Materials

• Analyzing why Bulbs in Series are Less Bright than Bulbs in Parallel: Developing Rival Explanations sheet
• Models for Series and Parallel Circuits overhead (optional)
• Analogies for Analyzing Series and Parallel Circuits sheet
• Holiday Lights sheet
• One string of holiday lights (optional)

Prep Step

• Review the lesson plan, background information, and understanding goals.
• Photocopy: Analyzing why Bulbs in Series are Less Bright than Bulbs in Parallel: Developing Rival Explanations; Holiday Lights; and Analogies for Analyzing Series and Parallel Circuits sheets for each student.
• Make an overhead of Models for Series and Parallel Circuits. (This is optional. It can be drawn on the board instead.)

Analyze Thinking

Step 1: Analyzing the WHY Behind What Happens

Hand out the sheet, Analyzing why Bulbs in Series are Less Bright than Bulbs in Parallel: Developing Rival Explanations and have students think about the questions. Explain that the purpose of generating a rival explanation is to keep your mind open to other possibilities and keep you from becoming too invested in your first idea.

As a group, discuss the rival explanations that students created to explain what is going on with bulbs in series and in parallel. Are there parts of the explanations that they agree with? Are there parts of the explanations that they disagree with?

Note to Teacher: This lesson does not have an "Explore Outcomes" or "RECAST Thinking" step because it follows up on the experimentation in the previous lesson.

Explore Causality

Step 2: Analyzing Series and Parallel Circuits with a Cyclic Simultaneous Model

Explain to students that they are going to use the Cyclic Simultaneous Model for thinking about what is going on.

Draw the following diagrams on the board or use an overhead of Models for Series and Parallel Circuits to project on an overhead projector. These overheads show conceptual diagrams of series and parallel circuits. In schematic circuit diagrams, such as those that electricians use, and in pictures of physical layouts that show actual bulbs and batteries, it can be hard to visualize electron flow, and particularly the resistance of the bulb, because the schematic diagram for a bulb does not intuitively look like an impediment to current flow. The conceptual diagrams show a bulb's filament (which is the part of the bulb that carries electron flow) as a narrowing in the circuit, indicating its role as a resistor and that it impedes flow of electrons. The battery is shown as a large wide area indicating its role as a repository of electrons. Explain to the students what each part of each circuit represents.

Ask the students to imagine electrons along the entire circuit. (Of course, there are protons too, but since they do not move, we won't focus on them right now.) When the battery is hooked up and the whole circuit of electrons starts to move at once, what will happen in the bulbs in a series circuit? The resistance of both bulbs will affect the entire circuit. What happens in the bulbs in a parallel circuit? When the battery in a parallel circuit is hooked up and the whole circuit of electrons starts to move at once, what will happen in the bulbs? The resistance of each bulb will only affect the circuit for which it is a part.

Step 3: Using Analogies to Apply the Cyclic Simultaneous Model

Let's use some analogies to help think about what is going on. Explain to the students that you'll be asking them to critique these analogies because there are some very popular analogies that can be helpful in some ways and be misleading in others. You may want to handout the Analogies for Analyzing Series and Parallel Circuits sheet to facilitate the discussion.

Cars on a Highway Analogy:

Using the diagrams of the series and parallel circuits, ask the students to think of the conducting and filament wires as lanes of traffic and the electrons as cars. Notice the places where the bulbs are; it's like a tunnel or narrowed lane and only a certain amount of traffic can get through at once-for the purposes of illustration, say that only one lane of traffic can get through and the lanes on either side have to merge. The cars in the side lanes can't merge or flow through the tunnel very well. How many lanes will be flowing in each type of circuit? The series circuit will have one lane of traffic flowing, and it will be tough going where the bulbs/tunnels are. Since the electrons/cars can't go through easily, traffic will be affected in the entire circuit/highway. Everyone is affected because fewer cars can get through the tunnels, so movement in the whole circuit is limited.

The parallel circuit will have two lanes flowing because electrons/cars can go through either tunnel/filament; there are twice as many lanes to go through as in the series circuit. Because the electrons/cars can't get through easily, the bulbs/tunnels will still affect the entire circuit (because fewer cars can get through the tunnels than through the circuit wire), but much less so than for the series circuit.

What are some ways that the Cars on a Highway analogy works?

• It captures the difference in amount of flow of current between series and parallel circuits.
• It builds on the Cyclic Simultaneous Model to show what is going on.
• It explains the difference in resistance between the two circuits to show why, given the same amount of voltage, one allows there to be more flow of current (parallel) than the other (series).
• It forces students to view the circuit as a system, because if cars in one part can't move, neither can cars in another part. It's like one big traffic jam.

