In this video, our topic is energy conversions. That is, we’ll be talking about how energy can change from one form to another. This kind of transfer takes place around us all the time. For example, whenever we see an object or objects in motion, chances are energy conversion is involved. In this picture, on the left-hand side, we see chemical energy stored in the rowers’ bodies being converted to the kinetic energy of the boat. While on the right-hand side, we see wind energy being converted to the boat’s kinetic energy.
As we get started talking about energy conversions, a helpful principle to keep in mind is the law of energy conservation. This law tells us that energy can’t be created and it can’t be destroyed, but it can be converted from one form to another. This is what we saw in the motion of those boats across the water. Energy was being harnessed and changing form to propel the boats forward. Even though energy can exist in many different forms, and we’ll talk about some of those forms in a bit, broadly speaking, it can be divided into two categories. Kinetic energy, energy due to motion, and potential energy, that is energy that’s stored up.
Even though we typically think of these two forms of energy separately, there are many practical examples of kinetic energy being converted to potential energy and back again. For example, consider a ball being thrown in the air and following an arc like this. When the ball is first released, it has a maximum of kinetic energy, energy due to motion, and a minimum of potential energy. But as the ball climbs higher and eventually reaches its apex, here its kinetic energy is at a minimum and its potential energy is at a maximum. And then, as it falls back down to earth, the exchange happens again. Now, potential energy is exchanged for kinetic energy. Until back down at the bottom of its trajectory, once again, its kinetic energy is at its maximum value and its potential energy is at its minimum.
There are many other examples of energy exchange, and to understand them better, we can begin to think about various kinds of kinetic and potential energy. Let’s start here on a very small scale. This is the nucleus of an atom, a collection of protons and neutrons. This nucleus and all atomic nuclei possess a kind of energy known as nuclear energy. This is a kind of potential energy that’s possessed by protons and neutrons, the constituents of a nucleus, due to forces they exert on each other. Nuclear power stations, also called nuclear power plants, take the nuclear energy of various atomic nuclei and convert it, ultimately, to electrical energy. This is one way that an energy conversion helps to support our daily lives.
Now, considering this nucleus further, we know that, often, the nucleus is surrounded by negatively charged particles called electrons. We know further that these negatively charged electrons are attracted to the positively charged protons in the nucleus. And they’re also repelled by other negatively charged objects, other electrons. This electrical attraction and repulsion helps to create a potential energy within an atom or within a molecule. It’s these interactions that are behind the form of energy we call chemical energy. As we saw, this is an electrical potential energy within atoms that’s due to the electrical forces between electrons and protons, the charged particles.
So, an atom, and let’s say that this is an atom, has nuclear energy and it also possesses chemical energy. And if we gather a bunch of atoms together, say that this is a gas enclosed in a container. Then, if we add up the energy of each one of these individual particles, each one of these individual atoms, then what we will have calculated is called the internal energy of this gas. This is the energy of the particles an object is made up of. The internal energy of an object is due to both the kinetic and the potential energy of the particles that make that object up.
Now, at this point, one important distinction to make is between internal energy and a form of energy called mechanical energy. Mechanical energy has to do with everyday-sized objects. For example, we can think back to our ball that we threw through the air. When this ball is in motion above the surface of the earth, it has both kinetic energy as well as potential energy. Its mechanical energy is the sum of those two energies. In general, the mechanical energy of an object is equal to the sum of that object’s kinetic and potential energy. And we say that this object is macroscopic; that is, on the size scale of everyday objects, just to distinguish it from the small particles that we consider when calculating internal energy.
Along with these kinds of energy, there are a number of others to keep in mind too. For example, if we have some object a certain distance above the surface of the earth, then we know that that object has gravitational potential energy. This is energy that an object has by virtue of its position in a gravitational field. Now, let’s say that this object is released and it falls to the earth and lands with a thud. That sound it makes is an example of another form of energy. Sound energy is defined simply as the energy transferred by sound waves. An example of sound energy is thunder.
Then, let’s imagine further that our falling object is an elastic object, something like a rubber ball. In that case, when it hits the ground, it will deform. It will change shape. When this happens. There’s a form of energy that’s stored in the ball called elastic potential energy. This is energy that, in general, is stored in a stretched or compressed object that tends to return to its original shape after being stretched or compressed. Often, when we talk about springs being compressed or stretched, this form of energy is involved.
