034 Why did bulb Y become dimmer?

For simplicity, let’s assume that the resistances of the bulbs are constant.

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In the original arrangement, by the potential divider principle, V_AB = V_BC = 2 V. So bulbs X and Y were equally bright.

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When bulb Z was added in parallel to bulb Y, the effective resistance between nodes B and C was halved. By the potential divider principle, the potential difference across bulb X has increased to 2/3 of 4 V = 2.67 V, whereas the pd across bulb Y (and bulb Z) has dropped to 1.33 V. This explains why bulb X now shines brighter than before, whereas bulb Y dims compared to before.

• 13 Current of Electricity and DC Circuits