Hands-on Activity: Will It Conduct?
Educational Standards :
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For the entire class to share:
Note: Many of the materials required for this lab can be reused in other electricity activities. When the batteries eventually wear out, dispose of them at a hazardous waste disposal site.
Introduction/Motivation (Return to Contents)
Before starting the activity, you may want to remind students that current electricity is the movement of electrons from atom to atom. You may also want to review that electrons carry a negative electric charge.
To begin, ask students, if they know where we get electricity from? (Possible answers: The socket in the wall, a power plant, from fossil fuels.) Explain to students that the current electricity we use in schools, businesses and homes comes from a power plant. The power plant sends electricity to substations, which are located in neighborhoods. The substations send the electricity to local businesses and houses.
Next, ask students if they know how current electricity is able to move from a power plant to substations and, finally, to businesses and homes? (Answer: Via electrical wires.) Now, ask students if they know from what material these wires are made? (Answer: Copper.) Show the class a few pennies and explain that the wires that link a power plant, to a substation, and then to businesses and homes are made from copper…just like a penny! Inform students that we use copper for electrical wires because electricity can easily flow through copper. Explain that current electricity can flow more easily through some objects than others. The materials that electrons can move through are called conductors. Most metals make good conductors because the electrons are loosely attached to the atoms. When this is the case, a negative charge buildup can push these electrons through the material. Now, ask students if only solids can conduct electricity? (Answer: No. Electrolyte solutions can also conduct electricity.) Explain that when certain solids are dissolved in a liquid, the resulting solution is able to conduct electricity; we call these solutions electrolyte solutions.
Ask students if they know what we call materials that do not allow electrons to flow through them? (Answer: Insulators.) In an insulator, the electrons are tightly attached to the atoms in a material and cannot be forced to move from one atom to another, so no electricity flows. Some good examples of insulators include plastic, cloth, air, rock and glass. Explain that they will learn more about conductors and insulators during the activity.
Finally, share with students that electrical engineers also use copper wires as conductors of electricity when they design electronic circuit boards (see Figure 1). The copper wires on a circuit board are called traces and are fixed on an insulated plastic board (many times green in color) called a substrate. The copper traces are carefully mapped on a circuit board by electrical engineers connecting electrical components (such as resistors, capacitors and microchips) on the circuit board and providing electricity to these components.
Procedure (Return to Contents)
Background — Solids
Metals are good conductors. Plastics, wood products, ceramics and glass are insulators. The graphite from a pencil conducts, but has a higher resistance than the metals. Graphite, like silicon, is a semiconductor; it has electrical properties intermediate between insulators and conductors. So, in the activity, depending on the length of the graphite, the light bulb may be considerably dimmer than when a metal object is used.
The resistance of an object depends not only on its composition, but its length, cross-sectional area and temperature. For example, a long piece of copper has a higher resistance than a short piece of copper of the same diameter. A piece of copper 1 m long with a diameter of 2 cm has a higher resistance than a piece of copper 1 m long with a diameter of 3 cm. If the temperature of a piece of copper is raised, its resistance also increases.
Engineers determine the most efficient means of utilizing materials for a given purpose. When an engineer designs an object with a particular resistance, s/he must consider how expensive the material is, how the shape of the object will affect its resistance, and to what temperature ranges it will be exposed. For example, an engineer may use a more expensive material to make a small, critical circuit element, and a cheaper material to make larger circuit elements.
Background — Liquids and Solutions
It is the presence of ions in a solution that enables it to conduct electricity. Positive ions move to the negative electrode and negative ions move to the positive electrode. The conductivity of a solution is proportional to the concentration of ions in the solution. Therefore, solutions with low concentration of ions only weakly conduct electricity; in cases like this, in the activity, the bulb of the conductivity tester may not light up or may glow dimly.
