It should be clear that our world runs on energy. Driving your car, washing your clothes, heating your house, and even running your computer (so you can read this post)—these things all require energy. But where does this energy come from? Does it even matter? Yes, it really does matter. Some forms of energy contribute to climate change and other sources are renewable. And since we are all living on the same planet, these energy choices can be quite important. This means that everyone should have a basic understanding of energy. Don't worry, I'm going to explain this at the level every human should understand.
What Is Energy?
Energy isn't actually real—it's just a way for us to keep track of interactions. (Humans deal with stuff that isn't real all the time. Words—words aren't "real," they are just ways that one human can share an idea with other humans.) If we keep track of all the energy changes in different interactions, we find that energy is conserved. This means that if you could measure all the energy before an interaction, you would find that the total energy is the same after the interaction. It's just in different places.
What about the units for energy? The most common energy unit in science is the joule. One joule is the amount of energy it would take to push with a force of 1 newton over a distance of 1 meter. That doesn't really help you get a good feeling for this unit though. How about this? If you pick up a textbook off the floor and put it on a table, that's about 10 joules of energy.
We also find it extremely useful to describe energy as different types. Here are the most common energies that you might might talk about:
Kinetic energy. This is the energy associated with objects in motion. The kinetic energy depends on both the mass of the object and the speed.
Electric potential energy. If you take two electric charges, they will of course interact. The electric potential energy is a measure of this interaction. This energy is actually very important. Since pretty much everything is made of electric charges (protons and electrons), a lot of other energies are based on this.
Gravitational potential energy. This is the energy associated with the gravitational interaction between objects that have mass (so pretty much everything).
Thermal energy. It takes energy to increase the temperature of the object—so we say objects have thermal energy. Since matter is made of particles, this is actually a combination of kinetic energy (due to the motion of the particles) and electric potential energy (in the interactions between atoms).
Chemical potential energy. When you have some type of chemical reaction that transfers energy, we call this chemical potential energy. This includes humans eating food, cars using gasoline, and chemical batteries. But really, this is just a fancy term for electric potential energy—again, because the interactions between atoms is almost exclusively an electric charge interaction.
Particle energy. OK, maybe that's not the best term—but I like it. Essentially, all particles have energy because of both their motion (the kinetic energy is technically part of this) and their mass. But this means that even a particle at rest still has energy. Mass is a form of energy.
One more thing to mention: power. If you take that textbook off the floor and move it to the table—we said that was 10 joules of energy. But clearly there is a difference if that move takes 1 second or if it takes 1 hour. Although the energy required is the same in both cases, the power is not. Here, we define power as the rate of change in energy.
Since power is actually a rate of energy use, we talk about the change in energy per change in time. If this change in energy is in units of joules and the change in time is in seconds, the power would be in units of watts. In the textbook example above, the first lift would require a power of 10 watts, the second one would only require 0.0028 watts. A typical (LED) light bulb uses around 20 watts of power, and if your car is electric, it uses about 20 kilowatts.
But you have to be careful. One unit you will see quite often is the kilowatt-hour. Although it might look like a unit of power, it's not. It's actually a unit of energy. Start with the definition of power above, but solve for ΔE, and you can see that the change in energy is equal to the power multiplied by the time interval. That means that we can describe a change in energy in units of power and time. That's the kilowatt-hour, where one kilowatt-hour is the amount of energy you get from a power of 1 kilowatt for a time interval of 1 hour (3,600 seconds). So, 1 kilowatt-hour is equal to 3.6 million joules.
Let's move on to the practical aspect of how we use energy. Fossil fuels are probably the most common energy source and one that humans have been using for the longest time. The basic idea of fossil fuel energy is to take some carbon and combine it with oxygen, forming carbon dioxide along with some energy (which is the part we want). Yes, it's true—you get energy from forming chemical bonds, not from breaking them. Of course the oxygen usually comes from the atmosphere (which is about 21 percent O2), but where do you get the carbon? Well, you could chop down a tree and burn it. Or you could dig up some old trees that over time turned into coal, oil, or natural gas. Since these are super old trees, they are fossils—boom, fossil fuels.
So, what's great about fossil-fuel-based energy? The best thing is that it's easy. You just need to get this stuff out of the ground and then burn it. It's like instant energy just waiting there for humans to use it. Most forms of fossil fuels also have a high energy density. There's quite a bit of energy in gasoline, which has an energy density of 46.4 MJ/kg. Even though an automobile is only 25 percent efficient, just 1 kilogram of gasoline can give you 11.6 million joules of energy. Remember, it was 10 joules to lift a textbook ofg the ground and onto a table. This is why you can get a car to drive 20 to 50 miles on just a single gallon of gasoline. You have to admit that's really impressive.
OK, then, what's not great about fossil fuels? Hopefully, you already know the answer to this question. When you burn a fossil fuel, you produce carbon dioxide. Carbon dioxide is a greenhouse gas, and it contributes to climate change. If we keep burning fossil fuels, the increase in carbon dioxide is going to change the climate in a way that will make it difficult for humans to keep doing things we have always done—like living near the coast or growing crops in certain regions. So, that's what's bad about fossil fuels.
