Colorado State University

Theory

It’s tough, as we said, to clearly define energy. Let’s call it a sort of a currency that can be traded for making things happen — you can “spend” some energy and get something you want in exchange, but the energy doesn’t vanish; it’s just transferred or transformed. Energy cannot be created or destroyed. We say that energy is conserved — you can turn one type of energy into another type (or even, in certain rather extreme circumstances, into mass), but you’ll never be able to increase or decrease the total amount of energy in the universe. We’ll clear up this picture and continue to emphasize conservation of energy as we go forward.

We’ll primarily consider four types of energy here: 

1. 1. kinetic energy — energy of motion; K = (1/2)mv², where m is mass (in kg), v is velocity (in m/s²), and K is kinetic energy (in Joules, J)
   2. potential energy — stored energy of position
      1. gravitational potential energy increases as an object gets higher above some equilibrium position; U_g = mgh, where m is mass (in kg), g is the magnitude of the acceleration due to gravity (9.8 m/s² on Earth), h is height above the equilibrium position (in m; h is negative for objects below equilibrium), and U_g is gravitational potential energy (in J)
      2. spring potential energy is present in a spring which is stretched or compressed beyond its equilibrium position; U_sp = (1/2)k∆x², where k is the spring constant of the spring in question (in N/m), ∆x is the displacement from equilibrium (in m), and U_sp is the spring potential energy (in J)
      3. electric potential energy is the energy present in an electric charge which is away from its equilibrium position in an electric field: U_elec = qV, where q is the amount of charge (in Coulombs, C), V is the electric potential (in volts, V), and U_elec is the electric potential energy (in J) — we won’t work with this much here; see Everything You Need to Know About Electricity for much more discussion!
   3. thermal energy — energy of molecular motion, with more motion corresponding to greater thermal energy; we won’t work with a particular equation for this, but rather consider it qualitatively
   4. chemical energy — energy stored in bonds between atoms; we won’t consider a particular equation for this either

Any or all of these types of energy may be present in any given system. A system is a set of interacting entities that we choose to consider when solving an energy problem. We know that the total amount of energy in the universe remains constant, so we could always pick the universe as our system, but that could get a bit complicated. Fortunately, conservation of energy applies across scales, so we can pick much smaller systems to analyze. Let’s consider a simple case: You pick up a heavy box. We’ll take our system as you and the box. Chemical energy in your body is transformed into gravitational potential energy of the box, plus some thermal energy in your body. 

This leads us to another important consideration: efficiency. Your students likely have an intuitive grasp of efficiency already — it’s about being able to achieve a desired outcome with as little “waste” as possible. This is essentially the definition of efficiency we’ll use here: the ratio of what you “get” to what you have to “pay.” In the example above, your efficiency was not 100%; what you “got” was gravitational potential energy in the box, but you had to “pay” more than this value in chemical energy, as evidenced by your increase in thermal energy. It turns out that your body is, as a general average for most activities, about 25% efficient. This means that, of the chemical energy you “paid,” about 75% was transformed to thermal energy.

Hopefully this is all sensible enough, but you may have spotted a concern with the initial scenario. What’s to stop us from taking just the box as our system? Or just the person lifting it? The answer is this: Nothing at all! Depending on what we know and what we want to know about the situation, either of those is a perfectly acceptable choice of system and consistent with conservation of energy. To understand why, we’ll have to consider a new idea: work. Work is a transfer of energy into our out of a system by means of forces applied by an object in the system to something outside it, or vice-versa. If we take just the box as our system, we say that the person lifting the box does work on it; this transfer of energy into the system is transformed into gravitational potential energy of the box. If the person lifting the box is the sole component of the system, we say that work is done by the system; this outward transfer of energy is equal to the chemical energy input, less thermal energy. 

Speaking of thermal energy: You know that you won’t stay warmer forever after you’ve lifted a box. Eventually your thermal energy will decrease back to its equilibrium value. Again, if we take the person lifting the box as the system, we could consider that they will transfer thermal energy to an object with a lower temperature than themselves (e.g., the surrounding air). This isn’t work, because there’s no force involved. We call transfer of thermal energy into our out of a system heat. One final point here: You know that you’d cool off faster after lifting the box if it was chilly outside than if it was a warm summer day, and that it would feel harder to lift the box quickly than to lift it slowly. There’s clearly something physically meaningful about the rate at which energy is transferred or transformed — we call this quantity power.

Application

Energy is a great topic for open-ended exploration and experimentation. We’ve developed and classroom-tested activities about the nature of energy and about conservation of energy. That said, we also have videos on experiments and demonstrations that can help you make energy interesting and fun for your students; these videos will be our focus here. We’ve organized the videos in terms of “the 5 E’s”: Engage, Explore, Explain, Extend, and Evaluate. This structure has worked well for us in teacher workshops, and we hope it works well for you too! However, we always welcome your feedback. Please contact us anytime to let us know what’s working, what’s not, and how we can make this resource more useful for you. Enjoy!

Engage

The purpose of the Engage segment is to pique students’ interest. We don’t need to directly transmit concepts at this stage, merely give students a chance to think and get excited about the topic we’ll be discussing.

Video 1: Liquid Oxygen Flaming Pretzels

Chemical energy in your body comes from food (which ultimately comes from plants, which get energy from the sun, in which mass is converted to energy as atoms fuse together). We can test the energy content of a food item by lighting it on fire — more chemical energy in the food means more fuel for the fire. Here we soak the interior of a pretzel stick with liquid oxygen (condensed from the air on the side of a container of liquid nitrogen); this increases the pretzel’s rate of combustion, to rather spectacular effect.