The light bulb will not light up when the electrodes are placed in distilled water. There may, however, be a very small current in the circuit due to the presence of H+ and OH- ions in the water. These ions are produced when water molecules spontaneously dissociate. These ions also recombine spontaneously to form water molecules. Because only a few water molecules dissociate in a given time, the ions make up a very small proportion of the particles in the distilled water. Therefore, distilled water is a very poor conductor. Tap water, on the other hand, can be a good electrical conductor due to the presence of many different ions such as Ca2+, Na+, Li+, Cl-, etc. However, at the low voltages used in this activity, tap water will not conduct electricity. Here are the approximate voltages of the following solutions: Salt and water is 2.0V, baking soda and water is 27.0V, Pedialyte is 7.9V and Gatorade is 13.0V.
Adding an ionic solid, or salt, to distilled water produces a solution that conducts electricity. When an ionic solid such as table salt, NaCl, is added to water, it dissociates — breaks apart into oppositely-charged ions — into Na+ and Cl-. Salts are strong electrolytes — they dissociate completely. Increasing the amount of salt in a solution increases the conductivity of the solution.
Acids and bases also break down to form ions when dissolved in water. Therefore, a solution of an acid or a base conducts electricity. Strong acids, such as sulfuric acid or hydrochloric acid, and strong bases, such as sodium hydroxide or potassium hydroxide, are strong electrolytes because when they dissolve in water, almost every molecule dissociates to produce ions. On the other hand, weak electrolytes, such as weak acids and weak bases, produce relatively few ions when dissolved in water. Citric acid and acetic acid (in vinegar) are weak acids. Baking soda and ammonia are weak bases. When weak electrolytes dissolve in water, the solution is a poor conductor. Increasing the concentration of a weak electrolyte in a solution increases the conductivity of the solution. Increasing the amount of acid or base in a solution increases the conductivity of the solution, allowing charge to move through the circuit and light the bulb.
Materials that dissolve in water without producing any ions are nonelectrolytes. Sugar, ethanol, and kerosene are non-electrolytes. Non-electrolytes produce solutions that do not conduct electricity when dissolved in water.
Before the Activity
With the Students
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
Remind students to be sure to replace the aluminum foil electrodes each time they test a different solution because the electrodes may become contaminated with the previous solution.
Students should be careful to hold the foil electrodes slightly apart when placed in a liquid; they will get a false positive if the two electrodes touch.
Assessment (Return to Contents)
Discussion Question: Solicit, integrate and summarize student responses:
Prediction: Have students predict the outcome of the activity before the activity is performed.
Activity Embedded Assessment
Worksheet: During the activity, have students use the Will It Conduct? Worksheet to record their observations and answer questions.
Prediction Analysis: Have students compare their initial predictions with their test results, as recorded on the worksheets. Ask the students to explain why some solutions conducted electricity while others did not.
Inside-Outside Circle: Have the students form into two concentric circles (an inner-outer circle), so that each student has a partner facing him/her from the other circle. The outside circle faces in and the inside circle faces out. Three people may work together if necessary. Ask the students a question (see below). Have partners consult each other to discuss the answer. If they cannot agree on an answer, they can consult with another pair. Call for responses from the inside or outside circle or the class as a whole. Repeat until all the questions have been answered correctly. Questions:
Activity Extensions (Return to Contents)
Choose one conducting material to test with the conductivity tester. Obtain samples of the material having different lengths and cross-sectional areas. Challenge students to use their conductivity tester to show that resistance increases with increasing length and decreases with increasing cross-sectional area.
Have student's research acids and bases. What are some common acids and bases, and how are they used?
Have students conduct research on the use of different materials in electric circuits: copper, gold, aluminum, paper, plastic, etc.
Activity Scaling (Return to Contents)
References (Return to Contents)
Experiments in Electrochemistry, Fun Science Gallery, accessed March 2004. http://www.funsci.com/fun3_en/electro/electro.htm
Where Electricity Comes From, Southern California Edison, accessed March 2004: http://www.sce.com/
ContributorsXochitl Zamora Thompson, Sabre Duren, Joe Friedrichsen, Daria Kotys-Schwartz, Malinda Schaefer Zarske, Denise Carlson
Copyright© 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.
Supporting Program (Return to Contents)Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Last Modified: March 1, 2013