Simple Circuits Curriculum

Section 4: Lesson Plan

Materials

• "D" cell battery, 2 per pair of students
• Wire, (insulated copper wire with plastic coating, apx. 6 inches long with copper ends exposed), 3 per pair of students
• Lumps of clay or play dough, 1-2 lumps per pair of students
• Re-sealable plastic bags, 1 per pair of students, with the following items:
  • 2 inch piece of #26 and #32 Nichrome wire
  • 2 inch piece of #32 steel wire
• What Contributes to a Good Filament?: Testing Variables sheet (PDF 72 KB), 1 per pair of students
• For resistance demonstration:
  • Marbles, apx. 100 white and 100 black
  • Clear plastic tubing filled with marbles from Lesson 3, approximately 3 feet long, 1 ½ inches in diameter
  • Small plastic tubing that is approximately half of the circumference of the larger tube.
  • One set of objects that were tested in Lesson 3.

Prep Step

• Review the lesson plan, background information, and understanding goals.
• Collect materials and put 2 inch piece of #26 and #32 Nichrome wire, and 2 inch piece of #32 steel wire in a re-sealable plastic bag. (Nichrome and steel wires can be purchased at a hardware store; however, they may be difficult to find. Plan ahead as you may need to go to a specialty hardware store to buy these.)
• Photocopy What Contributes to a Good Filament?: Testing Variables sheet, 1 sheet for each pair of students

Analyze Thinking

Step 1: Analyzing Initial Thinking about the Filament

Ask students to think about what is in a light bulb. This is a review from Lesson 2. What do they think is going on along the filament that causes the light bulb to light? Have them draw a model of their ideas. You may see some of the ideas explained in the introduction (where protons and electrons clash or attract and so forth).

Explore Outcomes

Step 2: Creating Filaments

Explain to your students that they are going to create filaments in class today. Students should work in pairs. Demonstrate how to build a circuit for testing the filaments. The sheets Diagram of a Circuit Tester (PDF 78 KB) and Photograph of a Circuit Tester (PDF 141 KB) illustrate how the circuit should look. The students will use lumps of clay or play dough to secure the wires in an upright position. Pass out the sheet, What Contributes to a Good Filament?: Testing Variables. Students should make the comparisons in steps 1-3 below to see what happens when they hook up a wire as a filament.

Students should work step-by-step exploring answers to the following questions:

1. Does the thickness (width or diameter) of the filament wire matter?
   Have students first wrap a 2 inch length of #26 of Nichrome wire between the two supporting wires and test it with the battery. Then, have them wrap a 2 inch length of #32 wire and compare. Explain that #26 is actually thicker than #32. Discuss findings.
2. Does the type of material of the filament wire matter?
   Have students compare a 2 inch length of Nichrome wire to a 2 inch length of steel wire of the same thickness (#32 Steel to #32 Nichrome). Discuss findings.
3. Does the temperature of the filament wire matter?
   When students first hooked up any of the wires, did the effects happen immediately, or did it take a while to notice the effects? Did the effects appear to increase as the wire got hotter?

Step 3: Thinking About Resistance

After the students have had a chance to test each variable, gather them together as a group and review their results. They should have found that the #32 Nichrome wire glows a bright red and that it takes a few seconds to turn bright red. The #26 wire gets hot and appears a little white. The steel wire warms up some but the least of all.

Discuss what is happening using the Cyclic Simultaneous Model to think about the circuit. Use the shower curtain model and the plastic tubing model as part of the discussion, or draw the concepts on the board. Remind students that the circuit has electrons all along it. Review the idea that the electrons are all moving at once like a bicycle chain. Focus on the filament. Point out that the filament is very hard to get through and this makes it harder for the electrons to flow. We call this resistance. Resistance is measured in ohms. Try to stay away from the idea of resistance as having to do with speed. It really has to do with the possibility of pushing electrons along the filament and the difficulty of doing so.

