Materials & Prep
Gather batteries, bulbs, wire, and bulb holders. Place students in groups of three or four with one battery, one bulb, one holder, and two or more wires per group. Prepare a recording sheet with columns for Setup, What we observe, and What we infer. Remind students to follow school procedures for low-voltage electrical materials and confirm local safety procedures before class. Do not allow students to connect the two battery terminals directly with a wire.
Opening: Connected routes
Display two simple drawings: Route A shows a continuous road from a house to a park and back to the house. Route B shows a road from the house toward the park that stops at a gap. Ask students to point to or circle the route that lets a traveler leave the house, reach the park, and return. Students should succeed using their existing understanding of connected paths.
Turn and talk“What does a traveler need in order to get all the way there and back?” Listen for “a connected route,” “no gap,” or “a way back.”
ConnectSay, “You just used the key idea for today. Electricity also needs a connected route. We call that route a complete circuit. Today we will find out why one wire from a battery to a bulb is not enough.” Keep the road drawing visible as a single analogy for the idea of a complete path.
Mini-lesson and modeling: Trace the path
DisplayDraw a battery, bulb holder, and wires. Label the battery’s two ends as the positive terminal and negative terminal. Explain that electric current can travel through the bulb only when there is a path from one battery terminal, through the bulb, and back to the other terminal.
ModelFirst connect one wire from the battery’s positive terminal to one metal contact on the bulb holder. Ask students to predict whether the bulb will light. Test it. The bulb stays off. Think aloud: “The wire reaches the bulb, but the path stops there. There is no connected route from the bulb back to the battery’s negative terminal. One end of the path is open.”
ConfrontName the class misconception directly: “A single wire touching the battery and bulb may look connected, but it does not make a complete circuit.” Ask students to show with a finger where the path stops in the one-wire setup.
Worked exampleAdd a second wire from the bulb holder’s other contact to the battery’s negative terminal. Test it. The bulb lights. Trace the route aloud: “The path starts at the positive terminal, travels through the first wire, enters the bulb, leaves through the second holder contact, and returns through the second wire to the negative terminal. There are no gaps, so the bulb lights.”
ClarifyThe bulb is not lit simply because wires touch it. Both contacts of the bulb must be part of the path, and the path must return to the battery’s other terminal.
Guided practice: Predict, build, explain
Have students study three drawings on the board: A, one wire from the positive terminal to the bulb; B, two wires connecting the positive and negative terminals through the bulb holder; C, two wires attached to the positive terminal, with no wire returning to the negative terminal. Students first predict “on” or “off” for each drawing and point to the gap or missing return path.
Check for understandingAsk, “In setup C, the bulb has two wires touching its holder. Why is it still off?” Require the sentence frame: “It is off because the path does not from one battery terminal to the other.” Expected answer: “return” or “connect.” Quickly correct any answer that focuses only on the number of wires.
ModelBuild setup B with one student volunteer. Have the student trace the path with a finger while saying, “From the battery, through the bulb, and back to the battery.”
Investigation: Find the complete path
Students work in groups of three or four. One student builds, one observes the bulb, one traces the path, and one records. Rotate roles after each setup. Students use the same battery, bulb, holder, and wires to test these configurations:
- One wire from the positive battery terminal to one holder contact. Leave the other holder contact and negative terminal unconnected.
- Two wires, one from each battery terminal to the two holder contacts.
- Two wires attached to the same battery terminal, with the other battery terminal unconnected.
- A complete circuit, then carefully remove one wire and observe what changes.
Students record for every setup: “The bulb was on/off.” “I observed .” “I infer the path was complete/incomplete because .” They must draw an arrowed path for at least one setup that lights and one that does not. The group checklist is: both bulb contacts connected, both battery terminals included, path returns to the other terminal, observation recorded before explanation.
ScaffoldGive groups a path-tracing card with three prompts: “Where does the path start?” “Does it pass through both bulb contacts?” “Does it return to the other battery terminal?” Students may trace the wires with a finger before writing. Accept a labeled drawing or dictated observation as well as a sentence.
CirculateLook especially for students saying, “It has two wires, so it must work.” Ask them to trace the entire route and identify the exact gap. Distinguish observation from inference: “The bulb is dark” is an observation; “the circuit is incomplete” is an inference supported by tracing the path.
ExtensionStudents design a fifth setup that looks connected but leaves one gap. They predict its result, test it, and explain why the bulb stays off. Students ready for a deeper challenge may explain why touching both battery terminals with one wire is not a useful circuit through the bulb and must not test that arrangement.
Closing check: Claim, evidence, reasoning
Exit ticketStudents answer independently: “Why does a bulb light only when there is a complete path?” They must include one claim, one piece of evidence from today’s investigation, and the path in order. Sentence frame: “The bulb lights only when . In setup , I observed . This shows that current must .”
DiscussInvite two students to trace the path aloud, one for a lighting setup and one for a nonlighting setup. Collect the exit tickets and check that students explain both the route through the bulb and the return to the other battery terminal, not merely that wires are touching.