5.3 Conservation of Energy

Critical Questions:

  • What is energy?
  • What do all of the different types of energy have in common?
  • How do we use food to move ourselves around?

Now that we’ve described a bunch of different types of energy (in the previous post), we can talk about how they behave. Following on Mr. Feynman’s comments, each of these types of energy has a very specific mathematical formula, so we can calculate exactly how much of each type of energy we have in a given situation.

The most interesting thing about energy is this: it cannot be created or destroyed; it can only change forms.

I’ve already hinted at this idea when talking about the examples from the previous post. Any kind of potential energy can turn into kinetic energy when the object in question is free to move. The simplest example involves one of those wind-up toys you probably had as a kid, or at least you probably saw them in old cartoons.

caterpillar wind-up toy
I like this one because it’s a happy caterpillar.

To make these toys work, you turn a little crank; this tightens a spring inside the toy, which means that you have increased the toy’s elastic potential energy. When you let go of the crank, the spring turns some gears, which cause the toy to spin, or walk, or drive away. Whatever it does, it’ll probably move somehow – in other words, it’s gained kinetic energy. But meanwhile, the spring is no longer wound up, so it’s lost all its potential energy.

One way to think about this is to say that the potential energy (in the spring) transformed into kinetic energy. And it turns out that the amount of kinetic energy you end up with is exactly equal to the amount of potential energy you started with.

Imagine energy as a bunch of ping-pong balls. These balls are kept in a series of boxes labelled ‘kinetic’, ‘gravitational potential’, ‘thermal’, and so on. At any moment in time, you can count up all of your ping-pong balls and find that you have, say, twenty. Now you can take as many of the balls from any box and move them into other boxes, but you know that at the end of the day, you’ll still have twenty ping-pong balls.

If somebody came over to your house and saw this setup, and they were nice enough not to give you concerned looks, they might reach into their own pockets and hand you a few more. At this point, you might count again and find that you now have twenty-three balls, but you’d be certain that these extras did not appear out of thin air – your friend is now missing three from his pocket. The total number of ping-pong balls in the universe remains unchanged.

ping pong balls
When people come over and say, “What’s with the ping pong balls?”, you just smile knowingly.

When you wound up the toy, it took energy to do so – your body has lost exactly as much energy as the toy gained.

Similarly, if you woke up the next morning and found that you only had fifteen ping-pong balls, you would most likely grab a baseball bat and head out into the city in search of the thief.[1. Of course, this analogy relies on the idea of a collection of ping-pong balls that never get broken or crushed. However, I find the idea of indestructible ping-pong balls so appealing that I’m going to leave it like this.]

Once you know that energy can change form like this, many of the processes you see around you every day might start to make a bit more sense. For example, when you eat food, you store its energy inside of your body in the form of chemical bonds; then, when you need to move, your body transforms it into kinetic energy. Batteries store another kind of chemical energy, which is released as electrical energy when they’re connected to a circuit. A ball falling out of the sky is losing its gravitational potential energy and gaining kinetic.

Now we arrive back at the why, and I hope you can understand more of what I was saying earlier. What is energy? We can define it mathematically, but it isn’t any one thing. It can take many different forms – even more than what I’ve described thus far. And it can change shape and transfer from one object to another. So it isn’t any one thing but some abstract quantity that can be measured and passed on.

And there’s another important question here: Why is energy conserved? Why can’t we just create more energy when we need it, or get rid of it when we have too much? This is another question that seems, for the moment, to be too fundamental to answer. Someone who knows a lot of the math involved might tell you that this is a consequence of the fact that the laws of physics don’t change over time. For now, we’ll have to accept it as what seems to be the truth, because this simple fact can reveal a lot more to us about how the universe works.

Big Ideas:

  • Energy is a measure of something’s ability to create motion in something else, or to make itself move.
  • Energy cannot be created or destroyed, but it can change from one form to another and transfer between objects. The total amount of energy in the universe doesn’t change.

Next: 5.4 – Perpetual Motion Machines

Previous: 5.2 – What is Energy?