Give students an analogy to help them think about what is going on, such as; "Think about how it feels when you run along a beach. Now think about how it feels when you run in the water. Which one feels harder?" Nichrome wire is like "running in the water" for electrons, and copper wire is like "running on the beach" for electrons. There is more resistance to movement through water than through air.

Illustrate each of the variables that determine how easy it is for electrons to move along the filament.

• The thickness (width or diameter) of the filament wire: Show your students the wide tubing with marbles in it. Next, show the small piece of tubing that represents the filament. Electrons have a harder time moving through the narrower wire than the wider wire, so forward flow is impeded.
  
  Draw a 2 inch square on board. Then draw a 1 inch square that fits in one corner. The #32 Nichrome wire is much smaller that the #26 Nichrome wire. (Remind the students that the larger number goes with the thinner wire.) This shows that the smaller wire (though not hollow) makes the flow harder because there are not as many atoms available (i.e. electrons to flow). Make sure students understand that while the pipe and the tubing in the analogies allow things to flow within them, a wire allows things to flow along it. The electrons that make up the wire are moving.

• The type of material of the filament wire: Discuss the concept of insulators and conductors on a continuum. In an insulator, there is not much opportunity for electrons to move. In a conductor, there are many opportunities for electrons to move, allowing the current to flow easily. Explain that the Nichrome wire is somewhere in between. Because of the way that the atoms are bonded, there are places where electrons cannot move, and these places become obstacles to the overall flow of electrons. Electrons can move along the wire, but it is very difficult. Impeding the flow of electrons results in an energy transfer (the electrons bounce all around instead of moving forward, further impeding flow) that heats the fine wire, which becomes hot enough to glow and give off light. Show students some materials along the rough scale of conductors to insulators (from their experimentation in the last lesson).
  
  Ask, "Where would we put these materials on the scale?"

Make sure that students realize that the circuit wire has less resistance than the filament wire. The circuit wire is typically made of copper. Resistance is an impediment to the flow of electrons. The resulting amount of current throughout the entire circuit depends upon the amount of resistance in the circuit.

• The temperature of the filament wire: The hotter the wire gets, the more the electrons bounce all around, raising the temperature and giving off heat and light energy. With an increase in how much the electrons are bouncing around, resistance increases. Imagine trying to move in a forward direction in a crowd where everyone is bouncing back and forth in random directions. It would be pretty tough to maintain your forward path!

Explore Causality

Step 4: Reflecting on Passive Causality

Explain to the students that when we think about a circuit, it is easy to forget about resistance. The role of the electrons is easier to think about because they are doing something active—they are agents! But resistance is a very important part of what determines the amount of current flowing in the circuit. Later lessons will further address the role of resistance. Passive causality can be understood by thinking of other variables or structures that somehow cause an effect but do not actively "do" anything. For instance, a high fence keeps people out of certain places and a bridge allows people to cross a river, but neither a fence nor a bridge is moving or active.

Pose the following question:

"Some students want to talk about resistance as a push in the opposite direction. Do you think that this is a good idea or not?" Gather students' thoughts. A problem with this is that it could lead students to incorrectly think that the push is there when there is no current flow. Instead, it helps to focus on resistance as an impediment to flow.

Review, Extend, Apply

Step 5: Making Connections: Explaining a Short Circuit

Now that students have thought more about resistance, ask them how they would explain a short circuit. What's going on? Students should realize that a lot of current moves through the circuit because there is nothing to make it more difficult for electrons to move.

Step 6: Making Connections: Blowing a Bulb

Show what happens when you burn out a bulb by using too high a voltage. You can see material from the filament wire splattered across the inside of the bulb. Metal atoms that make up the filament are now inside the glass bulb. The very high current created so much heat in the filament that the metal vaporized. Atoms got jostled so hard that they literally blew apart! Even though there was resistance, the voltage was so great that the electrons really moved, overheating the filament.