What is energy?

It's one of the most basic concepts in physics, but also one of the hardest to define. Cathal O'Connell has a go.

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Any physics textbook will tell you energy is “the capacity to do work”. Then it usually goes on to explain that “work” is the action of moving something against a force. But isn’t this definition kind of unsatisfying? It’s a bit like Plato’s definition of man as a “featherless biped” – it’s hard to poke holes in the reasoning, but you can’t help but feel something is missing.

The reason energy is so hard to define is because it’s an abstract notion. In physics, the concept of “energy” is really just a kind of shorthand, a tool to help balance the books. Energy is always conserved (or converted into mass) so is incredibly useful in working out the results of any kind of physical or chemical process.

There is no physical “essence” of energy, and no such thing as “pure energy”. Energy is always carried by something, usually in the form of movement.

The classic example of kinetic energy is a billiard ball rolling across a table. The heavier the ball, the faster it moves, the more energy it carries. In other words, the more painful it will be if it pops off the table and lands on your little toe.

Another form of kinetic energy is known as heat. The temperature of something is a direct measurement of how fast the atoms inside it are moving. In a hot cup of coffee, the water molecules are racing around at a fast clip, slowing down as the cup cools.

Throw an iron bar into the fire and its atoms start moving faster too, although in this case the atoms are bound in position, and so the movement is the form of a jiggling vibration.

Sometimes an object is pulled or pushed in a particular direction, but its movement is stopped by some other force. In this case, the object is said to have potential energy. Potential energy means the potential to move.

It’s a bit like a racing car driver pressing the accelerator with the handbrake still on – nothing much happens until she releases the brake.

A glass sitting on a table is being pulled down by the force of gravity. But any movement is being stopped by a much stronger force – the electrical repulsion of the atoms in the table. Give the glass a nudge off the table, though, and it falls.

What about chemical energy, electrical energy, or nuclear energy? These are a bit more complicated, but in the final analysis, all these forms of energy also involve a type of movement or a potential to move.

For example, lots of energy is locked, like a coiled spring, inside atomic nuclei. This energy can be released when a uranium nucleus splits in two. The two halves are both positively charged, and so just after the split they are electrically repelled by other another and fly apart. Thus the nuclear potential energy ends up as kinetic energy.

As the Russian physicist Lev Okun said, “The more basic is a physical notion, the more difficult to define it in words.” For energy, the best we can do is say it’s the capacity to cause movement.

And that should do us poor featherless bipeds just fine.

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Cathal O'Connell is a science writer based in Melbourne.