Find the Resistance of a Lightbulb

A typical older incandescent lightbulb was 60 W. Assuming that 120 V is applied across the lightbulb, what is the current through the lightbulb?

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Strategy
We are given the voltage and the power output of a simple circuit containing a lightbulb, so we can use the equation $P = I V$ to find the current I that flows through the lightbulb.
Solution
Solving $P = I V$ for the current and inserting the given values for voltage and power gives

$\begin{matrix} P & = & I V \\ I & = & \frac{P}{V} = \frac{60 W}{120 V} = 0.50 A. \end{matrix}$
19.51

Thus, a half ampere flows through the lightbulb when 120 V is applied across it.

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