So far, in this series of articles on scientific literacy, we’ve touched on scale, mass, gravity, atoms, molecules, chemical bonds and Fritz Haber, with the occasional tangent into other things. The purpose to all of this is to help people learn enough science to be able to understand public issues that are related to science and technology. We’re not trying to turn anyone into scientists—just scientifically literate people, which is just as important in this modern age as been able to read and write and add and subtract.
I’m thinking that the next step on the journey is to start looking at something that is so fundamental to everything else that it will come up over and over again as we go forward. In fact, we’ve already discussed it a little, without really making a big fuss about it. The subject is energy and it’ll probably take two or three articles to cover the basics.
Quite simply, energy is why everything happens. We mentioned in the articles on bonding that one atom with form bonds with other atoms if the bond makes the atoms more stable. “More stable” is really just another way of saying “lower energy”. The universe, and everything in it, will always move towards a lower energy, more stable condition. You might have heard the term for this, which is “entropy”. If someone held a gun to your head and told you they wanted a one-word answer to why anything happens, “entropy” would probably be your best choice. The guy with the gun is probably not a physicist, and he’s probably looking for a more specific answer to a specific situation, so the end result may be messy, but at least you’ll know that you were probably right.
Let’s define some terms. “Energy”, according to the standard definition, is the ability to do work. We’re not talking about energy in the sense that it usually comes up, which is how a person feels about doing something. This is the definition that is one of the foundational concepts of physics. Energy, as a rule, comes in two basic varieties, kinetic energy and potential energy. You might remember your elementary school science where your teacher said something like, “kinetic energy is energy in motion. Potential energy is energy at rest”. While that statement isn’t wrong, it’s not what one would call complete. If all you were talking about were marbles sitting at the top of a ramp or rolling down the ramp, that definition would pretty much cover it, but energy exists in many different forms. There is, like with the marbles on the ramp, what is called “mechanical energy”. There is also “chemical energy”, which is the energy stored in chemical bonds. This is like the bonds between the atoms in the gasoline in your car. The gasoline isn’t moving, but it definitely contains lots of energy. There are, depending on who you ask, several other “flavors” of energy. One that most people agree is a separate type of energy is nuclear energy. We talked about this a little bit when we were talking about atoms and the particles that make up atoms.
Remember that there are four fundamental forces in the universe: gravity, electromagnetism, the strong nuclear force and the weak nuclear force that have different strengths and which operate over different distances. Gravity is, by far, the weakest force, but it works over infinite distance. The weak nuclear force holds together the smaller subatomic particles that make up protons and neutrons. It’s both weak and only operates over tiny distances (as inside a proton). It is important, however, because it’s part of why stars “burn”. The electromagnetic force can be quite powerful, particularly over fairly short distances, but it can be felt at very large distances, too. We’ll talk about electromagnetism quite a lot, going forward. It’s the force that we are most aware of. The most powerful fundamental force is the strong nuclear force. This is the force that binds protons and neutrons together in the nucleus of an atom. It’s also the force that is released during a nuclear reaction. It’s not chemical energy, and it’s not mechanical energy, so most physicists give it its own category. There are some definitions of energy that add 2 or 3 or 7 more specific types of energy to the list, but for most of us, I think that just overly complicates things. We’ll go with three types of energy: chemical, mechanical and nuclear.
But getting back to kinetic and potential energy, we can think of kinetic energy as energy that is being transferred, while potential energy is energy that is stored, waiting to be turned into kinetic energy. Kinetic energy can be turned into potential energy and vice-versa. Perhaps you’ve heard something like, “energy can be neither created nor destroyed”. Would you be surprised to know that you now know the First Law of Thermodynamics? You probably thought that anything about “thermodynamics” was way too complicated to understand. As I mentioned at the beginning, basic science can be understood by anyone. Just don’t be intimidated by the words.
