Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. To make the electrodes, cut two 4-cm pieces of 16-gauge copper wire for each student group. Straighten the electrodes if necessary.
2. Strip about 1 cm of insulation from each wire lead of the buzzers and the battery clips.

Safety Precautions

A 9-V battery has a low current and is considered safe. Do not use household current. The battery will get hot with extended use. Remind students to disconnect the battery when not in use. Wear safety glasses when using the conductivity tester. Students should only conduct experiments approved by the instructor. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Disposal

The audio conductivity tester may be disassembled and stored for future use.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. The assembly and testing of the apparatus can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the questions may be completed the day after the lab.
• If a class has a hearing-impaired student, a high-brightness red LED (clear bulb, 100-mW, 50-mA) and a 1 K-ohm resistor may be added to the circuit. The LED will light up when the circuit is complete and will get dimmer with increased resistance.
• The thin wire leads connected to the copper electrodes may occasionally slip out of the Fahnestock clips due to their size difference. Place a small piece of electrical tape over the clips to secure the connection.
• If quantity of materials is a problem, set out materials to be tested at various stations around the room and have the groups rotate from one station to the next.
• The following materials are suggestions for testing with the audio conductivity tester. Distilled water is recommended, since most students may assume that all water conducts electricity.
  {12706_Hints_Table_3}
• Liquids may be dispensed in small beakers, medicine cups (Flinn Catalog No. AP5442) or in well plates (Flinn Catalog No. AP1725). Make sure any containers used are clean and have been rinsed with DI water. Label each liquid.
• Place a beaker and a wash bottle with distilled water next to the liquids to be tested for rinsing of the electrodes between tests.
• The copper electrodes will last through many investigations. Additional 16-gauge wire is available from Flinn Scientific, Catalog No. C0146.

Teacher Tips

• This is a great hands-on activity to use as an introduction to electricity or as a component of a unit on metals and nonmetals. The conductivity tester has many other applications in electrochemistry and even earth science—testing the conductivity of various rocks and minerals, one characteristic used in their classification. Those with high metal content will conduct an electric current.
• Substances that conduct an electric current when dissolved or melted are called electrolytes. Electrolytes contain moveable free ions that can carry an electric charge through a solution. Salt water is an electrolyte because table salt is an ionic compound that dissociates (breaks up into simpler constituents) in water, forming positively charged sodium ions and negatively charged chloride ions. Sugar on the other hand, is a molecule formed by covalent bonding and does not dissociate in water. Thus sugar water is a non-electrolyte.
• Students may be surprised to learn that graphite from their pencils, a form of the nonmetallic carbon, is a conductor. This has to do with the arrangement of the atomic bonds. Each graphite atom has three strong bonds that connect the atoms in layers. One of the four valence electrons is then free to conduct an electric charge between the layers. Diamond, another form of carbon, is a non-conductor because all four valence electrons are bonded together, with no free electrons to conduct a charge.
• Even though the human body is a conductor, skin is not a good conductor unless wet. Safely demonstrate the danger of handling electrical appliances with wet hands by firmly holding the copper electrodes in each hand between a dry thumb and forefinger, then repeat with wet fingers. A faint sound should be heard when the fingers are wet. This is safe with the low voltage supplied by a 9-V battery. Do not try this with household current!

Answers to Prelab Questions

1. Label each diagram below as a closed circuit or an open circuit.
   {12706_PreLabAnswers_Figure_3}
2. What would happen if the lightbulb in Diagram A were replaced with an insulator material? Explain.

   An open circuit would result since electrical charges cannot flow through an insulator.

3. During a thunderstorm, all the lights in your house go out—no electricity! You get a flashlight, push the switch, but nothing happens. Describe at least three possible reasons why the flashlight doesn’t work and how each would affect the flow of current.

   The filament in the bulb may be broken; this would create an open circuit. The battery may be “dead,” thus the circuit would have no voltage. A connecting wire may be broken or disconnected creating an open circuit.

Sample Data

Data Table A

Answers to Questions

1. Based on the results of Part A, what property do the solid conductors have in common?

   All the solid conductors were made of metals.

2. Describe any differences observed in the relative conductivity of the liquids tested based on the volume and pitch of the sound produced.

   Salt water produced a loud sound, indicating it is a good conductor. Tap water is also a conductor, but not as good as salt water. Sugar water and distilled water produced no sound, indicating they are nonconductors.

3. Based on the results of Part B, describe the relationship between the length of a conductor and its resistance. What evidence led you to this conclusion?

   The longer the conductor, the greater the resistance. The further apart the electrodes were on the pencil drawing, the softer the sound, indicating a greater resistance to the current.

4. Comparing the results of testing Rectangle B to the same length of Rectangle A, what conclusion can be drawn regarding the width of a conductor and its resistance?

   The wider the conductor, the less resistance it has.

5. List the following copper wires in order of resistance, from greatest resistance to least resistance.
   1. 20 cm in length, 2.1 mm in diameter
   2. 20 cm in length, 1.0 mm in diameter
   3. 10 cm in length, 2.1 mm in diameter

      Wire B has the greatest resistance, followed by wire A and then wire C with the least resistance.

6. Explain how a volume control knob on a radio adjusts current flow.

   Turning the volume control knob increases or decreases the amount of resistance in the circuit. More current flows with less resistance, resulting in greater volume.

References

Special thanks to David A. Katz, retired, Wilmington, DE, for providing the idea and the instructions for this activity to Flinn Scientific.

