Energy is one of those words that we think we understand until we start thinking about it. Time is like that too. Saint Augustine famously said of time: “If no one asks me I know. If I wish to explain it, I know not.” The New Oxford American Dictionary says of energy: “1. The strength and vitality required for sustained physical or mental activity; 2. Power derived from the utilization of physical and chemical resources, esp. to provide heat or to work machines; 3. The property of matter and radiation that is manifest as a capacity to perform work.”
So, energy is not a thing, it’s a property of things. The last definition requires us to define “work,” since this is not the usual meaning of the word.
Here is an example. Your car breaks down in the middle of the road and needs to be pushed out of the heavy traffic. You need to apply a force on it to make it move. This force, making the car move, is performing or “doing” work. For that to happen, you need to spend energy. And where is your energy coming from? From your muscles. And the energy in your muscles? From metabolizing food. Food, in turn, needs to be planted by someone and, to grow, it needs energy from the Sun. (Cattle does too, but let’s say you’re a vegetarian.) And what about the Sun’s energy? It comes from nuclear fusion processes at its core that transform hydrogen into helium at a staggering rate of about six hundred billion pounds per second.
Ultimately, the protons that are fused in the Sun’s interior are responsible for making your car move. I know this knowledge doesn’t help you while you’re out there pushing your car under dozens of blaring horns, but at least you know you didn’t do it alone. You and the solar protons did it together, as a team.
In the example above, as you pushed the car you imparted some speed on it and did some work, moving it from a state of rest to a state of motion. This energy of motion is what we call kinetic energy, as I’m sure you remember from school. So, we can say that work is equivalent to the change in the car’s kinetic energy from zero (zero velocity) to its final value (moving car).
Everything that moves can do work. Let’s continue with your car saga to see how that goes. You and the solar protons are busy pushing your car out of the way when an unhappy soul driving on the wrong way hits you head on. Fortunately, he was going slowly and no one got hurt. But the collision bashed your fender in; the kinetic energies of the colliding cars was used up deforming the fender. This is not your day. And it's going to get worse.
Screaming, you leave the car and go in search of a public phone. (You also forgot your cell phone at home…) As you walk under a building, a kid on the fourth floor decides to test the law of gravity and let go of a water balloon. Bulls-eye! The balloon splashes right on your head. “Another one, you %$#^&@#!!!” you yell, thinking that at least it was only water. It could have been much worse. You can hear the kid laughing his head off behind the window.
Using physics as relaxation therapy, you think about what happened. As the boy suspended the balloon out the window, it wasn’t moving. But as soon as he let go, it started to fall, gaining speed from gravity’s acceleration. Gravity is doing the work here: the higher the balloon, the higher its impact energy. If the boy had been on the fortieth floor, the impact would have been much more violent.
Here, potential energy, the stored capacity to do work, is converted into kinetic energy. As the balloon hit the ground (or your head) all it had was kinetic energy; no more potential to keep falling. A spring, too, can store potential energy: as you release it, it moves into its relaxed position. In fact, some of the kinetic energy was used to rupture the balloon, some to move the air around it making noise, etc. Energy is a slippery mistress, always transforming itself.
You finally find a public phone (they hardly exist anymore…) and, of course, it’s out of order. You raise your fist to the sky and curse the gods, thinking of Job. You look down, and can’t believe your eyes: a beautiful woman, smiling at you. “Hi, would you like to use my cell phone?”
Incredulous and drenched, you accept. As you call your insurance company, you imagine the electric charges moving around in the cell phone’s battery, negative attracted to positive. The attraction creates a current that feeds the phone, making it work. Chemical energy turning into kinetic energy, charges in motion, the secret behind much of electronics.
You look at the pretty girl and ask, shyly, “can I buy you coffee? There’s a Café across the road.” To your surprise, she says yes. Immediately, another kind of energy transformation occurs in your body, as your heart starts pumping faster.
As you two sit down, you want to say, “Did you know that matter can turn to energy and vice-versa? I read it in this awesome blog on NPR, 13.7. Actually, we are energy and energy connects us to the rest of the Universe!” But you decide not to. Instead, you say, “You’re a life saver, really.”