A first PhET circuit lesson: why won’t the bulb light?

The bulb is off. Before adding another battery, ask: “Can you trace a complete path through the bulb and back to the battery?” That question gives this small science activity a purpose. You will compare an incomplete circuit with a complete one, changing a connection rather than several things at once.

We tested this sequence in PhET’s public Circuit Construction Kit: DC – Virtual Lab on October 26, 2026. It opened and ran in our desktop browser without signing in or paying. This is a tested browser procedure and an original parent-led activity, not a classroom trial, a full science curriculum or evidence of a child’s learning.

Open the activity and keep the goal small

Follow the official link above and select Play Simulation. Use the browser version for this activity; you do not need to buy an app. We did not test the apps, offline use or a physical phone or tablet.

Give the learner a few minutes to explore the toolbox before starting the comparison. PhET’s activity-design guidance recommends time for exploration and short prompts rather than instructions for every possible action. Our goal is just to explain the difference between an open and a closed path through one bulb. Leave resistance, brightness comparisons and current direction for another lesson.

Keep this activity inside the simulation. The drawing below is a simplified model, not instructions for wiring household electricity.

Build one path through the bulb

1. Drag one Battery and one Light Bulb from the toolbox into the construction area. Leave their settings unchanged.
2. Add a Wire. Drag its endpoints onto one battery terminal and the bottom connection point of the bulb. In our test the bulb remained off: the path was incomplete.
3. Add a second wire. Connect one end to the bulb’s other connection point, at its side. Leave the other end away from the remaining battery terminal.
4. Pause before making that last connection. Ask what will happen and why.
5. Drag the free endpoint onto the remaining battery terminal. In our test the bulb lit when the circuit was complete.

Each wire endpoint must connect to a component’s connection point. A wire that only looks close is not necessarily joined. Trace the path from one battery terminal, through a wire and the bulb, along the other wire, to the other battery terminal.

Break one connection, then reconnect it

Keep the same battery, bulb and wires. Select a joined connection point; in our desktop test this displayed a scissors control. Selecting it disconnected the join and the bulb went off. We then dragged the free wire endpoint back onto the battery terminal and the bulb lit again.

Ask the learner to explain the two states: open circuit means a break in this path; closed circuit means the path is complete. With the default working battery and bulb in this simple setup, the closed circuit allowed current through the bulb. Do not turn this into “every closed circuit lights a bulb”: different components, settings or faulty physical equipment can change what happens.

If the learner predicts that an extra battery is needed, keep that as a prediction to discuss. First change only the missing connection. That makes it easier to connect the observed result to the change you actually made.

If the bulb still does not light

• Trace both sides: check the return path, not just the wire leaving one terminal.
• Inspect the endpoints: connect to the bottom and side bulb connection points and to both battery terminals.
• Check the route: it must pass through the bulb. Do not join the battery terminals directly with a wire that bypasses it.
• Simplify: remove extra components or use Reset All to begin again. Reset clears the construction, so record anything you want to keep first.

For keyboard operation, the simulation offers a Keyboard Shortcuts control. Our test used pointer dragging; we have not completed a keyboard or screen-reader accessibility audit.

Where this fits

This is a small electricity investigation for a learner ready to discuss a cause and an observed effect, with adult help as needed. We do not assign a tested age range or claim alignment to a particular curriculum. For other subjects, our checks for understanding offer further question prompts. Add the activity to a flexible weekly plan when it fits the topic being studied.

Sources, illustrations and limits

Sources checked October 26, 2026: the linked official simulation and PhET activity-design guidance. The guidance is an older teaching resource; the browser sequence above was checked today. We observed the bulb off with an incomplete path, on after connection, off after disconnection and on after reconnection. We did not assess learning outcomes, every simulation feature or device compatibility.

All diagrams and prompts here are original Learning Shortlist work. We link to PhET rather than embed its simulation or reproduce its interface. See PhET’s current licensing information before reusing its assets in your own published materials. We are not a PhET affiliate and these are ordinary source links.