Video Transcript
In this video, we will learn how to
distinguish between categories of energy and understand how these categories depend
on the properties of objects.
We may have come across the word
“energy” in many different contexts. Indeed, the word “energy” is used
to mean some very different things. For example, a moving object has
energy. This kind of energy is called
kinetic energy. An object raised to some height
above the surface of the Earth also has some energy, even if it is not moving. This stored energy is called
gravitational potential energy. As well as energy that is stored,
we also have energy that is released. An example of released energy is
light from a light bulb. Now, these examples might all seem
somewhat unrelated. After all, what on Earth does a
light bulb have to do with a moving object?
The word “energy” is the important
connection here. Energy is a quantity described by a
single numerical value. So, in all these cases, that is,
the moving object, the object at a height above the ground, and the light bulb,
there is a numerical value that quantifies an energy associated with the
situation. Kinetic energy, gravitational
potential energy, and light are all examples of different categories of energy. To understand what is meant by an
energy category and why these are all examples of energy categories, we’re actually
going to start by thinking about how an object is defined.
An object is something made of
matter. All objects have a mass. That is, mass is a property that
all objects have. As well as mass, objects also have
various other properties. For example, the speed of an
object’s motion is a property of that object. Similarly, the temperature of an
object is also a property of the object. Then, there are also many other
properties, including but not limited to the color, opacity, shape, size, and
position of the object. There are some properties of an
object that when they change there is a particular category of energy that changes
correspondingly. Let’s now look at a few of these
properties and their corresponding categories of energy.
When the speed of motion of an
object increases, that is, when an object starts moving faster, the kinetic energy
of that object increases. Kinetic energy is the energy that
an object has due to its motion. When the temperature of an object
increases, or we could say as the object heats up, the thermal energy of the object
increases. Thermal energy is the energy
possessed by an object that is responsible for the object’s temperature.
As the height of an object above
the ground increases, for example, like a person who climbs from the bottom step to
the top step of a flight of stairs, the gravitational potential energy of the
object, or in this example the person, increases. Gravitational potential energy is
the energy that an object has due to its height above the surface of the Earth.
These are some of the most obvious
examples of properties of objects that can change and the corresponding categories
of energy that change when each of these properties changes. There are also several more
examples of properties and corresponding energy categories. Some others that we should be aware
of are that a change in chemical structure can correspond to a change in chemical
energy and that a change in magnetization corresponds to a change in magnetic
energy. With this information, we can
compare the energy of an object at different points in time.
We said earlier that energy is a
quantity that can be described by a numerical value. The value of the energy of a given
category for a given object, that is, the amount of that category of energy that the
object has, can either increase, decrease, or stay the same as time goes on. At any given time, the value of the
energy in a particular category can either be zero or it can be greater than
zero. If only a given property changes
for an object, with everything else remaining the same, then the value of the energy
of the corresponding category must also change, either increasing or decreasing.
For example, let’s think about the
object property speed. Recall that the corresponding
category of energy is kinetic energy. Earlier, we considered a ball
increasing its speed. An increase in speed, with all
other properties remaining the same, means an increase in the kinetic energy of the
ball. We also considered similar examples
in which an increase in temperature as water was heated in a pot corresponded to an
increase in the thermal energy of the water and likewise in which an increase in
height as a person climbed a flight of stairs corresponded to an increase in the
gravitational potential energy of the person.
As well as seeing how the energy of
a single object can change, we can also compare between two different objects. This time, let’s consider the
example of the object property height and the corresponding category of energy,
which is gravitational potential energy. We’ll consider two objects which
are identical in all respects, except that this left-hand object is at a greater
height above the ground than the right-hand object.
Because these two identical objects
have different heights above the ground and the object property height corresponds
to the energy category gravitational potential energy, then these two objects must
also have different amounts of gravitational potential energy. That is, they have different values
of energy of this category. In fact, the object with the
greater height above the ground will also have the greater gravitational potential
energy, or GPE for short.
So far then, we’ve looked in some
detail at a number of energy categories for which there is an associated property of
an object. Back near the beginning of this
video though, we said that light was also a category of energy. Now, to understand this, it’s
important to realize that not every energy category is directly associated with a
property of an object.
