The SCI 301 Puzzler

Here's a real puzzle that had me baffled for quite some time. (Don't worry, I'll give you information that took me a while to find.) You may have noticed that when the refrigerator compressor starts up, lights on the same circuit will temporarily dim. This occurs due to the heavy initial current drawn by the compressor that creates a voltage drop across the wires. (This is described more fully in the last part of the "Basic Electric Circuits" section of the Electricity module.)

But something very peculiar happens in my home when the water pump turns on. The light in the bedroom actually gets brighter! Several months later I was visiting a fellow UVI physicist and he had the same experience. We did some investigation and discovered an important clue. The pump and light were on different phases. That is, both share the center line (the white "neutral" wire) but are connected to opposite sides of the 240 VAC coming from the transformer. The simplified diagram below show before and after the pump turns on. Although the current is AC, it is easier to understand what happens if we look at the equivalent DC circuit. Imagine that the top wire is a 120 VDC, the neutral is 0 VDC, and the bottom is -120 VDC, at the transformer. The lamp is on and drawing about 1 amp. Remember that the household wires do have a bit of resistance, so the potential drop across the lamp will be a little less than 120 V. Now the pump turns on, drawing a much larger amount of current. Can you explain why the lamp would become brighter when the pump turns on? (Hint: The resistance in the wires is important.)

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