Energy Exercise 1

Exercise 1. A medium size glazed donut contains 255 Calories. How many kJ does this donut contain? How many hours could the energy in the donut light a 100 watt lightbulb? (1 watt = 1 J/s)

ANSWER:

Solution:


We know that 255 Calories is equal to 255 kcal and that 1 kcal = 4.184 kJ.

\(\displaystyle\require{cancel}255\;\cancel{kcal}\times\frac{4.184\;kJ}{1\cancel{kcal}}\;=\;\mathbf{1.07\times{10^3}\;kJ}\)

Next, we are asked to determine how many hours the energy in the donut could light a 100 watt lightbulb.

1.07 x 103 is equal to 1.07 x 106 J.

We are told that 1 watt is equal to 1 J/s, and the light bulb is 100 watts. There are 3600 seconds in 1 hour. We can now set up the problem.

\(\displaystyle\require{cancel}1.07\times10^6\;\cancel{J}\times\frac{1\cancel{s}}{100\cancel{J}}\times\frac{1\;hr}{3600\;\cancel{s}}\;=\;\mathbf{2.97\;hrs.}\)

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