What are some ways that the Cars on a Highway analogy doesn't work?

• It makes it look like there are a few "rows" of electrons when in fact, there are many. Electrons don't move neatly in lines the way cars do.
• It doesn't communicate the idea of how difficult it actually is for electrons to move through the filaments.
• It might give the impression that resistance has to do with the speed of electron movement. Resistance is really NOT about speed, it is about how difficult it is for electrons to get through and how many of them can get through.
• It doesn't account for voltage. The closest analogy is horsepower or speed, but again this leads to the idea that the effect of resistance is speed/slowness, not ease/difficulty.
• It may lead students to think that there is half as much resistance in a parallel circuit, because there are twice as many "lanes." This is not the case; there isn't a simple relationship between resistance in parallel and series circuits.

Water in a Hose Analogy:

Another analogy that people like to use is "water in a hose." The circuit is thought of as a hose. When you turn on the water, the amount that you turn on (or the pressure you create) is analogous to the voltage or amount of push that the battery has. If you clamp down on any part of the hose, it is analogous to adding resistance the way that a bulb adds resistance. The water has a harder time getting through the clamped part of the hose, and affects all of the water behind it. The current is represented by how much water flows, and is the result of how much water pressure/voltage there is and how much clamping/resistance there is. If you clamp multiple places in the hose, less water is able to get through the entire hose. How does this help in understanding series and parallel circuits? How much "water" will flow through each? The series circuit will have two or more clamped spots but only one hose, and it will be difficult for water to go through the clamped spots (bulbs). Since water can't go through the clamped parts easily, this will affect the water on both sides of the clamps. Less water will flow in the entire hose and less will trickle out the end of the hose.

The parallel circuit will have two or more clamped spots too, but it will branch from one hose into two (or more) hoses for the water to go through. The water can't get through the clamped spots easily, so it affects the water on both sides of the clamp, but less so than for the series circuit because there are more hoses to travel through.

What are some ways that the Water in a Hose analogy works?

• It turns students' attention to the systemic nature of what is going on.
• It shows the relationship between voltage (how much you turn the spigot), resistance (how many kinks in the hose), and current (how much water is flowing).
• It explains the difference in resistance between the two circuits and shows why, given the same amount of voltage, one allows for more current (parallel) than the other (series).
• Many students have experience with hoses, so they understand the components of this analogy well.

What are some ways that the Water in a Hose analogy doesn't work?

• It gives the erroneous idea that electrons are substance-like, rather than process-like.
• The hose is empty before the water enters it. This could inadvertently reinforce a Cyclic Sequential Model instead of a Cyclic Simultaneous Model.
• It does a nice job conveying that less water gets through, but doesn't convey the aspects of resistance that are related to difficulty of electron movement.
• In order to think about the result of resistance on current, one has to think about what comes out of the hose, but a circuit does not have a similar "ending." This could inadvertently reinforce a Linear Model.
• In reality, one clamp on a water hose would decrease the flow of water. However, it is not clear whether a second clamp would actually affect the system in the same way that a second light bulb adds additional resistance in a circuit. So increasing the number of clamps is not directly analogous to adding bulbs and therefore increasing resistance.

What questions do students have? What do they understand? What do they find confusing about the analogies? Encourage them to explain what they think is helpful and not so helpful about the analogies.

Are there any other analogies that the students can think of? Ask the students to put their heads together with a neighbor and to think of other analogies to explain what is going on with parallel and series circuits. Share and discuss whatever ideas they come up with.

Review, Extend, Apply

Step 4: Analyzing Analogies for Series and Parallel Circuits

Ask students to choose one of the analogies presented, or one that they or others thought of in class, and explain it in their own words and drawings. They should also look for ways to fix the "problems" that we identified with the analogy they choose.

Step 5: Making Connections

Ask students to think about the examples of parallel and series circuits that they looked for around home and school in Step 3 of Lesson 5. Together, analyze a few of the examples in terms of the analogies discussed in this lesson. Ask, "Do you think that the lights in your school are wired in series or in parallel? Decide which you think it is and come up with an argument to support it."

Students might consider what happens when one light goes out in their room. Do the others go out, too? If it was a simple series circuit, they would. It would be inconvenient if one burned-out bulb affected the lights in the entire building! Perhaps they also considered how much resistance such a long series of lights would entail. A lot of voltage would be necessary to have enough light.