Now, let’s talk a bit more about entropy. Entropy is the tendency toward disorder in the universe. Since you are now on a roll of understanding important scientific principles, congratulate yourself that you now know the Second Law of Thermodynamics, which defines entropy. There’s a bit more to it, but that will do, for now. Think of a mound of sand in the middle of a parking lot. Over time, that mound of sand is going to get shorter and flatter, as wind and rain work on it. Eventually, the mound of sand will just be gone, spread out over the whole area. That’s a simple illustration of entropy, but it applies to everything. The reason for entropy is energy. Remember that “low-energy” tends to equate to “stable”, while “high-energy” tends to mean “less-stable”. Here’s an analogy, which is sort of related to the mound of sand, to help explain that. Let’s say you have someone dump a load of bricks in your driveway. If you don’t do anything with that pile, it’s just going to lie there, practically forever. In reality, the bricks will eventually crack and dissolve back into the dirt they were made from, but we won’t be here to see it. Anyway, the pile of bricks isn’t going anywhere, because the bricks are all lying on the ground (or on top of other bricks that are on the ground. The only force (more-or-less) that is working on them is gravity. Remember that gravity is a force exerted between any two (or more) objects that have mass. Gravity is pulling the bricks toward the ground, and the bricks are pulling the Earth toward them. The Earth is bigger, so the bricks fell out of the truck to the ground, due to gravity, rather than the Earth coming up to meet them. Since the bricks are on the ground, they can’t fall any further. This means they are in a low-energy state. They have zero potential energy, because they can’t move (which means they can’t do work, according to the definition of energy). However, let’s say we get ambitous and decide to build a wall out of our pile of bricks. If you bend down and pick up one of the bricks and stack in on top of another, that brick now has potential energy. The amount of potential energy it has is equal to the energy your muscles exerted to pick it up, against the force of gravity, and raise it up to stack on top of the other brick. The two bricks stacked together will probably stay that way, barring something really dramatic, like an earthquake. It’s still pretty stable, but it’s less stable than when both bricks were on the ground. Keep stacking bricks, one on top of the other. After a few, the stack gets wobbly. If you leave it and a strong wind blows, it’s likely that the stack will fall over and all the bricks will end up on the ground again. The stack of bricks contained all the energy (kinetic energy, since you were moving things) that you put into it with your muscles. Each brick had more and more potential energy as you stacked them higher, because you used more energy to lift it higher on the stack. For a good demonstration of the conversion of potential energy into kinetic energy, you could stand next to your stack of bricks and push the top brick off, letting it land on your foot. The potential energy in the brick, due to the force of gravity, will be transferred to the bones in your foot. The ultimate result of the kinetic energy of the falling brick being transferred to your foot depends on how far the brick fell and how strong the bones in your foot are, but at the very least, I expect it will hurt. Incidentally, if you were building a proper brick wall, with mortar and everything, that wall would retain all the potential energy you put into it for as long as it was standing. That wall, because of entropy, would also begin falling apart on the day you built it. If you don’t perform maintenance on the wall, over time, it will deteriorate more and more, until it collapses. That is because the wall is “ordered” and “high-energy”, due to the work you put into it. The universe, according to the Second Law of Thermodynamics (entropy), prefers “disorder” and “low-energy”, which means that eventually, the wall WILL fall no matter how well you built it, unless you continually put more energy into maintaining it.
All of that is mechanical energy. “Mechanical”, in this instance, just means that it involves objects and motion. Water flowing downhill, a marble on an incline, or a brick in a wall are all examples of mechanical energy. Let’s talk a bit about chemical energy. We’ll only touch on it, here, because we’re going to talk about it more in later articles.
