If you think about the history of humans (disclaimer, I'm a human) you could make the argument that history is about the quest for energy. Not just any old energy, we want it cheap or maybe even free. If you have nearly limitless energy, you can do pretty much anything. You can extract the materials you need from the air, water, or land. You can build stuff. You can go to space. Energy is the key.
But our quest for endless energy has caused some problems too. Let's go over the evolution of our energy consumption.
Everyone has a mental image of early humans. Maybe you picture them living in caves. Or maybe they are slowly stalking some animal so they can eat it. Just don't imagine them riding dinosaurs—that's not legit.
But it seems clear that early humans had lives that focused on getting and eating food (sometimes I feel like that too). Without food, you can't survive. With more food, you can do more stuff. The more you eat, the more energy you have (up to a point).
But it takes energy to get food. The food doesn't just fall out of the sky. On top of that, if a human spent more energy on procuring food than the energy equivalent of that food—that's bad. Eventually, humans found ways to get more food than was required. Farming is a more efficient way to get energy than hunting and gathering. Humans also found ways to store food to create a more steady supply of energy.
The point is that early humans found ways make food more readily available, and food is energy. The food isn't only used to meet humans' energy needs. You could feed an ox or horse and then have the animal do some work. Still, it's energy from food.
Let's say you want to heat up some water. Is it possible to use energy from food to heat the water? Absolutely. You can try this yourself. Go eat your favorite food and then take two pieces of metal. Rub the metal together (sliding back and forth). Don't stop. Keep rubbing the metal. You have to rub the metal until friction makes them warm up. Then you can put the warm metal in the water. Done.
True, that's not a very good way to heat up water. Wouldn't it be better to burn something? A wood-burning fire would work. I think this is the next big phase in the human quest for energy. You don't have to burn wood. You could burn coal or oil or just about anything.
The physics of burning can be summarized in a very simple manner. You take some carbon (like from wood) and use some energy to combine it with oxygen. The resulting product is even more energy and other stuff like carbon dioxide. Side note: you get energy from forming a chemical bond between carbon and oxygen. Don't fall into the trap of thinking energy is stored in bonds.
OK, you can't eat fire. But you can use the energy from fire to do some pretty awesome things. Of course you can heat up stuff—that can be useful, but you can also use it to cook food. Cooking food is a pretty big deal. First, humans can make food more efficient (greater transfer of energy) by cooking it before eating it. Second, cooked food can be stored longer than raw food. Plus it tastes better—but that might just be a result of eating cooked food for so long.
But here is the real problem with burning stuff—it's sort of awesome. Let's just skip to the most common thing to burn now (other than coal)—gasoline. The problem with gasoline is that it does a great job because of its high energy density. The energy density has two informal definitions. The first is defined as the amount of energy per unit volume (this is the real energy density). But it sometimes is used to mean the amount of energy per unit mass (technically the specific energy).
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For gasoline, it has an energy density of 34.2 megajoules per liter (MJ/L) . Compare that to something like a lithium ion battery at just 2 MJ/L or a sandwich (since we are already talking about food) at 5 MJ/L. This means that you can put that gasoline in your car and drive great distances.
Why is being awesome so bad? Because it's difficult for humans to stop using it. We like gasoline because we can fill up the tank in our car and drive for hours and hours before filling up. This also means that we are turning carbon and oxygen into carbon dioxide. Massive amounts of carbon dioxide that humans put into the atmosphere is how we get climate change—and that's not so good.
Using the Sun
We can get energy from the Sun—I'll refer to this as solar energy. You're probably thinking about photovoltaics right now. But food and fossil fuels are also types of solar energy if you want to be super technical about it. We get energy from plants, but plants get their energy mostly from the sun. Fossil fuels also come from plants.
Before we get to modern solar energy (like the solar panels you see on rooftops), there are other forms of solar energy that humans have been using for a super long time. In particular, I am thinking of water power and wind power.
When water moves downhill (either naturally or from a dam), you can create something to harness this downward-moving water. Like a watermill: The moving water turns a giant wheel and that wheel does something useful. But where does this water get its energy? The sun. When sunlight shines on water, it heats it up and causes evaporation. The water moves up into the air and eventually falls back down as rain. If the rain lands on ground that is higher than where it started, you get an increase in gravitational potential energy.
What about wind power? A windmill is just like a watermill except that it uses wind. Wind is caused mostly by regions of different atmospheric pressures around the planet. When you have a region of high pressure next to a low pressure one, the air wants to move from high to low pressure. Boom. That's wind. How do you get changes in atmospheric pressure? It's based on the landscape, the rotation of the Earth, clouds, and yes—the sun.
Now for your favorite—photovoltaics. These remarkable devices have no moving parts, but they can still get energy from the sun and convert it into electric energy. They aren't perfect, but once you have them it's almost like free energy. I know, it's not actually free energy. But they are amazing.
How about an energy source that isn't connected to the sun? That's what we have with nuclear power. It just so happens that we can also get some energy by doing stuff to atoms—actually the nucleus of an atom. There's a lot here to look at, so let's just start with the atom.
Every atom you find on Earth is made of just three things: electrons, protons, and neutrons. The electrons are negatively charged particles in the outer cloud of the atom—they don't really matter in this situation, so just ignore them. The protons and neutrons are in the nucleus of the atom and they are what make that atom a particular element. If you add or take away a proton, you will change the element to something else. Changing the nucleus is what happens in a nuclear reaction.
OK, so take an atom like Uranium-235. It's uranium because it has 92 protons in the nucleus. It's uranium-235 because there are 143 neutrons in the nucleus (and 92 plus 142 is equal to 235). Wait, where are you going to get this uranium? Don't worry about that for now. But here is the cool part. If you hit the uranium with another neutron, something magic happens. The uranium atom breaks apart into two other atoms—krypton-92 and barium-141 (and some extra neutrons). No big deal, right? Wrong.
It turns out that if you were able to measure the mass of the starting stuff (the neutron plus the uranium) and then you measured the final stuff (the krypton, barium, and the neutrons) you would find that you lost mass. Yes, the mass of the stuff before the interaction is slightly more than the mass of the stuff after the reaction. But don't worry—all is not lost. This is where a version of Einstein's famous equation comes into play. Yes, you've seen this before: E = mc 2 . Mass is just a form of energy. So the loss in mass means you get energy. In fact, for just a little bit of mass you get A BUNCH of energy.
If you start a nuclear reaction, you can use this mass converted to energy to then do some useful stuff—like heat up water to make steam and then have the steam turn a turbine or something. But is this "free energy"? No, but it's close. Oh sure, you still have to build the nuclear reactor and get the uranium and stuff. Also, there is the question of what to do with the left over stuff after the nuclear reaction. It might still be radioactive and it's probably toxic. You can't just dump that into the ground. But still, it's a pretty good deal (also you don't produce carbon dioxide).
Oh, there is another type of nuclear reaction—one that's even better. When you take an atom and break it apart (like the uranium example), that's called nuclear fission. It's also possible to take lower mass elements and combine them together. In the case of lower mass atoms, you can get energy from combining instead of breaking. This is called nuclear fusion. You could start with stuff like hydrogen and end up with helium. That's good, right? Yes, it's good—but it's also way more difficult to get this to work. Humans are still trying to make this nuclear fusion process such that it outputs more energy than it takes to start the fusion.
Right now, nuclear fusion is going to be our next best energy source. Maybe something else will come along, but if we could get fusion to work—that might be the next "cheap energy". It's going to be great. What are you going to do with all that energy? I think I'm going to use it to build a space elevator.
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