“No Free Lunch” in physics form. The law of conservation of energy states that in a closed system, energy is never created or destroyed. It can change forms (from chemical potential in gasoline to kinetic in a moving car to heat in our brakes, for example), but it’s always there in the same amount. This is one of the most profound, useful, and often depressing laws in physics.
It’s depressing because it means we can’t get something for nothing. Any energy we use has to come from somewhere and at the expense of something else that would otherwise have that energy. Now, often times this is acceptable. Stealing sunlight from the ground of a desert probably costs little ecologically. Stealing a bit of kinetic energy of water in a hydroelectric plant probably does little harm to the river. But it does come at a price and the price may be higher than we wish.
Conservation of Energy is also a useful principle. It can be used as a shortcut if you know where all of the energy is in a system. (For example, those classic physics problems where the energy is all gravitational potential or kinetic are so much faster to do with Conservation of Energy than the straight up way.) It is widely used by physicists to solve problems that would be a hundred times uglier without it.
And the principle is profound because what it says about the universe. At first blush, there’s no reason why the universe would have had to conserve energy (or anything else). Now, it turns out that if the laws of physics are the same in time (that is, laws like gravitation aren’t changing from what they were a billion years ago) , Conservation of Energy is the natural result. In and of itself this is awfully profound and not just a little bit cool. There’s no a priori reason why this would have been the case, at least not with a casual glance. Time-symmetry isn’t obviously connected to energy until you look under the proverbial hood of the physics engine, be down inside, connected they are.
All that is as may be, but what we most care about here is how you can use the principle of Conservation of Energy as a tool. Here’s how: any time some suggests a source of power for anything humans get up to, ask yourself where the energy comes from. Sometimes it’s well-known and obvious: solar cells use sunlight energy. Gasoline is refined from oil, the by-product of the breakdown of living matter millions of years ago. But what about electric cars? Where do they get their energy? Sure, it’s electricity right before they turn it into kinetic energy and drive off. But where does that energy come from? For a true electric car (not a hybrid), it comes from the same place the electricity in your home does. Depending on where you live, this could be from renewable energy sources, coal, or nuclear. So how “green” are electric cars? It depends a lot on which of those sources supplies the electricity.
How about hydrogen fuel cells? The energy there isn’t electric, at least not when it’s stored. It’s stored as chemical potential energy in the form of hydrogen. Hydrogen loooooves to react with oxygen and make water (a by-product we don’t worry about as much as carbon dioxide). In reacting, it releases energy, which can be harnessed to make electricity (or heat, if you prefer to make that instead). Clean, right? Well, maybe. Where do we get the hydrogen? Remember, hydrogen likes to react with oxygen, that’s what makes it a good fuel. It doesn’t exist here on Earth as hydrogen gas very much. We have to make it from water. Oh, dear, wait: if we start with water and end with water, we’ve put back as much energy into the chemical bonds as we’ve taken out. So where does the energy come from? Answer: not from the hydrogen. We need to put energy into the system to break apart the water. Where does that energy come from? Well, that depends on where you live and who builds the plant. It could be electricity from the grid or solar power, for example. But in any case, the energy has to come from somewhere. (In the end, the hydrogen is basically a form of a battery, storing energy for later use. Then again, in a sense, so is gasoline.)
As our energy consumption drives us to use more and more energy, we worry more about the sources. (This is for all kinds of reasons, ranging from environmental to geopolitical.) Conservation of Energy is a valuable tool to help us citizens navigate the rhetoric (some of it dishonest) about energy policy. While it might be a physicist’s favorite principle, it’s time we shared it with everyone else.