Video Transcript
A food manufacturer determines that
the energy content of one slice of their new bread is 400 kilojoules. What value is this in
kilocalories? Give your answer to one decimal
place.
Energy is a word you may have heard
used in a variety of contexts. But in a strict scientific sense,
energy is the capacity for doing work. An important concept is that it
cannot be created nor destroyed; it can only be transferred to other forms.
Let’s look at how we measure
energy. The SI unit for energy is a joule,
which is defined as the amount of thermal energy needed to raise the temperature of
one gram of water by one divided by 4.184 degrees Celsius. Now that we have discussed energy
and joules, let us begin to answer the question in order to convert 400 kilojoules
to kilocalories.
Quite often, the numbers we are
dealing with are very large, so we can use the kilojoule. One kilojoule equals 1000
joules. We can use dimensional analysis and
set up conversion factors. Remember, when you set up the
conversion factor to convert kilojoules to joules, put the like unit on opposite
sides of the fraction so that they will cancel out.
Another common unit used to measure
energy is the calorie. Calories are often used in
nutrition and can be found on food labels. The calorie is the quantity of
energy needed to raise the temperature of one gram of water by one degree
Celsius. It is equivalent to 4.184 joules
exactly. One calorie is equivalent to
exactly 4.184 joules.
Now we can convert joules to
calories using the following conversion factor. Remember, when you set up the
conversion factor to convert joules to calories, put the like units on opposite
sides of the fraction so that they cancel out. We could also convert calories to
kilocalories using a similar conversion factor as the one used to convert joules to
kilojoules. Remember one kilocalorie equals
1000 calories.
If we follow the equation on the
top, we will take 400, multiply it by 1000, divide by 4.184, and then divide by
1000. But you may notice that there is a
little redundancy there, and we can actually convert directly from kilojoules to
kilocalories, just as you can directly convert from joules to calories. Doing this calculation gives us our
answer of 95.60229 in units of kilocalories. However, we need to give our final
answer to one decimal place. Rounding to one decimal place gives
us our final answer of 95.6 kilocalories.