But let me just be clear. It's not just the use of gasoline in automobiles. We also burn fossil fuels for the production of the electrical energy used in houses and stuff. The basic idea is to burn the fossil fuel to heat up water and convert it to steam. This steam then pushes on the blades of an electric turbine engine to spin it. These spinning turbines create electrical energy through an electromagnetic interaction (using loops of wires and magnets). A number of energy sources use spinning turbines, actually.
If you just go outside during a sunny day, you can feel it. You can feel your body warming up as a result of the interaction with the light from the sun. In fact, at our location in the solar system, the sun gives us about 1,000 watts per square meter of power. Of course, the trick is to get this energy into something more useful like electrical energy. One way to do this is with a solar panel (photovoltaic cell). This is essentially a solid-state device (with no moving parts) for which light can cause an electron energy transition to produce electric current. Yes, that's an over simplification—but you get the idea. It turns light energy into electrical energy.
But wait! There's another way to use solar power. It's called a concentrated solar power plant. The idea is to arrange a bunch of mirrors to all reflect sunlight to a central point. The object at this solar focal point will then get extremely hot, and you can use that hot thing to heat up water to produce steam and then turn an electric turbine. Oh, usually the extremely hot thing will be a liquid—maybe like molten salt. That way you can heat up some stuff and then move it to make some steam while still heating up other parts of the liquid.
OK, but is solar power also renewable? It's fine if you say that it's a renewable energy source, but technically it's not. The solar energy comes from the sun (that's probably obvious). But the sun produces energy mostly due to nuclear fusion reactions in the core. Guess what? In 5 billion years, the sun is going to run out of energy. So it's not technically renewable, but in the time span of the life of the sun, it's practically unlimited.
I would like to call this "hydropower" instead of hydroelectric, but that's the common name that everyone uses. The thing is that we have been using some form of hydropower for a long time—the water wheel is much older than the invention of electricity. In terms of electrical energy, it's not too complicated. In fact it's mostly like the electrical energy from fossil fuels. However, instead of using steam to turn an electric turbine you use falling water, or, technically, moving water resulting from a change in height.
The key to all forms of hydropower is that water wants to move down closer to the center of the Earth. When 1 kilogram of water moves down 1 meter, the change in gravitational potential energy is about 10 joules (yes, it's the opposite of lifting up a textbook). That might not seem like a lot of energy, but now imagine moving an entire lake lower by 1 meter. That's a bunch of energy.
But would we run out of hydropower? What if we take all the water that is above sea level and move it down to sea level? Well, that would be the end of hydropower. But it won't happen because of solar power. Yes, hydropower is a form of solar power. The Sun heats up water and causes evaporation. This means there is water vapor in the air which eventually comes back down to the surface of the Earth to fill up all those lakes and stuff. Hydropower is as renewable as solar (as long as the Sun keeps shining).
I wonder what else we could get to turn one of those electric turbines. What if we put a large blade on the front of the turbine so that wind could turn it. Yup. That's wind power.
Since energy is conserved, that must mean that when you get electrical energy from the wind turbine then something else must decrease in energy. Yes, this is true. As the wind moves through the turbine, it pushes the blades to make them spin. In the process of this interaction, the air decreases in speed—even if just a little bit. That means the air decreases in kinetic energy, and that's where the electrical energy comes from.
Why does this air move in the first place? Well, that's what we call "weather." But maybe you won't be too surprised to learn that the sun is at least partially responsible for wind. As the sun warms the atmosphere, the air increases in pressure and expands. Now you have regions of air with higher pressure and regions with lower pressure. The air moves into the lower pressure regions, and that's wind. Wind power is as renewable as solar (as long as the sun keeps shining).
Oh, I hate to say this—but I guess I should add it for completeness. No, wind turbines don't cause cancer.
Technically, there are two kinds of nuclear power—fusion and fission. Humans have not quite figured out how to make an efficient fusion reactor, so let me focus on a fission reaction. The fission reactor starts with a heavy atom like uranium-235 and hits it with a neutron. The atom absorbs this neutron to become uranium-236. However, U-236 is unstable and breaks into smaller parts—maybe palladium-117 and a palladium-118 plus some neutrons. But here's the weird part: If you look at the mass of the original U-236 and the mass of all the pieces, they aren't the same. The mass of the two palladium atoms plus neutrons is less than the mass of the uranium. So, mass isn't conserved—but energy is. This is where the famous E = mc2 equation comes from, where c is the speed of light (about 3 x 108 m/s).
A loss in mass in the system of particles just means that there is an increase in some other type of energy. In this case, it's an increase in kinetic energy of the resulting particles. Since the value of c is so large, a tiny decrease in mass results in a huge increase in kinetic energy. Now you can use that energy to … wait for it … heat up some water and make steam. This steam is then used to turn an electric turbine (don't pretend to be surprised). But remember, you are getting this energy just by converting mass into energy—it's so crazy, it almost looks like you are getting energy for free.
Although it might seem like a perfect way to get some electrical energy, there is one small problem. You now have this leftover palladium and stuff that's both radioactive and chemically active. No one wants to be around dangerous radioactive chemicals, but this nuclear waste has to be stored somewhere safe. It's not as simple as putting it in a box and burying the trash, because it can remain radioactive for thousands of years. Imagine building a box that keeps stuff contained for that long and you see the problem. But it's the price you pay for seemingly "free" energy.
So, which power plant is the best? At this point, there's no clear answer—except that fossil fuels are probably the worst choice. Hopefully we can figure something out for the future.