Materials

• Re-sealable plastic bag, 1 per pair of students, containing:
  • "D" cell battery, 3 per pair of students
  • Flashlight bulb, 4 per pair of students
  • Wire, (insulated copper wire with plastic coating, apx. 6 inches long with copper ends exposed), 10 per pair of students
  • Bulb holders, 4 per pair of students
  • Battery holders, 3 per pair of students
• Overhead of Models for Series and Parallel Circuits
• For voltage demonstration:
  • 9 volt battery
  • "D" cell battery
  • Household bulb

Prep Step

• Review the lesson plan, background information, and understanding goals.
• Gather materials for demonstration and put all other materials in the bags.

Analyze Thinking

Step 1: Checking Current Understanding

Ask students to explain, in their own words, what voltage, resistance, and current are. They can write their explanations down or discuss them in small groups. Gather their ideas.

Remind the students that voltage can be thought of as a "push" from the battery. Remind them about the drawing of the battery and how we discussed the power of the battery to push the electrons away from the protons (where they are attracted) and towards the negatives (where they are repelled). This power is voltage. It is measured in volts. Resistance can be thought of as opposition to the flow of electric current. It is measured in ohms. Current flow results from the amount of voltage and resistance, and is measured in amperes, or amps.

Find out what ideas students have about how voltage, resistance, and current relate to each other based on their explorations with parallel and series circuits. What do they think will happen in a circuit if any of the variables are increased or decreased?

Note to Teacher: This lesson does not have an "Explore Outcomes" or "RECAST Thinking" step because it advances understanding by analysis of the causality involved—the constraint-based relationship between voltage, resistance, and the resulting current.

Explore Causality

Step 2: Introducing Ohm's Law

Show the symbols in Ohm's Law: V = Voltage; R = Resistance; I = Current.

Pair the students up and give each pair a bag of materials. Have students set up a simple circuit.

Ask, "What can you do to increase the voltage?" Have students explain how they would increase the voltage. Add more batteries in series. What is the result in terms of resistance and current? The resistance stays the same because the students didn't add any more bulbs (or other resistors), and current increases.

Then ask, "What can you do to increase the resistance?" Have students show how they would increase the resistance. Add more bulbs or other resistors in series. What is the result in terms of voltage and current? Voltage stays the same because they haven't changed it, and current decreases.

Then ask, "What if you wanted to increase the current?" Have students show what they would do. Add more voltage or decrease the resistance. Ask, "Can you directly change the current? Why or why not?"

Step 3: Calculating with Ohm's Law

Introduce Ohm's Law: I = V/R.

Calculate some examples together:

• What is the current if the voltage is 9 volts and there is a resistance of 4.5 ohms? 2 amps.

• What is the current if the voltage is 220 volts and the resistance is 40 ohms? 5.5 amps.

• Can you figure out the voltage if you know the current and resistance? Yes. You just adjust the equation to V = I x R. Calculate some examples together.

• What is the voltage if the current is 5 amps and the resistance is 25 ohms? 125 volts.

• What is the voltage if the current is 12 amps and the resistance is 4 ohms? 48 volts.

• Can you figure out the resistance if you know the current and voltage? Yes. You just adjust the equation to R = V/I. Calculate some examples together.

• What is the resistance if the voltage is 110 volts and the current is 5 amps? 22 ohms.

• What is the resistance if the voltage is 60 volts and the current is 5 amps? 12 ohms.

Students do not need to memorize the three equations; if they know one, they should be able to figure out all three equations without any memorization. They can deduce any equation from the others.

Have students calculate the answers to some math questions about Ohm's Law.

• If the voltage is 100 volts and the resistance is 4 ohms, what is the current? 25 amps.

• If the voltage is 100 volts and the resistance is 50 ohms, what is the current? 2 amps.

• If the voltage is 300 volts and the resistance is 50 ohms, what is the current? 6 amps.

• If the voltage is 300 volts and the current is 10 amps, what is the resistance? 30 ohms.

• If the voltage is 300 volts and the current is 50 amps, what is the resistance? 6 ohms.

• If the current is 30 amps and the resistance is 10 ohms, what is the voltage? 300 volts.

• If the current is 50 amps and the resistance is 10 ohms, what is the voltage? 500 volts.

Ask, "How can we conceptually assess our answers as a check on our computation?" They can check to see if their computation goes in the right direction by knowing that current goes up if there is higher voltage and the resistance stays the same. The current drops if there is lower voltage and the resistance stays the same. The reverse is also true. The current goes up if there is lower resistance and the voltage stays the same. The current drops if there is higher resistance and the voltage stays the same.

Review, Extend, Apply

Step 4: Thinking About Which Batteries Can Light Which Bulbs

Show the students a "D" cell battery (1.5 volt), a 9 volt battery, and a large (household size) light bulb. Demonstrate that you can light the bulb with the 9 volt battery but not with the 1.5 volt battery. Ask, "What do you think is going on?" Help students to realize that the 'D' cell does not have enough voltage, or "push," to light the large bulb given the bulb's resistance. It cannot move enough electrons through the wire at one time. Therefore, the bulb does not light. A 9 volt battery does have enough push.

Step 5: Applying Ohm's Law to Series and Parallel Circuits

Go back to the Models for Series and Parallel Circuits and have students describe what is going on in each case. In the case of series circuits where you are adding more bulbs, you are increasing the amount of resistance in the circuit. In the case of series circuits, where you are adding more batteries, you are increasing the amount of voltage. In the case of parallel circuits where you are adding more bulbs, you are keeping the amount of resistance for each circuit the same. A parallel circuit is just like parallel loops of single-bulb circuits, with each bulb directly connected to both ends of the battery, so each circuit only has the resistance of one bulb, no matter how many parallel bulbs you add. In the case of parallel circuits where you are adding more batteries, you are keeping the amount of voltage for each circuit the same.²

Have students draw a model to show what is going on in at least one set of cases. If there is time, have them draw a model for both sets of cases (where you manipulate resistance and where you manipulate voltage.) You could use this activity as a good embedded assessment to see what the students understand.