can become a part of the circuit, too. Charge flowing through a person is dangerous and sometimes deadly. Current follows the path of least resistance. Materials with low resistance, such as certain metals, are good conductors. Charge will flow through a copper wire but not the plastic coating that covers it because copper is a good conductor and plastic is not. Water is also a good conductor when mixed with salt from a person’s skin. That is why it is dangerous to use electrical devices near water. Short Circuits A short circuit is an unintended path connecting one part of a circuit with another. The current in a short circuit follows a closed path, but the path is not the one it was intended to follow. The illustration below shows a functioning circuit and a short circuit. 1 The charge flows through one wire, through the light bulb, and then back through the second wire to the outlet. 2 The cord has been damaged and the two wires inside have formed a connection. Now the path of least resistance is through one wire and back through the second wire. Functioning Circuit Short Circuit coating wires 1 RESOURCE CENTER CLASSZONE.COM Explore resources on electrical safety. 2 In the second case, without the resistance from the lamp, there is more current in the wires. Too much current can overheat the wires and start a fire. When a power line falls, charge flows along the wire and into the ground. If someone touches that power line, the person’s body becomes part of the path of charge. That much charge flowing through a human body is almost always deadly. check your reading 378 Unit 3: Electricity and Magnetism Why are short circuits dangerous? Grounding a Circuit Recall that when lightning strikes a lightning rod, charge flows into the ground through a highly conductive metal rod rather than through a person or a building. In other words, the current follows the path of least resistance. The third prong on some electrical plugs performs a similar function. A circuit that connects stray current safely to the ground is known as a grounded circuit. Because the third prong grounds the circuit, it is sometimes called the ground. In this illustration, green represents the path that connects the appliance and the outlet to the ground. Orange is used in this illustration to represent the path that connects the appliance’s circuit to a power source and back. ! SAFETY TIPS • Never go near a fallen power line. • Never touch an electrical appliance when you are in the shower or bathtub. • Always dry your hands thoroughly before using an electrical appliance. • Never use an electrical cord that is damaged in any way. ground wire connects to ground wire Normally, charge flows through one prong, along a wire to an appliance, then back along a second wire to the second prong. If there is a short circuit, the charge might flow dangerously to the outside of the shell of the appliance. If there is a ground wire, the current will flow along the third wire and safely into the ground, along either a buried rod or a cold water pipe. check your reading • Never bend or cut a ground prong in order to make a grounded plug fit into an ungrounded outlet. What is the purpose of a ground wire? Safety devices control current. Suppose your living room wiring consists of a circuit that supplies current to a television and several lights. One hot evening, you turn on an air conditioner in the living room window. The wires that supply current to the room are suddenly carrying more current than before. The lights in the room become dim. Too much current in a circuit is dangerous. How do you know if there is too much current in a wire? Fortunately, people have been using electric current for over a hundred years. An understanding of how charge flows has led to the development of safety devices. These safety devices are built into circuits to prevent dangerous situations from occurring. Chapter 11: Circuits and Electronics 379 How Fuses Work If you turn on an air conditioner in a room full of other electrical appliances that are already on, the circuit could overheat. But if the circuit contains a fuse, the fuse will automatically shut off the current. A fuse is a safety device that opens a circuit when there is too much current in it. Fuses are typically found in older homes and buildings. They are also found in cars and electrical appliances like air conditioners. new fuse A fuse consists of a thin strip of metal that is inserted into the circuit. The charge in the closed circuit flows through the fuse. If too much charge flows through the fuse, the metal strip melts. When the strip has melted and the circuit is open, the fuse is blown. The photographs on the left show a new fuse and a blown fuse. As you can see, charge cannot flow across the melted strip. It has broken the circuit and stopped the current. blown fuse How much current is too much? That varies. The electrician who installs a circuit knows how much current the wiring can handle. He or she uses that knowledge to choose the right kind of fuse. Fuses are measured in amperes, or amps. Remember that amperage is a measure of current. If a fuse has blown, it must be replaced with a fuse of the same amperage. But a fuse should be replaced only after the problem that caused it to blow has been fixed. Fuses How can you stop a current? SKILL FOCUS Making Models PROCEDURE 1 Use the alligator clips to clip one end of each wire to the steel wool strand. 2 Place the steel wool strand in the jar. Tape the wires to the sides of the jar. 3 Clip the free end of one wire to the negative terminal of the battery. 4 What do you predict will happen when you complete the circuit? Clip the free end of the remaining wire to the positive terminal of the battery and observe the steel wool strand. WHAT DO YOU THINK? • What did you observe when you completed the circuit? Why did that happen? • How can you stop the current? CHALLENGE How is the setup in this activity similar to a fuse that would be found in a home circuit? How does it differ? 380 Unit 3: Electricity and Magnetism MATERIALS • 2 pieces of insulated wire with alligator clips • single strand of steel wool • glass jar • tape • 6 V battery TIME 15 minutes Other Safety Devices Most modern homes do not use fuses. Instead, they use safety devices called circuit breakers. Circuit breakers, unlike fuses, do not have to be replaced every time they open the circuit. Like fuses, circuit breakers automatically open the circuit when too much charge flows through it. If the circuit becomes overloaded or there is a short circuit, the wire and the breaker grow hot. That makes a piece of metal inside the breaker expand. As it expands, it presses against a switch. The switch is then flipped to the off position and the current is stopped. Once the problem is solved, power can be restored manually by simply flipping the switch back. The illustration on the right shows a circuit breaker. check your reading open circuit circuit breaker How are circuit breakers similar to fuses? The photograph at the bottom right shows another safety device—a ground-fault circuit interrupter (GFCI) outlet. Sometimes a little current leaks out of an outlet or an appliance. Often it is so small you do not notice it. But if you happen to have wet hands, touching even a small current can be very dangerous. groundfault circuit interrupter GFCI outlets are required in places where exposure to water is common, such as in kitchens and bathrooms. A tiny circuit inside the GFCI outlet monitors the current going out and coming in. If some of the current starts to flow through an unintended path, there will be less current coming in to the GFCI. If that happens, a circuit breaker inside the GFCI outlet opens the circuit and stops the current. To close the circuit again, you push “Reset.” KEY CONCEPTS CRITICAL THINKING 1. Describe three parts of a circuit and explain what each part does. 4. Apply Suppose you have built a circuit for a class project. You are using a flat piece of wood for its base. How could you make a switch out of a paperclip and two nails? 2. Explain the function of a ground wire. 3. What do fuses and circuit breakers have in common? 5. Communicate Draw a diagram of a short circuit. Use the symbols for the parts of a circuit. CHALLENGE 6. Evaluate A fuse in a home has blown and the owner wants to replace it with a fuse that can carry more current. Why might the owner’s decision lead to a dangerous situation? Chapter 11: Circuits and Electronics 381