Along with these forms of energy, there are a couple more we can keep in mind. Whenever we have a lot of electrons, negatively charged particles, moving together in a certain direction, then we have what’s called an electrical current. And associated with electrical current is what’s called electrical energy. This is simply the energy that’s transferred by an electric current. The flow of electrons is one very common physical event. Another common one is the movement of electromagnetic waves, light. The energy associated with the movement of electromagnetic waves is called radiant energy.
Radiant energy is what we experience when we stand out in the sunlight or stand near to a campfire and can feel its warmth. Of course, the term radiant energy applies to all electromagnetic waves, not just the ones we can experience through our five senses. Now that we’ve defined a number of different types of energy, and we’ve seen examples of how these types can be converted from one kind to another, let’s get a bit of practice with these ideas through an example.
Which of the following is the term used to refer to the energy of an object due to the energy of its constituent particles? A) Chemical energy, B) Nuclear energy, C) Internal energy, D) Electric potential energy.
All right, so what we want to do is figure out which of these four terms correctly describes the kind of energy that an object has due to the energy of its constituent particles. That is, the particles that make that object up. To figure this out, a good place to start is with the nucleus of an atom. Let’s say that here is our nucleus, a collection of protons and neutrons. Just considering this nucleus all by itself, there’s a form of energy stored here called nuclear energy. This is energy that’s possessed by the protons, we’ve drawn them in blue, and the neutrons, we’ve drawn them in green, due to the forces that they exert on one another in this nucleus.
Nuclear energy comes not from the energy of the protons and neutrons by themselves, but because of the forces they exert on one another. Therefore, we wouldn’t say that nuclear energy is the energy an object has due to the energy of its constituent particles. In the case of nuclear energy, it’s the interactions between those particles that are important. So, we’ll cross that off our list of candidates. Moving on, let’s draw a few electrons around this nucleus, as they normally would appear for an uncharged atom.
Now, these electrons, which are negatively charged, feel an attractive force to the positively charged protons in the nuclear core. And it works both ways; the protons are attracted to the electrons too. And then, the negatively charged electrons push one another away or repel each other, as do the positively charged protons to one another. We can say then that this atom, just by virtue of having positive and negative charges in it, possesses some electric potential energy.
Now, in general, electric potential energy describes energy that is stored in electrically charged objects. An important thing to notice about the term is that it’s strictly a potential energy. That is, it’s not energy associated with motion. But if we consider our problems statement again and talking about these constituent particles, the particles that make up our object of interest, these particles could have potential as well as kinetic energy. And based on this description, we want our term to be able to account for them both. So, considering option D, electric potential energy, because this only accounts for a potential and not kinetic energy, we won’t choose this as our answer.
And then, getting back to our atom, when we talk about the electrical potential energy of the charges in an atom, that’s a description of that atom’s chemical energy. We could say that the term electric potential energy applied to the charges in an atom is equivalent to the chemical energy of that atom. This means that option A, chemical energy, is also a description of potential but not kinetic energy types. Therefore, it, too, offers an incomplete description of the energy of an object due to the energy of its constituent particles, which may involve kinetic energy. This means we’ll cross option A off our list as well.
And we’re left with one remaining choice, internal energy. If we were to model our atom, shown here as a single dot like this. And then, if we were to collect lots of these atoms together and enclose them in some container. Then at that point, if we were to add up all the energy of each one of these atoms, both the kinetic and the potential energy of each one. Then, that sum would be the energy of these constituent particles. And we could say that these constituent particles are the atoms that make up our object. And that is indeed our object’s internal energy. This is the term that refers to the energy of an object due to the energy of its constituent particles, the particles that make it up.
Let’s summarize now what we’ve learned about energy conversions. Starting off, we saw that energy can take many different forms. And these forms can be converted from one to another. But though it can be converted, as far as we know, energy can’t be created and it can’t be destroyed. This is called the law of energy conservation. And when people say that energy is conserved, this is what they mean. We saw further that energy, as a category, can be broadly divided into two types: kinetic energy, energy having to do with motion, or potential energy, that is energy that is stored up.
And lastly, we studied and defined a number of different forms of energy. Starting with nuclear energy, we then considered chemical, then electrical potential energy, internal energy, mechanical, sound energy, gravitational potential energy, elastic potential energy, electrical energy, and finally, radiant energy. These are all forms that energy can take when being converted from one form to another form.