Here’s my rant about energy and how we’d typically treat it in an introductory physics class. We might say something about “energy” and say that it has something to do with “work,” and then we’ll turn around and define “work” as having to do something with “energy.” This is circular and non-helpful. More importantly, it doesn’t really tell you anything that you would really want to know about energy and why it’s important.

So I start with food.

What did you eat for dinner most recently? If you’re lucky, you had something that filled you up, maybe tasted good, and ultimately served some kind of purpose. In fact, you can’t be alive and function if you don’t eat. If you have some spaghetti with meatballs and tomato sauce, you can eventually tie any of these back to the Sun. Pasta was once wheat that grew because of the Sun; and even the beef in the meatballs sustained itself from eating alfalfa which got its energy from the Sun. Almost anything you eat has a similar history.

We also know that all of those foods have some kind of “Calorie” value. These are “food Calories,” or “kilocalories,” which is 1000 calories (what I’d call “chemistry calories”). We can compare these to other units that measure similar things, but for now we simply know that we eat food for these Calories, and these Calories are supplied by the Sun.

So here’s an interesting table I got from a textbook for another course. In it, you’ll see a lot of confusing and strange information about different materials and processes. It’s worth staring at for 5 minutes or an hour or a lifetime.

The table and the comparison of different “energy densities” brings up a lot of questions. What it starts to drive home, though, is that this thing called “energy” comes in lots of different forms. Coal, butter, ethanol, and chocolate chip cookies all have similar amounts of energy per gram, but they are wildly different in how we use them. Or are they? We can only eat a couple of these, and others we use in other processes that warm things up or move things around, but apparently they’re pretty similar in terms of how much energy we get from them. Also, they, too all get their energy from the Sun if you go back far enough.

(You can think about that last sentence for a while. It’s worth it. I’ll wait.)

So, we still don’t really know what energy is all about, but we can play with the units of Calories, joules, watt-hours and the like. You might know that you get charged for electricity by assessing how many “kilowatt-hours” you use in a month. This is worth thinking about for a bit. A “watt” is a rate, like a speed, for energy, and if we trace it do its fundamental roots it’s shorthand for joules per second.

You can think about this for a bit, too. It’s important. You’ll also get to practice with this later.

We then play with this simulation, particularly to show that “height” is a quantity that is conserved for something skating back and forth on any variation of a “U” you can imagine. This means that the height is what we can use to keep track of what the skateboard can do. Similarly, the higher the height, the fast the speed at the bottom. These go hand in hand.

If you haven’t had a chance, you really ought to play with this. There are features we didn’t use in class but are really useful.

Great. At this point you can open up a book or ask a wisened physics professor to express how we keep track of energy in terms of stored energy (potential), moving/acting energy (kinetic), and processes by which we transfer energy (work). There are equations for this, but the big idea is that energy is something we account for, and by doing this we can solve for really complicated features of some system. Like for example, no matter what path a skateboard takes on the way down a (frictionless) ramp, its starting height will exactly predict a certain speed at the bottom, as well as a height on the other side. This is A Really Big Deal. You’ll use it a lot now, basically by just keeping track of this conserved quantity of energy in all its different forms.

But then we can look again a energy in the real world, maybe the food you eat or the energy table above, or this look at the U.S. electric grid and its sources of energy, courtesy of Dan Schroeder. This is worth playing with, zooming in and out and scrolling around the country, as well as going back in time. You can see how energy is used (remember the coal in the energy table?) and many other sources for energy — even though most of these, too, come from the Sun, in one way or another.

If you poke around the grid, you’ll notice that each power plant has information about its capacity, owner, etc. This is worth taking a look at, especially as you notice how energy use changes over time.

What’s the point of all this? I want to make sure we recognize not only that “energy” is some chapter in a textbook that gives us a way to solve problems, but that it’s a real quantity that we use through our own food as well as the electricity that runs our homes, and lots of other things as well. We can find these and then start to make sense of them. Energy is a quantity that is not just some abstract concept in physics, but something we use across all disciplines in science to figure out if and how anything functions. This is just our first step in putting this all together.