What is Energy? - Definition and Significance in Nature

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  1. 0:04 What is Energy?
  2. 1:50 Where Does Energy Come From?
  3. 5:46 Why is Energy Necessary?
  4. 7:07 Types of Energy
  5. 9:03 Lesson Summary
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Taught by

John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

This lesson describes the nature of energy and how it is transferred from one source into another. Additionally, it will describe the significance of energy in natural systems.

What is Energy?

I like to go for runs, and when I do, I think of energy, or perhaps more accurately, my lack thereof. When I run, I also think about matter. Matter is anything that has mass and takes up space, and as I get older, I seem to accumulate more matter, which makes it more difficult to run. Energy and matter are everywhere, and everything is composed of energy and matter. Energy, however, is more abstract, as you can't really see it. Nonetheless, energy has been around since the beginning of time. In fact, theory holds that energy existed before matter. According to the Big Bang Theory, the universe began when an infinite amount of energy suddenly began to expand. Energy cooled as it expanded, and, as it cooled down enough, the particles were able to come together and form matter eventually, giving rise to the universe as we know it today. It can be said that the universe is composed of two interchangeable properties: energy and matter. Energy can be converted into matter and matter can be converted into energy.

Okay, so what is energy? Energy is usually defined as the capacity or ability to perform work. I'm sure you can appreciate the fact that energy is needed to do work, but what's work? Work can be defined as the movement of mass when a force is applied to it. In other words, work is done when a force moves some object some distance. Simply put, work requires energy, and energy spent performs work.

Where Does Energy Come From?

Now that we know what energy is, let's consider where it comes from. The answer to this question is really pretty simple. Energy comes from matter. Now you know why we've been talking about these two properties of energy and matter. This makes sense when we examine the definition of work. Work can be defined as the transfer of energy to or from matter. In fact, the total amount of energy contained within matter is directly proportional to the mass. Additionally, energy can be transferred from one system to another.

So what's a system? A system is a collection of objects organized into a whole - or, to put it another way: whatever you are considering as the whole for the purpose of a study. Let's talk about a few examples of systems. For example, you can consider the entire universe as a system. You may consider the universe as a system for the purpose of studying the relationship between different galaxies within the universe. Another example of a system could be your body. You may consider the human body as a system for the purpose of studying the interaction between the various organs that make up your body.

Energy transfer from one system to another can be as simple as moving a mass from one system to another system. For example, when a lion eats a zebra, the energy contained within the mass of the zebra is transferred into the mass of the lion - or, perhaps, a whole bunch of lions. On the other hand, energy transfer can be more complex. When energy is transferred from one system to another system by some means other than movement of mass, the second system will change as a result of work done to it.

Let's look at an example of that. We've all heated water on a stove, or perhaps we've watched somebody else do that. When we heat the water on the stove, energy is transferred from the heated element of the stove into the water. As that energy is transferred, the water molecules are forced to move faster. Similarly, a system may transfer energy to another by coming into direct contact with it. My son loves to play baseball; he's played ever since he was a little kid, and he's really pretty good at it. When he hits the ball, that ball goes pretty far. In this example of energy transfer, the energy of motion in the bat is transferred into the baseball, which, in turn, now has energy of motion as well.

In the physics world, energy is measured in units that we call joules. This is important because you'll see this unit attached to numbers as a measurement of how much energy there is in a system. In other fields, such as industry and nutrition, energy may be measured in different units. For example, in industry kilowatt-hours are used as a measure of energy. In nutrition, calories are used as a measure of energy. You've probably heard of calories before; in fact, if you look at any wrapping paper or box of food, it's required by law that the amount of energy contained within that food be expressed in terms of calories.

Why is Energy Necessary?

Hopefully you now have a better understanding of what energy is and where energy comes from. Let's shift gears here a bit. Now let's talk about the significance of energy as it exists in nature. As energy is defined as the capacity to do work, we can appreciate that energy is needed in order to move anything in nature. This can be as obvious as energy derived from the burning of fossil fuels to move your car, or it can be as subtle as energy derived from the breakdown of nutrients by our cells to do work. For example, our muscle cells have to break down nutrients to obtain energy that is necessary to contract. That contraction of the muscle is the work performed by the muscle, and the energy needed for that work comes from the breakdown of nutrients within the muscle cell. In either case, energy is released when some mass is broken down. Once released, energy, in turn, can be used to perform work. As we've talked about before, energy is everywhere. Throughout the universe, energy is released from matter and used to perform work.

Examples of Types of Energy

Mechanical energy can be converted into electrical energy.
Energy Converted to Energy

Let's shift gears again. Now let's talk about different types of energy. There are various types of energy. One example of an energy type is thermal energy. Thermal energy is the internal energy of a system as a result of its temperature. We discussed this a little bit before when we talked about heating a pot of water on the stove. That heated pot of water has a certain temperature, which is a reflection of the internal energy contained within the water. That's thermal energy.

Another type of energy is mechanical energy. Mechanical energy is the energy of an object as a result of its location or motion. A good example of mechanical energy might be water held back behind a dam. There it has a certain position. Then the water moves over the dam. Now that water has motion. So that's a good example of a system containing mechanical energy - water moving over a dam.

It's really important, however, to keep in mind that all energy, regardless of the type, is still simply the capacity to do work. Any form of energy may be transferred into another form of energy. For example, let's go back to the dam. Water flowing over a dam has energy of motion, and that energy of motion is used to generate electrical energy. When these energy transformations occur, the total amount of energy remains the same. It is important to note as well that what we may call 'man-made' energy is simply energy that has been transferred from nature to some man-made source.

Lesson Summary

Let's summarize. In review, energy is the capacity to do work and work is accomplished when a force moves an object. Energy is measured in units referred to as joules. Energy is necessary within systems in order to do work within that system. Finally, energy exists in different forms and energy can be transformed from one form of energy into another form of energy.

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