• Assess students’ understanding of how electrical current flows through cells by observing and asking questions during the lab activity and checking students’ answers to the lab follow-up questions.
• Assess students’ understanding of the relationship between resistance, current, and voltage during the Day 2 explicit instruction discussion of the warm-up and the circuit and by asking questions during the example problems.
• Collect the Ohm’s Law Practice Problems and the Vocabulary Practice worksheets or go over them in class to check for understanding.

Day1

In preparation for the lesson, gather all materials and divide them according to the number of lab groups. Cut the wire into 4-inch strips and strip the insulation from the wires. Use sandpaper lightly on the ends of the nails and wire. Set up a potato battery in advance as a demonstration, as shown below.

The objects inserted into the potato should be as close together as possible, without touching. If they touch, the battery will be shorted and no voltage will be produced. Use the DC setting on the voltmeter. If the voltmeter shows a negative number, switch the wires around. If the potatoes are large, they can be cut in half just before the experiment.

Background: A potato battery works because it provides a potential difference similar to how a car battery works. When two different metals are placed in the potato, its juice acts as an electrolyte. The nails and wire act as the positive and negative terminals (i.e., cathode and anode). When the voltmeter is hooked up, current flows through the electrolyte and electrons flow through the wire. If the two metals are the same, the chemical reactions do not occur, no ions flow, and no potential difference is created. The voltage produced in the potato cell is not strong enough to light a small light bulb.

The voltage can be increased by putting the potatoes in series. The increased voltage can be used to power small light bulbs. If time permits, connect several of the groups’ cells and measure the voltage. Connect a small light bulb or LED clock to test whether it works.

This experiment can also be performed using lemons or limes instead of potatoes. It may be interesting to have some groups use lemons and some use potatoes and then compare the voltage produced with each.

Lab: Producing Potato Power

Show students a voltmeter and explain that it is used to measure the voltage as electrical current flows between two points. Demonstrate how the voltmeter works using a C- or D-cell battery, and point out that the unit for voltage is volts (V). Explain that a battery is a type of cell, a device that generates electricity by means of chemical reactions. Define electrode and point out the electrodes on the ends of the battery. Tell students that the voltage difference between the electrodes is what makes a flashlight bulb work. Define electrolyte as a mixture of chemicals that conducts a current inside a battery.

Show students a potato and ask them if they think it can produce a current too. Divide the class into lab groups and give each group the lab materials. Give the groups about five minutes to discuss how a potato could work as a cell, without actually building one yet.

Then, give students the Producing Potato Power lab worksheet (S-6-6-2_Potato Power and KEY.docx). Be sure to show students the difference between the steel nail and the zinc plated nail. If desired, lead groups through the potato cell set-up step-by-step. Monitor students’ progress on the lab and answer questions as they arise. Instruct students not to write the conclusions to the lab yet. After students clean up all materials, call on several students to share their observations from the lab. Review how the potato cell works, especially the concept that a current is produced because chemical reactions in the electrolyte create a potential difference between the electrodes. Have students record definitions in their notes for battery, cell, electrode, electrolyte, potential difference, voltage, voltmeter.

Day 2

In preparation for the lesson, set up a simple circuit that includes a D-cell battery, a light bulb, and two wires, similar to the one shown below. Have a voltmeter, an extra battery, light bulb, and extra wires on hand for demonstration as you explain Ohm’s law.

Source: www.eia.doe.gov/energyexplained/images/battery2_small.jpg

Have students answer the following questions as a review and transition to Ohm’s law. Allow them to use their notes and lab worksheet for this activity.

1.   What are the units for measuring voltage? (Volts)

2.   What is the symbol for voltage? (V)

3.   Voltage is another word for ____________________. (potential difference)

4.   The higher the voltage, the more _________________ is released. (energy)

5.   The greater the voltage, the greater the electrical _______________. (current)

Review the answers to the warm-up questions above with the class. Emphasize the relationship between current and voltage: As voltage increases, current increases, and vice versa.

