Video Transcript
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.
