Two Bulbs, One Old Battery, One Bad Connection
A group builds a circuit with one D-cell battery, two wires, and a small bulb. The bulb lights brightly. They add a second identical bulb into the same single loop, and both bulbs now glow dimly. Next they swap the battery for an older one from a drawer, and neither bulb lights at all. Finally they put the first battery back, but one wire's bare end is resting on the glass of the bulb instead of touching the metal base, and again nothing lights. The group writes, 'The second bulb broke the circuit.'
1. Evaluate the group's claim using what happened at each step.
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The claim does not fit the evidence. With two bulbs in the loop, both bulbs still lit, just dimly. A dim glow means current is flowing all the way around, so the circuit was complete. If the second bulb had broken the circuit, nothing would have lit at all. The dimming has a different explanation: the battery's energy is now shared between two bulbs, so each gets less. The two failures that came later each had their own cause and should not be blamed on the second bulb.
2. Explain the two failures separately, using the idea of a complete path.
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Failure 1: the older battery. The bulbs did not light because the battery had little energy left to push current, not because the path was broken. A weak battery still makes a complete path; it just cannot drive enough current. Failure 2: the wire on the glass. Glass does not conduct electricity, so the wire end resting on glass left a gap in the path. Current cannot jump the gap, so the circuit was open and nothing lit. Touching the wire to the metal base would close the path again.
3. The group has the good battery, two bulbs, and three wires. Describe a test that would show whether one bulb is burned out, and predict what a third bulb would do.
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Test: connect the good battery to one bulb at a time with two wires. If one bulb lights alone and the other does not, the second bulb is burned out. Swapping bulb positions in a working circuit is an even quicker check, because a burned-out bulb will fail in either position. Prediction for a third bulb: with the good battery and three bulbs in one loop, all three should light but each will be dimmer than with two, because the same battery is now shared three ways. There is a limit - with enough bulbs the glow may become too faint to see, but the path is still complete.