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A room is supplied at 120 V through lead wires of total resistance 6 ohm. A 60 W bulb is already glowing. By how much does the voltage across the bulb drop when a 240 W heater is connected in parallel with the bulb?

1. zero Volt
2. 2.9 Volt
3. 13.3 Volt
4. 10.04 Volt

Correct answer: 10.04 Volt

Solution

Adding the heater in parallel lowers the load resistance, so more current flows through the lead wires and a larger fraction of the 120 V is lost across them, reducing the bulb voltage by about 10 V.

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