Chemical energy is, as the name suggests, energy stored in chemical bonds. We usually perceive chemical energy as heat. Kinetic and potential energy still pertain to chemical bonds, it’s just a little different than with mechanical energy. If you recall from the earlier articles, chemical bonds form by the transfer (in ionic bonds) or sharing (in covalent bonds) of electrons between two atoms. Whether or not the two atoms will form a bond at all depends on two things: the configuration of the electrons around the atoms and the energy that is present. Some atoms just won’t form bonds with other atoms, because the electron configurations won’t allow it. Argon, for instance, won’t form bonds with anything, because its electrons are already is their most stable configuration. Carbon, on the other hand, loves to bond with certain other atoms. Sometimes, two atoms will form a bond without any other input into the process. This is called a “spontaneous reaction”, because it will just happen when the atoms get near each other. During a spontaneous reaction, energy (in the form of heat) will be given off and the environment (the solution) will actually get warmer. An example of a spontaneous reaction is iron rusting. Iron will combine with oxygen in the air to form iron oxide. This type of reaction is called “oxidation”, for obvious reasons. A small amount of heat is generated by the formation of the bond. You won’t notice though, because, even though it is a spontaneous reaction, this particular oxidation reaction occurs very slowly. Another example of a spontaneous oxidation reaction is when certain nitrogen-containing chemicals, like TNT combine with oxygen under the right conditions. This reaction proceeds very quickly, releasing a lot of chemical energy all at once. It’s the same process, just occurring at different rates. In fact, you can burn a stick of dynamite (don’t try this at home), just like a piece of wood. It will burn, not explode. Gasoline is the same. If you throw a match into a pool of gasoline, it will burn, not explode (again, don’t try this, because much badness might ensue). As the stick of dynamite, or the puddle of gasoline burns, it will release exactly the same amount of energy that it would have if the dynamite exploded or the gasoline was vaporized and exploded. Just during the explosion, the energy is released very much more quickly. Rusting, burning and exploding are all examples of the same type of oxidation reaction. The only difference is in how fast the reaction happens.
Aside from spontaneous reactions, at other times, reactions will only occur if you add energy to the system (usually by heating it up). Oddly enough, these types of reactions are called “non-spontaneous”. The energy you add to the system (which is kinetic, since you are transferring heat energy from one place to another) is then stored in the chemical bonds that form, resulting in potential chemical energy, which will remain stored in that chemical bond until the bond is broken. A quick example is that gasoline. When you burn (oxidize) gasoline in your car engine, the fuel is vaporized, combines with air (oxygen) and, when pressurized and ignited with a spark, it will explode, providing power in your engine. As we just said, that reaction is spontaneous, under the right conditions in your engine. But how did all that energy get into the gasoline in the first place? The answer is that the energy in your gasoline came from the sun. Many millions of years ago, in oceans, there were all sorts of plankton and such floating around. Most of these plankton were simple plants, that, like the plants we have around us today, convert the energy of the sun into chemical energy through a process called photosynthesis. We’ll talk more about photosynthesis some other time. Anyway, photosynthesis converts the radiant energy in sunlight into chemical energy, stored in the bonds of carbohydrate molecules in the plant (or plankton). While the plant is alive, it uses these carbohydrates for energy. When these plankton died and fell to the bottom of the sea, the carbohydrate molecules remained, as did the energy in the bonds. Over millennia, more and more plankton accumulated and got covered up. Pressure got higher, and eventually, the plankton decomposed into the slimy black stuff we know as crude oil. Coal is basically formed the same way, but generally with larger plants dying and falling into swamps, getting compressed over time into a kind of black rock (coal). Anyway, all the energy that was put into the carbohydrate bonds by the plants by photosynthesis, is still there. It is that energy that is released when we burn gasoline or coal. The potential chemical energy in those bonds just sat there for millions of years, until we released it. Making the bonds required energy. Breaking those bonds released the energy.
To sum up: there are two basic categories for energy, kinetic and potential. Kinetic energy is displayed when energy is transferred. Potential energy is stored up and waiting to be released. In general, energy takes the form of mechanical, chemical or nuclear energy. The governing principle of the universe is governed by the Second Law of Thermodynamics, which describes the process of entropy, which is a tendency for everything to move from a condition of order to one of disorder (or lower-energy). The only way to go against entropy is to add energy to the system.
We’ll be talking about energy and entropy a lot in coming articles, because they are fundamental concepts that impact practically everything else. For now, though, you now might know a little more about these ideas (and TWO of the Laws of Thermodynamics) than you did.