EM SC 240N
Energy and Sustainability in Contemporary Culture

What is Energy?


Learning Objectives Self-Check

Read through the following statements/questions. You should be able to answer all of these after reading through the content on this page. After going through the content, check the boxes next to the questions/statements that you feel at least somewhat confident answering. I suggest writing or typing out your answers, but if nothing else, say them out loud to yourself. This is to help you reflect on important content and will help you prepare for this week's quiz. It will also help lay the foundation for future course content.

Considering that this course is called “Energy and Sustainability in Contemporary Culture,” let’s start by answering two fundamental questions:

  • What is energy?
  • What is sustainability?


Let’s tackle the second question first, since the answer is a little more straightforward, even if it’s not always easy to grasp: What, exactly, is energy?

To Read Now

The National Energy Education Development (NEED) Project is a non-profit organization that provides a lot of useful (and free!) information about energy and energy issues. Please read the first two pages of their Introduction to Energy, which provides a good overview of energy. Hopefully, much of this will be a review for you! (Note that we will go over more up-to-date energy use data than the document has - i.e., more recent than 2009 - below.). You are welcome to read the rest, but it is not necessary. (It is helpful, though.)

Energy is most commonly defined as "the ability to do work." This is a useful technical definition, but from a practical perspective, the NEED Project's indication that energy is also "the ability to produce change" is helpful. A similar way to think of energy is that it "makes things happen." Energy is required to make a TV turn on, a car to move, the sun to generate light and heat, water to vaporize, plants to add biomass, a power plant to generate electricity, and for you to think about this course content as you read it. And even if these things are not actually happening, energy provides the ability to make them happen.

As indicated in the reading, the two categories of energy are potential (stored energy) and kinetic (energy in motion), each of which have several forms. (Note that the categories are listed in parentheses below because they can either be included or not, e.g., chemical energy can be referred to as "chemical" or "chemical potential" energy. Generally, "kinetic" or "potential" is not included.):

  • Chemical (potential) energy is stored in the bonds between atoms and molecules. Common examples include the energy stored in food, fossil fuels, and batteries, but anything that is made of more than one atom has chemical energy. Practically speaking, basically everything made of matter has chemical energy.
  • Mechanical (potential) energy is "stored in objects by the application of a force." Common examples include a wound spring, a stretched-out rubber band, and compressed air.
  • Nuclear (potential) energy is "stored in the nucleus of atoms," and is what holds the nucleus together. Anything made of matter has nuclear energy, but most of the nuclear energy converted by humans comes from the fission (splitting) of uranium atoms and is used to generate electricity. Most of the energy used by humans, however, comes from nuclear fusion (fusing of atoms) in the sun.
  • Gravitational (potential) energy is "energy of position or place." Common examples include water (e.g., in a river) at a high(er) elevation, a ball sitting on top of a hill, and you sitting on your chair right now. If you see naturally flowing water, it is moving down hill (tides and waves notwithstanding), so hydroelectric energy (electrical energy generated from flowing water) starts out as gravitational potential energy.
  • Electrical (kinetic) energy is "the movement of electrons." The most common example of this is electricity moving through a wire, but discharging static electricity and lightning are also electrical energy.
  • Radiant (kinetic) energy is also called "electromagnetic energy." It travels in transverse waves and is produced by anything with a temperature above absolute zero. Common examples include light, sunlight, microwaves, radio waves, and radiant heat emanating in all directions from a fire.
  • Thermal (kinetic) energy is the vibration of the molecules of a substance. As an object or substance gets heated up, the molecules vibrate more rapidly, and they slow down as it cools down. Humans cannot see this vibration because it happens at a molecular level, but we can feel it, or at least the results of it. Have you ever accidentally touched a hot stove and gotten burned? That unpleasant sensation is the result of the quickly vibrating molecules of the stove imparting their thermal energy into your skin. Anything above absolute zero has thermal energy, so it is all around us all the time, including everything you see right now.
  • Motion (kinetic) energy is the energy in moving objects. Anything with mass that is moving has motion energy. Moving cars, flowing water, a falling object, and even wind (air is made of matter, after all!) are common examples.
  • Sound (kinetic) energy moves in waves and is produced by vibrating objects. When you hear something, it is the result of the bones in your ear absorbing and converting these waves into motion energy, which your brain then interprets as sound. Despite what you may have heard, if a tree falls in the woods and there is no one there to hear it, it does generate a sound! Well, it generates sound energy, at least.