Video 2: Sweet Fire

We’re playing with fire and food again, but this time there’s no liquid oxygen necessary. By blowing a cloud of powdered sugar over a blowtorch, we make sure there’s plenty of oxygen around for rapid combustion. As you’ll see, sugar is a rather concentrated source of chemical energy!

Explore

In the explore segment, students get to experience firsthand the principles we hope to teach them. We still don’t need to explicitly describe any concepts yet — the students will start working these out as they explore.

Video 1: Chilly Drilly

Since we’ve spent some time thinking about where chemical energy in your body comes from, let’s think about what you can do with it — let’s think about energy transfers and transformations. We can easily modify a battery-powered electric drill to make a simple hand-crank generator that students can use to explore these concepts. The “crank” part will be some sort of handle inserted in place of a drill bit. Make sure to clamp down the button you’d need to press to start the drill. Instead of putting a battery in the drill, you’ll want to attach a wire to each battery contact. You’ll connect these wires to a device you want to power. The series of energy transformations, then, is as follows: You use chemical energy in your body to do work. As energy is added to the drill, it is transformed into electrical energy. This electrical energy is transformed within the device you hook up to the battery compartment. Here we use a Peltier device, which, depending on the direction of current through it, moves thermal energy to one side of the device or the other. (This is a result of the thermoelectric effect, which is beyond the scope of our discussion here.) A Peltier device is thus a heat pump, much like your refrigerator — it doesn’t “make cold,” it just “moves warm” away from out of a certain area (and thus into another). However, you could also connect the wires to a few mini holiday lights, or another small gadget of your choosing. With a double-pole, double-throw switch, you could even rig up a direct comparison between incandescent and LED bulbs — efficiency makes sense when you can feel the difference in the energy you must provide to light each!

Video 2: Photon of the Opera

Another take on a hand-crank generator, this one based on a salad spinner instead of a drill. We glued magnets onto the rim of the spinner’s basket and attached a solenoid to the exterior of the spinner’s bowl. To this solenoid we wired a couple of small LEDs. When you do work on the plunger of the salad spinner, the rotational kinetic energy of the basket increases. As the magnets on the basket move past the solenoid, the magnetic flux inside the solenoid is constantly changing, so an electric current is induced in the solenoid (see Everything You Need to Know About Magnetism for much more information on this topic). Electric energy in the current (see Everything You Need to Know About Electricity) is then transformed into light (photon) energy in the LEDs.

Video 3: Liquid Crystal

This experiment is a bit different from the other two in this section, but phase transitions — the application of energy conservation it explores — are both interesting and important. At room temperature, it’s possible to hold sodium acetate below its freezing point. A slight disturbance and a small surface to build on will quickly cause the solution to crystallize (freeze). As the atoms that were formerly sliding past one another in the liquid solution becomes locked into a crystal lattice, their kinetic energy decreases. You know, by conservation of energy, that this energy doesn’t just go away; in fact, it is transformed into thermal energy. Freezing actually warms the solution up!

Explain

Now it’s time to codify and formalize the students’ observations. The following videos can help you elucidate and demonstrate the basic concepts underlying the discoveries students made as they explored.

Video 1: To Pop or Not to Pop

Taking a blowtorch to a balloon sounds like a sure bet for popping. And for a balloon filled with air, it is… But not for a balloon filled with water. Transferring energy to air as heat simply causes the air to warm up and expand. The temperature of the balloon will quickly exceed the maximum value the rubber can withstand, and the balloon will pop. However, you’d have to add much more energy to bring an equivalent volume of water to the same temperature. In fact, you’d have to bring all the water to a boil and heat the water vapor! This is not likely — if you take a blowtorch to a water balloon, you’re going to bring the water in the balloon to a boil, at which point its temperature will stay constant at about 100˚C (the exact temperature varies a bit with your elevation). All the energy you add with the blowtorch goes into boiling the water (i.e., causing to to undergo a phase transition from liquid to gas), not to changing its temperature, so the balloon doesn’t pop!

Video 2: Energy in Food

We started with an interesting display of food on fire; let’s now think about what that meant. A quick review: You need food as a source of chemical energy for everything from basic cellular processes to running a marathon. We can get a sense of how much chemical energy is in a food item by burning it; we can expedite the burning process by adding liquid oxygen (condensed from the air using liquid nitrogen). Here we use this strategy to get a sense of the amount of energy in a cracker, a pretzel stick, and a cookie — it’s easy to see which one would provide you the most energy!

Extend

Once the basic principles are clear, we want to encourage students to expand their thinking and ask questions that go beyond the scope of what we’ve already discussed.

Video: Lightly Microwaved

If you try this yourself, make sure to follow the safety cautions noted in the video.

Waves, including electromagnetic waves (self-sustaining paired oscillations of electric and magnetic fields), carry energy (see Everything You Need to Know About Oscillations & Waves). Your microwave oven generates one such type of wave (microwaves, as you might expect); the electric field component of the microwaves pushes on water molecules in your food, thereby increasing the thermal energy of your food. We can demonstrate rather more dramatically that waves carry energy by putting some small fluorescent light bulbs in the microwave for a few seconds. The electric field component of the microwaves is sufficiently large to excite electrons around mercury atoms inside the bulbs. When these electrons drop back down to lower energy levels, they emit their excess energy in the form of a photon (a packet of energy) of visible light. Net result: The bulbs light!

Evaluate

It’s important to figure out what students understand after the first four E’s. Tests and quizzes are one option, but there are many others.

This one is up to you — what works best for you and your students? We, for example, have had good results with turning the tables and letting the students make a video at this stage. We’d love to hear what works in your classroom!