Question Video: Conversion between Kilocalories and Kilojoules Using the Energy Consumption of the Adult Human Body Chemistry

It is estimated that the average adult should consume around 2000 kcal per day to maintain a healthy lifestyle. What is this value in units of kilojoules?

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Video Transcript

It is estimated that the average adult should consume around 2000 kilocalories per day to maintain a healthy lifestyle. What is this value in units of kilojoules?

2000 kilocalories is an amount of energy, and kilojoules is another unit of energy. So we can ignore the per day. We can break down a kcal into k and cal. The k corresponds to kilo, which means 1000 of something, and cal stands for calorie or small calorie. So kilocalorie refers to 1000 calories. Meanwhile, the symbol for kilojoules is kJ. The J corresponds to joule. Our starting unit and our final unit both have kilo in them, so we don’t need to focus in on that. What we do need to remember is the relationship between calories and joules. It takes about one calorie to warm one milliliter of water one degree Celsius. But officially, one calorie is defined as 4.184 joules. If we scale both sides by 1000, we end up with the same numerical relationship between kilocalories and kilojoules.

Now we have all the information we need to convert our kilocalories to kilojoules. We start by multiplying our 2000 kilocalories by 4.184 kilojoules per kilocalorie. So our answer is 8368 kilojoules. Alternatively, you could have done it in steps, first, by converting from kilocalories to calories by multiplying by 1000 calories per kilocalorie, then convert it to joules by multiplying by 4.184 joules per calorie, and then finally convert it to kilojoules by multiplying by 10 to negative three kilojoules per joule. If we cancel common terms along this route, we end up with exactly the same conversion factor. So regardless of the route, 2000 kilocalories is the equivalent of 8368 kilojoules.

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