A 25-W bulb designed for 120 V is connected to 240 V. If its resistance stayed constant, what power would it draw?

1. At constant resistance, P = V squared / R, so power scales with the square of the voltage. 2. Going from 120 V to 240 V doubles the voltage. 3. Squaring that factor of two gives a factor of four in power. 4. So P = 4 times 25 W = 100 W. 5. 50.0 W is the trap of scaling power linearly with voltage instead of with its square. 6. In practice the filament runs hotter and its resistance climbs, so the real figure is a little under 100 W before it burns out. _Source: OpenStax College Physics (CC BY 4.0), Ch 20 "Electric Current, Resistance, and Ohm's Law", section Power in Electric Circuits_