Light is an example of such an
energy category. For light, there is no simple way
to describe a property of an object that changes when the value of this energy
category changes. Some other examples of energy
categories without an associated object property are heat and sound. Notice here that heat is not the
same thing as thermal energy, which is associated with the object property
temperature.
Now that we’ve learnt about some of
the main categories of energy, let’s take a look at an example question.
Two identical tennis balls are on a
flat tabletop. One of them is rolling along the
tabletop. They are at the same
temperature. Which of the following categories
of energy must the tennis balls have different amounts of? (A) Kinetic energy, (B)
gravitational potential energy, (C) thermal energy, (D) elastic potential energy,
(E) chemical energy.
In this question, we’re being asked
to compare categories of energy for two tennis balls. We’re told that these two tennis
balls are on a flat tabletop and that they are identical to each other. “Identical” means that everything
about these balls is the same. So they have the same size, the
same mass, and are made of the same material as each other. The only difference between the
balls is that one of them is stationary, while the other is rolling along the
tabletop. We need to work out which one of
these five categories of energy the two tennis balls must each have different
amounts of.
To work this out, we’ll need to
recall that many categories of energy, including the ones listed here, correspond to
particular properties of an object. Two identical objects with the same
value of a given property will have the same value of energy of the corresponding
category.
Let’s now consider what properties
are associated with each of the energy categories we’ve been given to choose
from. We’ll begin by looking at options
(D) and (E). The chemical energy of an object is
related to the object’s chemical structure, while the elastic potential energy is
related to the shape of the object, for example, whether it is being squashed or
stretched.
Since the two tennis balls are
identical, they must both have the same chemical structure. And there’s nothing to suggest that
either ball is squashed or stretched. So we can assume they also have the
same shape as each other. This therefore means that the
elastic potential energy of both balls must be the same and the chemical energy of
both balls must also be the same. Therefore, it is not true that the
tennis balls must have different amounts of either of these two energy categories,
quite the reverse in fact. This means we can rule out answer
choices (D) and (E).
Option (C) says that the tennis
balls must have different amounts of thermal energy. The thermal energy of an object is
associated with the property temperature. The question tells us that the two
balls have the same temperature. Since the temperature is the same
in each case, this means that both tennis balls must have the same amount of thermal
energy. That means we know that answer
choice (C) cannot be correct.
Option (B) says that the balls must
have different amounts of gravitational potential energy. The gravitational potential energy
of an object depends on its height above the ground. Since both the tennis balls are on
top of a table, they are both clearly at some height above the ground. This means that both balls do have
some gravitational potential energy.
Importantly, we’re told though that
the tabletop is flat. That means that the height of the
surface of the table is the same at all points. Both tennis balls therefore have
the same height above the surface of the Earth. Since their height is the same,
then this means that the gravitational potential energy of both these identical
tennis balls is also the same. Since this energy category is not
different for the two tennis balls, then we can rule out answer choice (B).
That just leaves us with answer
option (A), kinetic energy. Kinetic energy is the energy that
an object has as a result of its motion. The amount of kinetic energy that
an object has depends on the speed of motion of the object. In this question, one of the tennis
balls is not moving, while the other one is moving because it is rolling along the
tabletop. Therefore, the speed of each of the
tennis balls is different. This left-hand ball has a speed of
zero, while the right-hand ball has a nonzero speed. Since the two tennis balls have
different values of this property speed and since the kinetic energy of an object
depends on the object’s speed, then the two tennis balls must each have different
amounts of kinetic energy.
Our answer then is option (A). The category of energy that the
tennis balls must have different amounts of is kinetic energy.
Let’s now finish up by summarizing
what we have learnt in this video. We’ve been looking at different
categories of energy. We saw that energy can be
categorized into these different categories. We also saw that some of these
categories of energy are associated with a property of an object. When that object property changes,
the corresponding energy category also changes. We learnt that the amount of energy
in a particular energy category is defined by a numerical value, which can either be
zero or it can be greater than zero. Additionally, we saw how we could
compare the value of an energy category either between two different objects or for
the same object at two different points in time by considering the value of the
associated object property.
