Energy is the answer to so many questions. Pull back a spring-powered toy car and rolls forward. Why? Energy. You used your phone all day and it suddenly shut down. Why? No energy. A child eats six bags of M&Ms and runs around screaming and giggling because he has energy. But then he doesn’t want to clean his room because he has no energy. But what is energy?
Ah… that is the real question. To answer it, let me start with a story about three friends, Alby, Bobby, and Cami. I completely made those names up.
Alby has 10 dollars and gives it all away. Bobby ends up with $5 and Cami gets $4.99.
Do you see the problem? Yes, there is a missing penny. Where is it? When Alby gives away all of his money, that penny must end up somewhere. Even if he dropped it, it still exists. The total amount of money must add up to $10.
If you like, you can all this rule the conservation of money. No matter what happens, the amount of money must be the same before and after something happens. Oh, and here is something else to know about money: It’s not real. Oh sure, the bills and coins are real, and if you lost a $10 bill you’d be unhappy about it. But money merely represents something else. You could use a dollar to buy a ball or a snow cone, but those things exist even if you don’t have money. Money simply exists as a useful way for humans to trade things.
OK, what does this have to do with energy? Well, energy is just like money. Let me start with an example. Suppose I load a ball into a spring-powered ball launcher and fire that ball straight into the air. Here are the three phases of this action:
The launcher fires the ball with a compressed spring. That spring stores energy, called spring potential energy. Release the ball and it moves under some unspecified speed as the spring releases its energy. Now the ball has energy, called kinetic energy. When the ball reaches its maximum height, its speed is, for just an instant, zero. At that point, the ball has gravitational potential energy.
Now, if you calculate the potential energy before releasing the ball, it might be something like 10 joules. (A joule is a common unit of energy, named for British physicist James Prescott Joule.) Launch the ball and it might have nine joules of kinetic energy and 1 joule of gravitational potential energy because it is now aloft. As the ball rises, it has less kinetic energy and more gravitational potential energy. Once the ball reaches its maximum height, it has 10 joules of gravitational potential energy. Although the type of energy changes, the amount never does. Energy is just like money—it is conserved.
How about another example. The nucleus of an atom usually features some combination of protons and neutrons. A version of carbon carbon 14 has six protons and eight neutrons, but it is not entirely stable. Over times, the carbon 14 atom experiences radioactive decay (this explains how carbon dating works) through a process called beta decay. This decay leaves a nitrogen atom called nitrogen 14 and an electron. They both possess kinetic energy, because it turns out that mass is another type of energy from the famous expression E = mc2.
Now for the cool part. Based on the difference in mass of carbon and nitrogen, beta decay lets you find the total amount of energy. Let’s say this was 10 joules. You can also measure the speed and calculate the kinetic energy of the electron. Suppose you got a value of 3.99 joules. Doing the same with the nitrogen, you might get an energy of 6 joules. Yes, there is a missing 0.01 joule of energy. Does this mean that energy was not conserved? No, it means the missing energy must be somewhere else.
This actually happened, and in 1930, Wolfgang Pauli suggested that another particle in the beta decay process held that energy. He was right—that particle is called a neutrino. That’s crazy when you think about it. Humans arrive at this calculation because it is useful and because the total energy must be the same before and after something happens. When it appears that energy is not conserved, it is only because of some previously unknown factor.
There you go—a simple answer to the question, “What is energy?” I hope you understood it, because I wrote it for the Flame Challenge. That’s an international competition where scientists explain something in terms an 11-year-old would understand.