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Abstract

Objective

The objective of this science fair project is to build a simple circuit tester and use it to investigate how electricity flows through a household lamp.

Introduction

Electricity is like water in a river, it flows. For example, when you turn on a lamp, electricity flows in through the power cord, then it flows through the lightbulb, and finally, it flows back out through the power cord. Electricity flows through conductors. Most metals are good conductors. Copper is an excellent conductor, so it is used in power cords. To keep the electricity from flowing where it is not supposed to go, conductors that carry electricity are surrounded by insulators. Plastic and rubber are good insulators, which is why they are used to coat power cords. If electricity is like water flowing down a river, the conductors are like the sides of the river—they keep it within certain areas.

Electricity has to flow into and out of an object to provide power, and the path of the electricity is called a circuit. A circuit is a circular journey. In an electrical circuit, the electricity makes a circular journey through the device it is powering. For example, when a lamp is turned on, a circuit is formed from the socket in the wall, through the lamp, and then back to the socket. If the path of the electricity is broken, then the flow of electricity is stopped and the power to the device is turned off. The role of switches is to break the flow of electricity. A common type of switch has two pieces of metal that touch to make a circuit, and separate to break the circuit. More complex switches are used in electronic devices, but the basic idea is the same, they interrupt the flow of electricity.

You can test whether two objects are connected in a circuit using a device called a circuit tester (also called a continuity tester). In this electronics science fair project, you will make your own circuit tester. To determine if there is a path for electricity through a lamp, you will unplug it and attach probes to the prongs of the plug. When it is plugged in, electricity flows into the lamp from one prong and out through the other prong. By attaching your circuit tester to the two prongs, you can determine if there is a closed circuit for the flow of electricity. You will also determine how the lamp switch and the type of lightbulb affect the flow of electrical current.

Terms, Concepts, and Questions to Start Background Research

• Conductor
• Insulator
• Circuit
• Switch
• Circuit tester
• Continuity tester
• Closed circuit
• Current
• Circuit diagram
• Alternating current

Questions

• What word is used by scientists to describe the flow of electricity? Hint: Think of the word for "moving water."
• How many types of switches can you find in your house?
• What is the definition of an open circuit?
• What is voltage?
• How are current and voltage related?
• What are some examples of good conductors and good insulators?

Bibliography

• Marshall, B. and Lamb, R. (2009). How electricity works. Retrieved May 11, 2009, from http://science.howstuffworks.com/electricity.htm
• Energy Information Administration. (2007). What is electricity? Retrieved May 12, 2009, from http://www.eia.doe.gov/kids/energyfacts/sources/electricity.html

Materials and Equipment

• AA battery holder; available online at www.radioshack.com, part # 270-382A
• Snap connectors, 9-V; available online at www.radioshack.com, part # 270-325
• Wire strippers
• Insulated test/jumper leads; available online at www.radioshack.com, part # 278-1156
• Electrical tape
• Buzzer; available online at www.radioshack.com, part # 273-053A
• AA batteries, brand-new (2)
• Plastic container, 16-ounce (oz.) with a snap-on top
• Scissors
• Hole punch or screwdriver
• Lab notebook
• Incandescent lightbulb, 60-watt (W), preferably with clear glass (1)
• Energy-saving fluorescent lightbulb (1)

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Experimental Procedure

Important Safety Notes Before You Begin:

• All devices that are tested should be disconnected from a power source.
• Don't take any electrical appliances apart to test components inside.
• Do not go near the sockets in the wall with the circuit tester.

Figure 1. The partially assembled circuit tester. The connections have not been wrapped with electrical tape yet. The tester will buzz if current can flow between the two probes. The probes can be stored in the container when not in use.

