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Energy, Work, Power

Energy is basically the ability to do work. The standard unit of energy is the Joule. We can use the initially un-illuminating, but surprisingly useful, definition of a Joule of energy as...

“One Joule of energy is the amount of energy required to do one Joule of work”

Yes in a peak of laziness it was noticed that you could use the same unit for Work and Energy. It’s just like we take for granted we can use the same unit to describe a volume of water as we do to describe the capacity of a bucket we might put water in. An empty bucket by definition has no water in it, but we might still describe it as a 2 gallon bucket. It reflects an automatic mental trick that we do, where we recognise that to fill the water bucket we would need 2 gallons of water. Similarly we would need 1 Joule of Energy to do 1 Joule of Work. So we can also say...

“One Joule of work is the maximum amount of work that can be done with one Joule of energy”

Whilst this is true, it hasn’t really moved us very far forward! Actually, it’s in moving things forward (or in fact in any direction) that we can begin to get a definition for work and hence energy.

“One Joule is the amount of work done when a force of 1 Newton is active on an object for a distance of 1 metre.”

Which helps if you’re the kind of person who’s constantly thinking of forces in terms of ‘Newtons’ - there are people who do, but they are probably in a minority. An easy way to get the feel of a force of one Newton is to imagine yourself on earth (so think ‘kitchen’ rather than ‘moon’ or ‘orbiting space station’) looking at an object having a mass of roughly 100 grams (just under four ounces). The weight that we feel when we pick the object up is the gravitational force of the earth and the object attracting each other (we think of this as the object trying to fall to earth). To hold the object stationary we have to resist that gravitational attraction between the earth and the object. The force we need to exert to hold the 100 gram object in place and stop it falling is about 1 newton (it’s actually more accurately about 0.98 Newtons). So that’s what a force of 1 Newton feels like – not very big really. So now if you imagine something pushing, or pulling, or lifting, or squeezing, etc, with that kind of force for 1 metre of distance, then you have imagined one joule of work, and you have imagined one joule of energy being used to do that work.

As an aside, you can see that you are unlikely to get tired out by doing one joule of physical work. Because a joule is quite a puny amount of energy, we end up using kilojoule quite a lot. A kilojoule is 1000 Joules and is abbreviated as kJ.

Now imagine that force of 1 Newton pushing, pulling, or whatever it is doing for 1 metre, but now think of it doing this repeatedly every second. Maybe the force keeps moving over the same metre length, or maybe it’s like a model car that keeps pushing itself with a force of 1 Newton along a metre of track every second. Then it can be seen that one Joule of work is done every second. Or in other words, one joule of energy is being expended per second. A rate of working (or energy usage/production per second) is such a useful concept that we have a name for it, power. And we also have created a specific way of saying ‘one joule per second’, this is called a Watt. A process that consumes or generates energy, or does work, at a rate of one joule per second is operating at a power of 1 Watt.

Because a Joule is a fairly small amount of energy, a rate of energy usage per second of 1 Watt is not a massive amount of power. Therefore just like kilojoules often are used in practice, so are kilowatts (kW). As you would expect, a kilowatt is 1000 Watts, which is the same as converting energy at a rate of 1 kilojoule per second.

Of course there are lots of ways of using energy which are more interesting than moving 100 gram masses 1 metre. In fact whilst a fair proportion of our energy usage goes into moving things (i.e. transport), much energy usage doesn’t involve moving large masses at all. For example electrical energy is used to create heat and light. However it’s still possible to equate the use of energy in a light bulb to the definition in terms of force and distance.

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