Question

an led light bulb uses 9.0 watts (j/s) of power. how many hours would you be able to light the bulb with the energy from a 261 - cal candy bar? express your answer in hours to two significant figures. time = hr

Explanation:

Step1: Convert calories to joules

We know that 1 Cal = 4184 J. So the energy of a 261 - Cal candy bar is $E=261\times4184\ J$.
$E = 261\times4184=1.092024\times 10^{6}\ J$.

Step2: Recall the power - energy - time relationship

Power $P=\frac{E}{t}$, where $P$ is power, $E$ is energy and $t$ is time. We want to find $t$, so $t=\frac{E}{P}$. The power of the LED bulb $P = 9.0\ W=9.0\ J/s$.
$t=\frac{1.092024\times 10^{6}\ J}{9.0\ J/s}=\frac{1.092024\times 10^{6}}{9.0}\ s$.

Step3: Convert seconds to hours

Since 1 hour = 3600 s, then $t_{hr}=\frac{\frac{1.092024\times 10^{6}}{9.0}\ s}{3600\ s/hr}$.
$t_{hr}=\frac{1.092024\times 10^{6}}{9.0\times3600}\ hr$.
$t_{hr}=\frac{1.092024\times 10^{6}}{32400}\ hr\approx 34\ hr$.

Answer:

34 hr