Chapter 25 Problem 38

Problem

You buy a 75-W lightbulb in Europe, where electricity is delivered to homes at 240 V. If you use the lightbulb in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs? [Hint: Assume roughly that brightness is proportional to power consumed.]

Solution

This is a resistive load

$P={\frac {V^{2}}{R}}$

voltage is halved from Europe to United states, so the bulb will appear 1/4 less bright in United states.