Mentor: What comes to your mind when I say that a person has a lot of energy?
Student: That person is very active, doing a lot of things quickly, etc.
Mentor: Right, and the scientific definition of energy isn't very different from that idea. Basically, energy is the capability to do work.
Student: But energetic people are not always doing work. Is the scientific definition of work different from the normal definition?
Mentor: Exactly! In scientific terms, work is force exerted parallel to the direction of motion. Can you think of an example?
Student: When you lift a box, you are exerting a force upward, the box is moving upward, and you are doing work.
Mentor: Right. Given that energy is the capacity to do work, do you know what types of energy exist?
Student: Well, since energy has to do with moving objects, is there an energy of motion?
Mentor: Yes, this is called kinetic energy. Any object that is moving is said to have kinetic energy. Now, what happens to energy if you throw a ball up in the air?
Student: Let's see...As the ball rises into the air, the velocity of the ball decreases. Does that mean that the energy is decreasing? And then when it comes down and speeds up, is the energy increasing?
Mentor: The kinetic energy does decrease as the ball rises in the air and slows. Then, when the ball comes down and increases in speed, the kinetic energy increases. Do you know why I'm specifying kinetic energy?
Student: Are there other kinds of energy besides just kinetic energy?
Mentor: That's exactly right! According to the Law of Conservation of Energy, the amount of energy in a system must always remain constant. But we know that kinetic energy is decreasing and increasing when we throw a ball in the air. How do you think this happens?
Student: Kinetic energy is definitely decreasing and increasing, so I guess there must be another type of energy that the object gets as it rises.
Mentor: Quite correct, this type of energy is called potential energy. Remember how I said that energy is merely the capability to do work? As objects move higher in the air, their potential energy increases because of the effect of gravity - a ball high up in the air can be dropped, allowing it to do work.
Student: That makes sense. I guess that if you throw a ball up in the air, potential energy must increase as kinetic energy decreases, and vice versa, so that total energy is always the same.
Mentor: Yes. The potential energy has to do with how high an object is and how gravity is pulling on it. But enough with potential and kinetic energy, are there any other types of energy?
Student: I don't know of any other ways to do work other than moving objects.
Mentor: Well, if you can't think of any other type of energy, try using the Law of Conservation of Energy again. Why does a train stop if you put on the brakes? It's losing kinetic energy but not gaining potential energy.
Student: I've always heard that trains stop due to friction, so friction must be a type of energy.
Mentor: You're on the right track. Friction actually includes several different types of energy that you can perceive as a train stops:
- Thermal Energy, which is heat that can be created via friction.
- Sonic Energy, which is the sound you hear as a train screeches to a halt.
- Light Energy, which you might see in the form of sparks flying or the hot metal on the train glowing red.
Student: So this means that when you are checking to make sure that total energy stays constant, you have to include all of these other forms of energy as well.
Mentor: Correct! However, there are still a few more types of energy to consider.
Student: Wait a minute, how do physicists actually find all of these types of energy? How can you tell if something is energy or not?
Mentor: The easiest way to find other types of energy is to imagine a real-world example where it seems like energy is not conserved. The Law of Conservation of Energy is always correct, so there must be one or more other types of energy to make up for any shortfalls. For instance, what happens to the energies of someone as they jump on a trampoline?
Student: As they land, their speed and kinetic energy decrease, but they are going down so their potential energy also decreases. Trampolines do make sounds and give off a bit of heat when you jump on them, but it can't be enough to totally reverse your fall. Moreover, somehow when you land enough energy is stored to send you skyward once more. Hence, there must be some type of "spring energy" that trampolines gain as they stretch.
Mentor: Yes, the "spring energy" to which you refer is called mechanical energy and describes the stored energy of springs, rubber bands, and, yes, trampolines.
Student: That seems pretty easy. Are there any other types of energy?
Mentor: There are three more forms of energy. Can you figure out any of them from your life and experiences?
Student: Well, when I turn on a light something must be creating light energy, so is electricity a type of energy?
Mentor: Absolutely, electromagnetic energy - the energy of electromagnetically charged particles - is one of the most convenient forms of energy because it can be transferred over large distances and stored easily in the form of batteries. Basically, electromagnetic energy is the energy of attraction between positively and negatively charged particles. A flow of electrons circulating along a wire is like water rushing down a stream: the force of its motion can be used to do work.
Student: And we see that kind of energy in all of the electronics we use, right?
Mentor: Right! Although physicists are currently working on a so-called Grand Unified Theory that would combine electromagnetic and gravitational force, so your children may only have to learn one less type of energy and force.
Student: Wow, sometimes I forget that physics is still changing as new discoveries are made. You usually tend to think of mathematical studies as being fixed, but actually theories are changed all the time as new discoveries are made.
Mentor: Good point. In fact, one of the more modern forms of energy was first theorized in 1905 and was successfully used in the 1940s - Nuclear Energy. But there's still one known form of energy left. Consider the burning of wood. What are the energy inputs and outputs?
Student: Let's see, to burn wood you must first set it on fire, which requires heat energy, but then the wood produces a lot more thermal and light energy than the spark that started the fire. Maybe wood loses mass as it burns, since there is always less ash left after the fire than there was wood beforehand.
Mentor: That's an interesting idea, but the only way matter can be converted into energy is through a nuclear process, and clearly a bonfire does not cause a nuclear explosion. All of the missing mass you mention leaves the fire in the form of smoke and water vapor. No, the energy of wood is something entirely different: chemical energy.
Student: So chemical energy is the energy stored in anything inflammable?
Mentor: Anything inflammable has chemical energy, certainly, though chemical energy can also be stored in nonflammable objects. In general, chemical energy is the energy stored in the bonds between atoms. When wood burns, those bonds are broken and their energy is released in the form of light and heat.
Student: So chemical energy is substance-specific, like thermal energy.
Mentor: Yes, many of the types of energy are related to one another. The most important thing to remember, though, is the Law of Conservation of Energy. No matter what problem you are solving, the total energy of the system must always remain constant.