Setting Up the Circuit

1. Attach the 9-V snap connector to the AA battery holder.
   1. Even though the name says "9 V," you will use the connector to hook up the AA battery holder.
2. Strip about 2 cm from the end of the red wire from the 9-V snap connector.
3. Strip about 2 cm from the end of the red wire from the buzzer.
4. Twist the ends of the red wires together from the snap connector and the buzzer.
5. Wrap electrical tape around the exposed twisted red wires.
6. Attach a black alligator clip to the black wire from the buzzer.
7. Wrap electrical tape around the black alligator clip and the black wire from the buzzer.
   1. Even though the alligator clip is insulated, there is a chance the bare wire might touch another bare wire. The electrical tape ensures the bare wire is insulated.
8. Attach a red alligator clip to the black wire from the battery pack.
   1. Wrap electrical tape around the red alligator clip and the black wire from the battery holder to insulate the bare wire.
9. Place the batteries in the battery holder.
10. Touch the red and the black probes together. You should hear a buzz from the buzzer. If you don't, check the batteries and your connections and try again. See Figure 2.

Figure 2. Diagram for the circuit tester. When the two probes are connected to a conductor, current flows through the circuit, activating the buzzer.

Housing the Tester

1. Thoroughly clean and dry a 16-oz. plastic container and its snap-on top.
   1. Figures 1 and 3 show a salsa container. You can use whatever type of container you choose.
2. Use the scissors to cut two notches in the top rim of the base of the plastic container. Refer back to Figure 1, above. Have an adult help you with this step.
3. Place the buzzer, the battery pack and the wires inside the plastic base.
4. Rest the wire for the red probe in one of the notches, so that the probe is on the outside of the container.
5. Rest the wire for the black probe in the other notch, so that the probe is on the outside of the container.
6. Punch five holes in the lid, using the hole punch or a screwdriver. This will make it easier to hear the buzzer.
7. Place the lid on top of the container.

Testing the Flow of Electricity

Important Note: The circuit tester can be used to determine whether two things are electrically connected to each other. If two things are electrically connected, current can flow between them. The tester will buzz if current can flow between the two probes. Caution: Make sure anything you choose to test is not plugged into a wall socket or powered by batteries.

1. Check to make sure an incandescent lightbulb is in the lamp.
2. Plug the lamp in and turn it on to test that the lightbulb works. If it does not light, replace the lightbulb with a new incandescent lightbulb.
3. Turn the lamp off.
4. Unplug the lamp.
5. Attach one probe to each prong of the plug. See Figure 3.

Figure 3. Circuit tester attached to lamp plug. The two kinds of lightbulbs are shown: incandescent bulb (left) and energy-saving fluorescent bulb (right).

6. Does the buzzer make a noise? Record your observations in your lab notebook in a data table, like the one below.

7. Turn the lamp switch ON. Note: The lamp should remain unplugged at all times.
8. Does the buzzer make a noise now?
9. Remove the incandescent lightbulb.
10. What happened to the circuit when the lightbulb was removed?
11. Replace the incandescent lightbulb with the energy-saving fluorescent lightbulb.
12. What happens to the buzzer?
13. Turn the switch on the lamp OFF.
14. What happens to the buzzer now?
15. Explain your results.
    1. Look inside the incandescent lightbulb. Do you see why there is a circuit?
    2. For the energy-saving lightbulb, how would you explain your results? Hint: What is needed in this kind of lightbulb to make electricity flow?
16. Repeat steps 1–15 with two different lamps.

Troubleshooting

For troubleshooting tips, please read our FAQ for Is This Connected to That?

Variations

• Test a toaster. Unplug the toaster, attach the probes to the prongs of the plug. What happens when you push the toaster lever down?
• Test other appliances around your house.
• Test the connections in a USB cable (that is free at both ends). Attach the alligator clips to a small piece of wire; for example, a paper clip, so that you can make contact with the small wires in the cable. A helper can hold one of the probes. Make a diagram showing your results. Try other kinds of cables or cords.
• Test to see if the cold water pipes in your house are electrically connected. Use wire to extend the lengths of your probes. Attach the extended probes to the copper pipes that carry cold water under the kitchen and bathroom sinks.
• Try different kinds of buzzers in your tester. You may need to increase the voltage.
• What happens if the circuit is closed and electricity can flow, but there is resistance to flow? This could happen if the metal in a switch was corroded, for example. Model this by placing 1-, 10-, and 1,000-ohm resistors (available at RadioShack) across the probes.

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

David Whyte, PhD, Science Buddies

Last edit date: 2012-01-13 16:00:00