Video: Definition of Internal Energy

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.

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Video Transcript

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.

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