Show students the circuit you set up beforehand. Explain that a circuit is a complete, closed path through which electric charges flow. Ask, “What provides the voltage in the circuit?” (the battery) Ask, “How you could measure the voltage in the circuit?” (with a voltmeter) Use the voltmeter to measure the voltage in the circuit. Ask, “What does current measure in a circuit?” (the rate that electric charge passes through the circuit)

Tell them, “The unit for measuring current is amperes, or amps for short. Its symbol is A. What happens to the current if we increase the voltage?” Demonstrate by adding another battery to the circuit. (The current increases.) “What happened to the light bulb?” (It got brighter.) Remove the extra battery. “Now, what do you predict will happen to the light bulb if we add another light bulb to the circuit?” Add a light bulb as students answer. (It became dimmer.)

Explain that light bulbs are a type of resistance in a circuit. Define resistance as anything that opposes the flow of electric charge. Tell students, “The higher the resistance, the lower the current. The units are ohms, with the symbol Ω.”

Resistance depends on four things. Have students copy these factors into their notes:

1.      Conductivity: materials that are good conductors, such as copper, have low resistance; poor conductors, such as iron, have higher resistance.

2.      Thickness: A thick wire has less resistance than a thin wire.

3.      Length: A long wire has more resistance than a short wire.

4.      Temperature: Resistance increases as temperature increases.

Ask students how the class could increase the resistance in the circuit in each of the four ways above. Have students describe ways to alter the resistance in the circuit (e.g., use a thinner wire). Tell students that appliances in the home are all examples of resistors, and ask them for examples. (toaster, computer, hair dryer, lamp)

Review voltage, current, and resistance by having students fill in the chart below:

Explain, “In the 1820s, a scientist and professor named Georg Ohm developed a way to explain how voltage, current, and resistance are all related to each other. It is called Ohm’s law.” Ohm’s law is represented by the equations:

Have students copy the equation into their notes, and then add a column on the right of the chart in their notes and fill in the symbols for the Ohm’s law equation.

Explain that electricians and construction workers use Ohm’s law every day in their work. Examples:

• Electricians use the formula to install the correct gauge (thickness) of wires in circuits.
• Factory technicians use the formula when they install machinery and equipment.
• All construction workers must check that their extension cords are adequate for the power tools they are using.

Solve a couple of practice problems with students.

Example 1

You have a circuit with 25 volts and a resistance of 5 ohms. What is the current in the circuit?

Step 1: I = 25 volts/5 ohms

Step 2: I = 5 amps

Example 2

A 9-volt battery supplies power to a cordless hair dryer with a resistance of 18 ohms. How much current is flowing through the curling iron?

Step 1: I = 9 volts/18 ohms

Step 2: I = 0.5 amps

Independent Practice: Ohm’s Law

Hand out the Ohm’s Law Practice Problems worksheet (S-6-6-2_Ohm's Law and KEY.docx). Have students work independently to complete it.

Vocabulary Review

Assign the Vocabulary Practice worksheet as an in-class review or homework assignment (S-6-6-2_Vocab Practice and KEY.docx).

Extension:

• Students who might need more practice can use Version 2 of the Vocabulary Practice worksheet that includes all vocabulary terms and their definitions (S-6-6-2_Vocab Practice and KEY.docx).
• Students who might need an opportunity for additional learning while using the Ohm’s Law Practice Problems worksheet can pair with other students who can help them with the math. Allow students to use calculators to solve the problems.
• Students who might be going beyond the standards can design and/or build circuits that demonstrate the relationships shown in Ohm’s law (e.g., increasing the voltage to increase the current).
• Students who might be going beyond the standards can debate these questions: Was electricity invented or discovered? What is the difference between an invention and a discovery?
• Students who might be going beyond the standards can solve Ohm’s law problems for voltage or resistance, using the equations V = I × R and R = V/I. See Related Resources for Ohm’s law example problems.