Energy efficiency and conservation of energy will be addressed later in this lesson.

The gentleman in this video (4:35 long) also provides useful information regarding energy and illustrates many of the concepts from the reading above. (In case you are wondering, yes, he is this excited all the time. He also has a number of really good videos regarding many topics. His YouTube channel has over 6,000,000 subscribers, so he must be doing something right!). Please note that you can open this video in YouTube by clicking on the title of the video in the window below.

What is Energy?
Click Here for Transcript of What is Energy Video

As you probably know by now, we’ve been working with Google and YouTube to answer ten of the most popular science questions asked on the Internet. And I gotta hand it to you, because there are few questions that are as confounding and complex and fascinating and inspiring, as this one the collective consciousness has spewed forth:

What is energy?

I'm Hank Green, and this is the World’s Most Asked Questions.


Energy is everything. It’s everywhere. It’s one of the true constants of the universe because as long as there’s been a universe, there’s been energy. And while it comes in lots of different forms that can seem different to us, they all amount to the same thing: Energy is the ability to do work. And work is just the act of displacing something by applying force.

So, say you stomp on a stomp rocket. The force of your foot hitting the pedal is turned into the force of air leaving the cannon -- sending your rocket sailing. Or maybe you're enjoying a nice patty melt -- the energy from that food is broken down for all of the quadrillions of cells that you have to do all of the things that they have to do -- make copies of your DNA, assemble and repair proteins, transport materials from one place to another, make muscle cells contract -- you know, all the stuff of being alive. That rocket sailing, your cells toiling away, your phone or computer being on right now to watch me -- that’s all work being done. And the ability to do these things is inherent in everything around you. Even things that look inert, completely lacking in energy. Like this log.

This log, for example, is chock full of chemical energy because it’s made up of combinations of carbon and hydrogen and oxygen formed into lignin, which is the stuff that makes up wood. All of the bonds between all of those atoms, in every molecule of lignin, contain energy. How do I know? Because if I were to apply enough extra energy, like as heat, to break those bonds -- it would release that chemical energy as fire. That chemical energy is also the kind of energy you get from your patty melt -- your body is fueled by the chemical bond energy in sugars, fats, and proteins. But this log also contains nuclear energy! Each atom in this wood has a nucleus, made of protons and neutrons, and the energy that binds them together is one of the most powerful sources of energy in the universe. If you could split one of the atoms of carbon or hydrogen in this log, and rip those protons and neutrons apart, it’d release some of that energy. There’s so much nuclear energy in each atom that, if I could unleash all of it that's in this log? There’d be a giant smoldering crater where I’m standing and everyone in the town of Missoula, Montana, would be dead.

So, everything that’s made of atoms has nuclear energy locked up in it, but also, it turns out, that mass and energy are the same thing! You might have heard of this little equation that a German patent clerk came up with about a hundred years ago: E = mc2. And there are SO MANY OTHER KINDS of energy that I’d love to get into if we had the time ...  but even though they may seem different, they can all be used to do work, whether it’s driving a turbine, or moving an engine piston or allowing the screen on your tablet to glow. Or, if it’s that most mysterious of energies, dark energy, causing the universe to expand more than it seems like it should. But here’s the thing to remember.

Once the work is done, the energy isn’t done. Because energy never goes away. It can never be destroyed, and in the same way, it can never be created. It can only be transferred from one source to another -- like, how the energy in the plants and animals that were in the patty melt were transferred into you -- or it can be transferred from one form into another -- like the chemical energy in the wood being transferred to light and heat as fire. You could think of the universe as a constant flow of energy, and we are just little pit stops along the way. Everything your body is doing right now -- whether it’s your lungs absorbing oxygen, your heart pumping blood, your brain cells firing as you watch me and learn things -- all those things are using recycled energy that’s been around since the origin of the universe. And by simply being alive, you’re releasing that energy back into the environment around you, to be used by other things in other ways.

So Internet, to answer your question: Energy is everything. And for those of you who answered our questions on our SciShow survey, where you feel like you get your energy may be keeping you up at night. Survey takers who have a hard time falling asleep nearly every night get their energy from knowledge first, second from purpose. Least likely? From exercise. Of all the fascinating questions in the world, what question do you want to see answered most? Let us know on Facebook or Twitter or down in the comments below and we will aim to answer those questions in a new video at the end of the month.

