5.2
What is Energy?

Critical Questions:

• What is energy?
• What do all of the different types of energy have in common?

Despite its abstract and purely mathematical nature (as discussed in the previous section), we’re still going to have to come up with a good definition of energy for the purposes of this site.

The term ‘energy’ in physics means something very similar to what it means in everyday English. If you’re lying in bed and you don’t have the energy to get up, you may actually be lacking in something that could be equated with the scientific idea of energy. (Or you could just be a slothful person.) We would also say that it takes energy to light up our homes and to drive our cars, and this usage fits with the physics one as well.

The best kind of energy: magic (i.e. solar flares)

Already we can see that energy has a lot of different forms – your ability to get out of bed is quite different from a car’s ability to drive down the road, for example. And yet they all have some shared quality in order for us to be able to use the same word to describe them. We do have some excellent mathematics to help us define that similarity, but for now I’d like to propose a loose definition – one which is simplified almost to the point of inaccuracy, but which will nevertheless allow us to come to a better understanding of the topic.

This is the definition I’d like to use: energy is a measure of something’s ability to create motion in something else, or to make itself move.

To get an idea of the practical use of this definition, let’s look at some examples.

Example #1: A pool ball rolling along a table has energy because if it hits another ball, the second one will start to move.

This is the first type of energy that most teachers talk about in physics class. It is named ‘kinetic’ energy. We previously encountered the word ‘kinetic’ in the section about friction, wherein I mentioned that the term is derived from the Greek word for movement. The same idea applies here – kinetic energy is the energy of a moving object.

The mathematical expression for kinetic energy depends only on mass and speed. A big, heavy train has a lot of kinetic energy (even if it isn’t moving particularly fast) because it can give a lot of speed to something it runs into, like a car. A bullet, on the other hand, doesn’t have a lot of mass, but it still has a lot of kinetic energy (it can do a lot of damage) because of its high speed.

Example #2: An object held at some height has energy because if it is dropped, it will gain speed.

This is called gravitational potential energy. It is ‘potential’ energy because it could potentially cause some motion to occur, as soon as the object is let go of.

Example #3: A coiled spring has energy when it is compressed or extended because if it’s released, it will spring back to its regular shape.

I used the example of a spring here, but the same could be said of a stretched rubber band, or a bent piece of wood, or a squished tennis ball: these objects can be held in that position for as long as you’d like, but when you let go, they’ll rebound with some motion.[1] These are all examples of what we call elastic potential energy.

You may be seeing a pattern here. In classical mechanics (i.e. the kind of things we’ve been talking about so far), there are really only two categories of energy – kinetic and potential. But there are many different types of potential energy. Potential energy can be thought of as “stored” energy, because it is hidden away inside something else, ready to become kinetic energy as soon as somebody lets it loose.

Much like a caged Liam Neeson.

Example #4: Two magnets have energy when they are help apart from each other because if they are released, they will either snap together or get pushed farther apart (depending on how they’re oriented).

If you’ve been following these ideas, then you’ll know that this is another variety of potential energy. As I mentioned earlier, the force at work here is the electromagnetic force, so we can say the magnets have electromagnetic potential energy when they’re held apart.

Example #5: Radiation from the sun has energy because it can move the particles in your body or in a solar panel – increasing temperature, changing the shapes of molecules, or pushing electrons around and thus creating electrical current.

To be honest, this one opens up a whole can of worms. You see, the sun emits all kinds of things – light, x-rays, ultraviolet (UV) rays – but all of them are actually just different varieties of what we call electromagnetic (EM) radiation. And what exactly EM radiation is will take a while to explain. But for now, we can at least say it has energy, because it can move things it runs into.

Example #6: A high-temperature object has energy because it can pass that heat on to another object, make water boil, and so on.

This is another one that deserves more discussion, and it’s going to get it in just a few sections. You might have noticed that while the rest of these examples closely follow the definition of energy as being related to motion, this one sounds a bit different. In fact, temperature and motion are intimately related, but for now I’d like to keep things simple.

The energy of high-temperature objects can be called ‘thermal’ energy.

Example #7: The chemical bonds holding particles together at the molecular level have energy because, when those bonds change in chemical reactions, light or heat energy may be released.

This is a brief introduction to the world of chemistry, which will be discussed in chapter 9. I wanted to bring it up now because I’ve already talked about the energy a car uses, and this is where it comes from: cars are able to move because of the way chemical bonds change when gasoline burns.

This is technically a form of potential energy, but we can also think of it as chemical energy.

In the next section, we’ll tie these very different types of energy together, figure out what it all means, and see how this idea of energy can help us understand just about everything.

Big Ideas:

• Energy is a measure of something’s ability to create motion in something else, or to make itself move.
• There are many different forms of energy: kinetic, gravitational potential, electromagnetic, etc.
1. Actually, all of these examples behave quite differently in a mathematical sense, but the basic principle is the same.