We have already talked a fair amount about “energy,” noting that there are two main sources of energy that drive the processes we seek to understand in Earth system science. But what exactly is energy? In the coming chapters, we’ll need to have a more precise way of talking about energy in order to look deeper into Earth’s systems. We’ll also need to understand the distinction made in science between energy and “power.”

Brief Definitions of Energy and Power

You’ve probably learned about energy in your past science courses, in which case you will know that it’s surprisingly difficult to come up with a simple definition of “energy.” In fact, if you look it up, you’ll find that different dictionaries (or other sources) define energy in a variety of different ways. Nevertheless, for most purposes, we can define energy in science in this fairly broad way:

• Energy is something that can make matter move or heat up.

In everyday life, we often use the terms energy and power interchangeably, but in science the term “power” has a slightly different meaning:

• Power is the rate at which energy is being used, released, or generated.

As a simple example of the difference between energy and power, think about a light bulb (Figure 4.41). A particular light bulb will be labeled with the amount of power that it uses when turned on (for example, 15 watts). This amount of power is always the same for that particular light bulb. In contrast, the amount of energy the light bulb uses depends on how long you leave it on. If you leave it on for 10 minutes, it will use twice as much energy as if you leave it on for 5 minutes.

Basic Types of Energy

In everyday life, we talk about many different kinds of energy. For example, we talk about the energy we get from food, the energy needed to drive a car, the energy of sunlight, electrical energy, the internal energy of the Earth, and more. Looking more deeply, however, scientists have found that all forms of energy can be classified into just three basic types:

1. Energy of motion, which is more formally called kinetic energy (kinetic comes from a Greek word meaning “motion”).
2. Energy carried by light, or radiative energy (the word radiation is often used as a synonym for light).
3. Stored energy, which is more formally called potential energy , a name it gets because it has the potential to later be converted into energy of motion or radiative energy.

Energy can change its form among these three types (Figure 4.42), but it can never be created or destroyed (an idea embodied in what is called the law of conservation of energy ).

Energy Drivers of the Earth

In high school and beyond, you will learn much more about these basic types of energy. Here, let’s just look at how the two main energy drivers for Earth fit into these basic types:

• The first major driver, energy from the Sun, is radiative energy, since that energy reaches us in the form of sunlight.
• The second major driver, Earth’s own internal heat, is less obvious. However, as you may recall from physical science, on a molecular level heat is created by the motions of atoms and molecules: the higher the temperature, the faster the atoms and molecules in any substance — such as air or water or a rock — are moving (Figure 4.43). In other words, Earth’s internal heat is actually a form of kinetic energy, sometimes called thermal energy .

Figure 4.43 – Temperature is a measure of the average kinetic energy of atoms or molecules.

Temperature and Heat

There’s an important subtlety here, which is that temperature and heat, or thermal energy, are not quite the same thing. The following discussion should help you understand this idea.

Figure 4.44 illustrates what you should have found with the above discussion. The higher temperature of the oven air means that these molecules are moving faster and therefore strike your skin harder, on average, than the molecules in the boiling water. However, the much higher density of water means that many more molecules are hitting your skin (in the places where the water touches you). While each individual molecule hitting your skin from the boiling water transfers a little less energy than a molecule of air from the oven, the sheer number of water molecules hitting you means that more thermal energy is transferred to your skin. To summarize:

• Temperature is a measure of the average kinetic energy of the atoms and molecules in a substance.
• The thermal energy content (or heat content) of a substance is the total amount of kinetic energy of all its particles in a substance.

“Useful” Energy (Energy Transfer)

Objects generally contain energy in more than one form, and in any particular situation, only some forms are “useful” in the sense of being available for transfer to other objects or for conversion into some other form.

As a simple example, imagine you have a piece of popcorn and you toss it up and let it fall into your mouth. Figure 4.45 shows that the piece of popcorn has energy in several different forms, including:

• It has kinetic energy due to its motion as it falls into your mouth.
• It has thermal energy that depends on its temperature: If it is hot when you eat it, it contains more thermal energy that if it is cold.
• It has potential energy stored in its chemical bonds (“chemical potential energy”), which can release this potential energy when the bonds are broken or changed in particular ways.

In the context of eating the popcorn, however, we care only about the chemical potential energy that your body can make use of (through metabolic processes that break down or change the molecules of the popcorn). In other words, we can ignore the thermal energy and the kinetic energy of motion, because they don’t provide useful energy to our bodies when we eat the popcorn.

The popcorn example illustrates the idea that when we discuss energy in science, there are usually multiple types of energy involved, but we focus only on the type of energy that is “useful” in a particular context. For example:

• For any kind of food, the “useful” energy is the chemical potential energy available to our bodies.
• For objects perched on a ledge where they might fall down, or particles coming together through gravity to form a planet, the “useful” energy is the gravitational potential energy that that can be released as they fall (see this “I Was Wondering” box).
• For engineers trying to learn how to make cars safer in the event of a car crash, the “useful” energy in this context is the kinetic energy of the moving car (and its passengers).

For our current purposes, we need to understand what kind of energy is “useful” in the context of Earth system science.

• In the case of the solar energy reaching Earth, it is almost all “useful” in the sense that it could in principle be absorbed by the ground, oceans, or atmosphere. (However, some of the sunlight is reflected back to space, so this part does not actually get “used” by Earth.)
• For Earth’s internal thermal energy, the “useful” part is part that can cause movement or heating of other parts of the system. In other words, when we talk about the internal energy of Earth (or any other world), we are talking about the thermal energy (or heat) that can be transferred from one place or object to another.

Units of Energy and Power

We are now ready to discuss how we quantify energy and power in science. As with other quantities that can be measured, we need defined units of measurement, and here they are:

• For energy: The standard international unit of energy is called the joule.
• For power: The standard international unit of power is called the watt, defined as 1 joule per second:

1 watt = 1 joule/s
 

We’ll talk about both energy and power in our Earth science discussions, but when we are doing quantitative comparisons, we’ll more commonly talk about them in terms of power. Moreover, because we will often discuss large amounts of power, we’ll often modify watts with one of the following common metric prefixes:

For example, total world power consumption is approximately 15 terawatts, meaning that as a civilization, we use about 15 trillion joules of energy each second. As we’ll see in later chapters, we can use a number like this to evaluate how different energy sources, such as wind or solar, might be useful to us in the future.

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