Credit: SciShow. "World's Most Asked Questions: What Is Energy?." YouTube. October 28, 2014.

Okay, so if you ask a physicist or energy expert what energy is, she will likely tell you that energy is the ability to do work. This sounds straightforward enough, but you may be thinking, “what is work?” Ask the same (or another) expert, and you will likely hear: “Work is the transfer of energy.” The video below from Kahn Academy (3:16) is optional but does a good job of explaining what this means. If you are still a little confused after watching it, you may want to read through the rest of the energy lesson, then go back to it. The formulas are not important for this course, but the concept of how work is related to energy is important. One thing to note: the narrator uses the term "Joule" a lot in this video. A Joule (J) is the international unit of energy and is simply a way to quantify energy. (More on quantifying energy shortly!)

Optional Viewing

Work as the transfer of energy from the Khan Academy (3:17 minutes)

Work as Energy Transfer
Click Here for Transcript of Work as Energy Transfer Video

One way to find the amount of work done is by using the formula Fd cosine theta. But this number for the amount of work done represents the amount of energy transferred to an object. For instance, if you solve for the work done and you get positive 200 joules, it means that the force gave something 200 joules of energy. So, if you have a way of determining the amount of energy that something gains or loses, then you have an alternate way of finding the work done, since the work done on an object is the amount of energy it gains or loses.

For instance, imagine a 50-kilogram skateboarder that starts at rest. If a force starts the skateboarder moving at 10 meters per second, that force did work on the skateboarder since it gave the skateboarder energy. The amount of kinetic energy gained by the skateboarder is 2,500 joules. That means that the work done by the force on the skateboarder was positive 2,500 joules. It's positive because the force on the skateboarder gave the skateboarder 2,500 joules. If a force gives energy to an object, then the force is doing positive work on that object. And if a force takes away energy from an object, the force is doing negative work on that object.

Now imagine that the skateboarder, who's moving with 10 meters per second, gets stopped because he crashes into a stack of bricks. The stack of bricks does negative work on the skateboarder because it takes away energy from the skateboarder. To find the work done by the stack of bricks, we just need to figure out how much energy it took away from the skateboarder. Since the skateboarder started with 2,500 joules of kinetic energy and ends with zero joules of kinetic energy, it means that the work done by the bricks on the skateboarder was negative 2,500 joules. It's negative because the bricks took away energy from the skateboarder.

Let's say we instead lift the bricks, which are 500 kilograms, upwards a distance of four meters. To find the work that we've done on the bricks, we could use Fd cosine theta. But we don't have to. We could just figure out the amount of energy that we've given to the bricks. The bricks gain energy here. And they're gaining gravitational potential energy, which is given by the formula mgh. If we solve, we get that the bricks gained 19,600 joules of gravitational potential energy. That means that the work we did on the bricks was positive 19,600 joules. It's positive because our force gave the bricks energy.

This idea doesn't just work with gravitational potential energy and kinetic energy. It works for every kind of energy. You can always find the work done by a force on an object if you could determine the energy that that force gives or takes away from that object.


In order for an object to gain or lose energy, work must happen. If you pick up a book from the ground and put it on a table, the book gained gravitational (potential) energy. You performed work on the book, and the amount of work is equal to the amount of potential energy gained. When you pull your car or bike out from a parking spot, the car/bike has motion energy, but when it was parked had none. That energy gain is the result of work done by the car engine (then drivetrain and wheels) or your legs (then pedals, chain, and wheels), and you can figure out the work done by considering the velocity and mass of the moving object. When the vehicle stops, the bike/car performs work on the road and tires, resulting in them heating up.

The sun is constantly generating massive amounts of radiant energy. That energy is provided by hydrogen atoms fusing together into helium and releasing nuclear energy. The amount of radiant energy generated in this process is equal to the amount of work done by the hydrogen atoms on the sun. When this sunlight hits your skin (or any object), it performs work on it, resulting in a gain in thermal energy. This gain in thermal energy is equal to the amount of work done.

I could go on and on, but the key thing to remember is that energy transfer requires work. Any time energy is transferred from place to place or from one form to another, work must be done, and the amount of work is equal to the amount of energy gained or lost.

Optional (But Strongly Suggested)

Now that you have completed the content, I suggest going through the Learning Objectives Self-Check list at the top of the page.