Berenato, G. and Maynard, D. F. A Simple Audio Conductivity Device; Journal of Chemical Education; April 1997; Vol. 74, pp. 415-417.

Introduction

Flip a switch and a light turns on. Push a button and the car radio comes on. Electricity is taken for granted in our everyday lives, but many may not know how it works. Explore the conductivity of different materials and investigate factors that affect the flow of electricity by building and experimenting with an audio conductivity tester.

Concepts

• Conductors versus insulators
• Open versus closed circuits
• Resistance

Background

A continuous flow of electric charges is called a current. In order for a current to flow and do work a complete circuit is needed. An electrical circuit has three components—a source of electrical energy (voltage), such as a battery or generator, a conducting path, such as wires, and resistance (e.g., a lightbulb, motor, speaker, toaster). The path through which the electricity flows must be continuous or closed. If a break occurs anywhere in the circuit, it is considered an open circuit, and the current no longer flows.

Some materials allow electric charges to flow through them freely. Such materials are good conductors of electricity. Other materials allow electric charges to flow, but not as easily as conductors. These are considered semiconductors. Very poor or non-conducting materials that prevent the flow of electric charges are called insulators.

Resistance is a property of matter that opposes the flow of electric charges in a circuit. Electrical resistance depends on the type of material, the length, the diameter and the temperature of the material. The filament in an incandescent lightbulb—usually tungsten—has a high resistance that converts some of the electric energy to heat and light energy. Good conductors have low resistance, but even the best conductors offer some resistance to current flow.

Experiment Overview

The purpose of this activity is to construct an audible conductivity tester. The tester will be used to investigate the conductivity of various solids and liquids and then to investigate the effect of a material’s length and width on its conductivity.

Materials

Prelab Questions

1. Label each diagram below as a closed circuit or an open circuit.
   {12706_PreLab_Figure_1}
2. What would happen if the lightbulb in Diagram A were replaced with an insulator? Explain.
3. During a thunderstorm, all the lights in your house go out—no electricity! You get a flashlight, push the switch, but nothing happens. Describe at least three possible reasons why the flashlight doesn’t work and how each would affect the flow of current.

Safety Precautions

A 9-V battery has a low current and is considered safe. Do not use household current! Wear safety glasses when using the conductivity tester. Only conduct experiments approved by the instructor. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Preparation: Audio Conductivity Tester Assembly

1. Obtain a piezo buzzer, three Fahnestock clips, two 16-gauge copper electrodes, a battery clip and a 9-V battery.
2. Using one Fahnestock clip, connect the red (+) wire from the battery clip to the red (+) wire of the buzzer. Note: Insert the two wires into the clip from opposite directions. Make sure the bare wires are secure in the clip.
3. Using another Fahnestock clip, connect the black wire of the battery clip to one copper electrode. Make sure the bare wire and the copper electrode are secure in the clip.
4. Using the third Fahnestock clip, connect the black wire of the buzzer to the second copper electrode (see Figure 1).
   {12706_Procedure_Figure_1}
5. Connect the battery clip to the battery.
6. Briefly touch the two electrodes together. A loud beep should sound. If no sound is heard, check the connections and try again. Make sure the battery works. Caution: The battery, wires and electrodes will get hot with prolonged use. Unclip the battery when not in use.

1. Your instructor will give you several materials to test. List each item in Data Table A on the Conductivity Tester Worksheet.
2. Predict which items will conduct electricity and which will not by writing “C” for conductor or “I” for insulator next to each item in the data table column labeled Prediction.
3. Test each solid item by briefly touching both copper electrodes to the item, making sure the electrodes do not touch each other. Also make sure the metal Fahnestock clips are not touching each other. If the buzzer sounds, write “C” for conductor in the data table column labeled Conductor or Insulator. If the buzzer does not sound, write “I” for insulator. Note the relative volume of the buzzer as well (e.g., soft, medium or loud).
4. Test one liquid by inserting the tips of both electrodes into the liquid, making sure the electrodes and clips do not touch each other.
5. Remove the electrodes from the liquid, rinse with distilled water and pat dry with a paper towel. Record the results as in step 3.
6. Repeat steps 4 and 5 for the other liquids. Note: Be sure to rinse electrodes with distilled water and pat them dry with a paper towel between each liquid test to avoid cross-contamination of the liquids.
7. Disconnect the battery.

8. Find Rectangles A and B on the Conductivity Tester Worksheet.
9. Using a #2 pencil, completely color in both rectangles. For best results, color them in solidly and very dark.
10. Connect the battery to the conductivity tester.
11. Place one electrode at the zero end of Rectangle A, making sure the electrode is touching the pencil drawing.
12. Place the other electrode 1 cm away from the first electrode in the middle of the rectangle width. If no sound is heard, color over the rectangle again with a pencil and retest. Remove the electrodes from the rectangle and record your observations in Data Table B.
13. Place one electrode on the zero end of Rectangle A again, and the other electrode 2 cm away. Record your observations, comparing the sound emitted in this step to the sound emitted in step 12.
14. Repeat step 13 keeping one electrode on the zero end and placing the second electrode 3, 4, and 5 cm away, respectively, recording your observations each time.
15. Place one electrode at the zero end of Rectangle B, and the second electrode at the other (1-cm) end. Compare the sound emitted to the previous sound emitted when the electrodes were the same distance apart on Rectangle A. Repeat step 12 if necessary for a better comparison. Record your observations.
16. Wipe the electrodes clean with a paper towel and disconnect the battery.
17. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12706_Student1.pdf