Two Ways to Wire Two Bulbs
Students have two identical bulbs, two D-cell batteries in a holder, and connecting wire. They build two arrangements.
Arrangement 1: one single loop - battery, then bulb 1, then bulb 2, back to the battery.
Arrangement 2: the same battery, but bulb 1 and bulb 2 sit side by side, each with its own separate path to the battery terminals.
In Arrangement 1 both bulbs glow dimly. In Arrangement 2 both bulbs glow brightly. The students then unscrew bulb 1 in Arrangement 1, and separately unscrew bulb 1 in Arrangement 2. Bulb 2 in Arrangement 1 goes dark; bulb 2 in Arrangement 2 stays lit.
1. Explain why the bulbs are dim in Arrangement 1 and bright in Arrangement 2.
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In Arrangement 1 every charge that leaves the battery must pass through both bulbs in a single path. Two bulbs in one path means twice the resistance, so less current flows, and each bulb gets only part of the battery's push. Result: two dim bulbs.
In Arrangement 2 each bulb has its own complete path back to the battery. Charge moving through bulb 1 does not have to pass through bulb 2, so each bulb gets the full push of the battery and draws as much current as it would alone. Result: two bright bulbs. The battery is also supplying current to two separate paths at once, so it delivers more total current and drains faster than in Arrangement 1.
2. The students unscrew bulb 1 in each arrangement. Predict what happens to bulb 2 in each case, and explain the prediction using the ideas of complete circuits and current paths.
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Arrangement 1: bulb 2 goes out. The two bulbs share one loop. Unscrewing bulb 1 opens that loop, so there is no complete path from one battery terminal to the other. Current stops everywhere in the loop, and bulb 2 is dark even though it is undamaged.
Arrangement 2: bulb 2 stays lit. Bulb 2 has its own path to the battery that does not pass through bulb 1. Breaking bulb 1's branch only removes that branch; bulb 2's circuit is still complete.
This is the key test: if removing one bulb kills the other, the bulbs were in series; if the other keeps working, they were in parallel.
3. A student claims, "Arrangement 2 gives the same total amount of light as Arrangement 1, because both use the same two bulbs." Evaluate this claim.
Read the worked answer
The claim is incorrect. Same bulbs does not mean same light output.
In Arrangement 1 the bulbs are dim, so the pair produces less light than two bright bulbs. In Arrangement 2 each bulb gets the full battery push and glows brightly, so the pair produces more light - roughly twice what one bright bulb gives.
The extra light is not free: the battery in Arrangement 2 must push current through two separate paths, so it delivers more total current and will run down sooner. A fair statement is: Arrangement 2 produces more light but drains the battery faster, while Arrangement 1 produces less light and lasts longer.