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Units of energy
True or false? A kilojoule is a larger unit than a joule.
Imagine someone asks you how many metres away the nearest store is, or how many kilograms a bag of tomatoes weighs. You can probably answer these easily. But what if someone asked you how much energy you’d need to lift 1 kilogram of tomatoes 1 metre up? Perhaps the answer wouldn’t come to you right away. Energy is often less intuitive to grasp than, say, distance or weight.
Energy is the ability to perform work, such as moving an object. Therefore, the basic unit of energy is the same as the unit of work — a joule. If something has one joule of energy, it is capable of performing the work of one joule, such as moving an object one metre, by a force of one newton. So, one joule is equal to one newton times one metre. That’s roughly what it takes to lift an average tomato one metre up against the pull of Earth’s gravity.
What about lifting that one kilogram of tomatoes? You need to apply a force of 9.8 newtons over the distance of 1 metre. This requires 9.8 joules of energy. Energy can also exist in other forms that have nothing to do with moving physical objects. It can be related to heat, light, or electricity.
No matter what form it takes, we can express the amount of energy in joules. Bringing a glass of room-temperature water to the boil takes about 80 000 joules. A tomato, when digested, releases chemical energy of about 70 000 joules. The energy Earth receives from the sun every day is ten thousand billion billion joules. These examples show that the joule is a rather small unit, which can be inconvenient in many situations.
So, energy is often given in a unit that’s 1000 times larger than joule — a kilojoule. One kilojoule supplies enough energy to bring half a teaspoon of cold water to the boil. Digesting that tomato releases about 70 kilojoules. But when it comes to food, we tend to use another unit of energy. You can often find this unit on food packaging in a table that shows nutritional value.
This unit is a kilocalorie. A kilocalorie is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius. One kilocalorie is equal to 4.2 kilojoules. The 70 kilojoules of energy in that tomato equals about 17 kilocalories. There is one more unit of energy you might have heard of.
It usually appears on electricity bills that show how much electrical energy a household has used. That unit is the kilowatt-hour. It expresses the amount of energy used to run a certain electrical device for a period of time. If you have a 1000-watt high-speed kitchen blender and you run it for an hour, you’ll use exactly 1 kilowatt-hour of energy. A 100-watt light bulb turned on for ten hours also uses 1 kilowatt-hour.
And so does a 10-watt LED turned on for one hundred hours. 1 kilowatt-hour is equal to 3600 kilojoules. Energy is the ability to perform work. Joules are the standard, universal unit for measuring energy. But in many situations it is more convenient to use other units, such as kilojoules, kilocalories, or kilowatt-hours.
Why don’t you try and find out how many joules it takes to lift your school bag? Maybe, you can even look at the wattage of an electrical device. If you know how long you use this device at a time, you can calculate the kilowatt-hours!