Shorting a Bulb: What Happens When You Bypass One

1 · Predict

Two bulbs (lampA, lampB) are in series. A switch, wired in parallel across lampB, is initially open. If you close it — creating a zero-resistance shortcut around lampB — what happens to lampA's brightness?

• LampA dims when the switch closes.
• LampA's brightness doesn't change.
• Closing the switch bypasses lampB, so lampA now gets the full circuit current — it brightens.

2 · Set Up

1. Open the short-a-bulb preset and press Reset. Both bulbs start at equal resistance (100 Ω); the switch starts open.
2. Enable the current readout on both bulbs.
3. Record lampA's current with the switch open, then close the switch and record it again.

3 · Collect Data

Make a bar chart comparing lampA's current in the open-switch and closed-switch cases.

4 · Analyze

1. With the switch open, both bulbs are in series: I = E/(2R) using E = 9 V, R = 100 Ω. With it closed, lampB (100 Ω) is bypassed by the 0 Ω switch, leaving just lampA: I = E/R.
2. Explain why a zero-resistance path in parallel with lampB carries all the current, leaving zero current through lampB itself once the switch closes.

5 · Extend

1. LampA's power goes up four times when the switch closes — the current doubles, and P = I²R goes as the square of it (the same reasoning as switched-lamp's). Where does that extra power come from — was the battery working equally hard in both cases?
2. A real switch has a tiny but nonzero resistance when closed. Would that change the qualitative result here (lampA brightening), or only the exact numbers?

The Physics Behind This Experiment

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