Playing With Energy Pre-­‐Lab: Conservation of Energy A Bit of History A modern physics student learns that the joule and the calorie are two different units of energy and an energy given with one of the units can easily be converted to the other using the fact that 1 cal = 4.18 J sometimes called the mechanical equivalent of heat. Historically, the calorie was a unit reserved strictly for dealing with heat, thought to be related to a massless fluid called caloric. Hotter objects contained more caloric. The predecessors of the joule (like the foot-­‐pound) were used to measure work (also referred to in those days as effort, force, or living force). Thus, that equal sign above is, in a sense, a loose statement of conservation of energy. It’s worth taking a brief look at how this conversion factor was discovered. As early as the 1820’s scientists were looking into the conservation of energy, though they themselves weren’t quite aware of this. These scientists were investigating how one kind of effect could be converted into another kind of effect. For example, a battery could take a chemical effect and produce an electrical effect that could produce a magnetic effect that could produce a mechanical effect. Things would also heat up along the way, so thermal effects were also being produced. It seemed that there may be some great truth lurking somewhere, but it took until the 1840’s for any great progress to be made. It was in 1847 that amateur physicist James Joule performed his now-­‐famous paddle wheel experiment, which started convincing scientists to take that conversion factor above seriously. This experiment was a very elegant descendant of other experiments that Joule had performed over the previous five years, all with the goal of measuring the mechanical equivalent of heat. Within about a decade of Joule’s monumental announcement, the theory of caloric was dead and the law of conservation of energy was well established. It’s no real wonder then that the SI unit of energy bears James’ name. Except…it turns out that in 1842, another amateur physicist named Robert Mayer published a value for the mechanical equivalent of heat that was not too far off. He had performed a not-­‐so-­‐elegant experiment involving work and temperature changes with gases. Very few people listened. (The lesson there is that beautiful experiments are persuasive!) Mayer subsequently became mentally ill and attempted suicide. Happily, he eventually recovered both physically and mentally (though at one point he was, much to his chagrin, proclaimed in a local newspaper to have died). In 1871, Mayer was finally recognized by the Royal Society of London as having made important contributions to the study of physics. (Most of the facts in this history come from Great Physicists by William H. Cropper, The Ten Most Beautiful Experiments by George Johnson, and Genius of Britain by Robert Uhlig.) 1 Feynman’s Introduction The Pre-­‐Lab exercises here will consist of two very different halves: the first a highly abstract (but fun) introduction to the conservation of energy with the second half being a look at the very practical matters of energy storage. Richard Feynman, one of the great minds and personalities of the twentieth century, delivered an introductory physics course to students at the California Institute of Technology in the mid 1960’s. These lectures were later collected into a text aptly titled The Feynman Lectures on Physics. The way in which Feynman introduces energy is quite charming and is well worth reading, which you are now asked to do at http://www.feynmanlectures.caltech.edu/I_04.html. Do This: Read Feynman’s introduction to energy at the above website. You just need to read the first section with the heading What is Energy? PL1. What are the nine different forms of energy that Feynman lists in his introduction? (Note that this is not necessarily an exhaustive list of all forms of energy.) PL2. Briefly explain the importance of the window in Feynman’s story. How does the window relate to the notion of a system being isolated? Storing Wind and Solar Energy For reasons too many and controversial to explore in detail, electrical energy produced by renewable means has been given much attention in recent years. Two of the leading renewable sources of energy are wind and solar. These sources have many obvious advantages over nuclear plants and power plants that burn fossil fuels. However, there are clear drawbacks, as well. One of the most severe of these drawbacks is the intermittent and unpredictable nature of wind and sunlight. Oftentimes during the peak hours of energy consumption, the demand for energy far outweighs the supply. It’s pretty easy to see how this situation is undesirable. In addition, there are other times when the supply far outweighs the demand (called the off-­‐hours). This is also a problem! Due to the way the electrical grid is designed, energy must be consumed as it is produced. Almost unbelievably, the excess production can be such that power companies pay consumers to take the extra energy! Wouldn’t it be great if there were some way that the overflow of energy produced during off-­‐hours could be stored and later used to supplement the supply of power during peak hours? It turns out that there are a lot of ideas floating around that try to do just that. We’ll take a look at a few of them right now. (You can check out the article “Gather the Wind” in the March 2012 issue of Scientific American for even more information. There’s an optional link on the Pre-­‐Lab Links page.) Pumped Hydro Storage One such solution uses a technique known as pumped hydro, a beautifully simple technique that stores wind or solar energy in the gravitational potential energy of a reservoir of water. During off-­‐hours, 2 renewable energy is used to pump water up to a high elevation. Later, when energy demand is high, the water falls downward through turbines, much like in a hydroelectric dam. One pumped hydro development that is in the works in Southern California is called the Eagle Mountain Pumped Storage Project. They have a nice video on their website that demonstrates their technique. Do This: Go to http://www.eaglemountainenergy.net and click on the link for “ECE Virtual Tour.” Watch the 90-­‐second video attentively. There will be some follow-­‐up questions. It’s okay to mute the video; all of the information is written out on the screen. PL3. What percentage of solar power is produced on the weekend? Prove that this number is reasonable. PL4. According to the video, what percentage of wind energy is produced at night? PL5. What is the relationship between motors and generators? Supercapacitors Capacitors store energy in an electric field, something you’ll learn about next semester. Supercapacitors store a lot of energy in an electric field. Current applications of capacitors are not that different from the current applications of flywheels (which you will read about shortly). A capacitor powers the flash in your camera because the battery in your camera can’t provide energy quickly enough. The battery slowly charges the capacitor. Then the capacitor quickly discharges through the flashbulb. These devices are also used to store energy on small scales, like protecting your computer against a brief power outage. There is some hope that large banks of supercapacitors could eventually store energy on larger scales. The Flywheel A third, refreshingly simple method for saving renewable energy for later is the flywheel. A flywheel is just a spinning disk. Energy is stored as rotational kinetic energy of the flywheel. These devices are already used all over the place for mechanical reasons, like in your car’s clutch. Flywheels are also sometimes used when a huge amount of energy must be delivered very quickly (i.e. a lot of power is needed). A low-­‐power energy supply (like a wall outlet) gets the flywheel spinning over a relatively long time. Later the flywheel is decelerated over a relatively small time, delivering lots of power, maybe to gigantic lasers. (Really! This technique is used at Lawrence Livermore National Lab.) Researchers are trying to develop flywheels that are practical for domestic use. One company doing this is Velkess. Check out velkess.com and watch the two videos on the homepage. PL6. The second video says that an electric motor transforms electrical energy into mechanical energy. What energy transformation takes place in an electrical generator? 3 Part I: Feynman’s Marbles The Story In the Pre-­‐Lab, you read a story about a boy and his blocks as an introduction to energy. Here you will play a game with marbles that is inspired by that story. Equipment •
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Bowl of 12 marbles. The marbles have a diameter of 1 inch. Graduated cylinder with 166 mL of water Toy box Digital balance The Rules There are three places that marbles are allowed to go: the bowl, the box, and the graduated cylinder. If you want to know how many marbles are in the bowl, all you have to do is look and count. If you want to know how many marbles are in the box, you can’t open it; you have to weigh it like the mother from Feynman’s story. If you want to know how many marbles are in the graduated cylinder you have to look at the level of the water. 1. Time to Play 1.1. These marbles cannot be created, destroyed, or divided. However, they can be placed in the bowl, the box, and the graduated cylinder. Write a general algebraic expression of the law of conservation of marbles. (That is, use a variable for things like the mass of a marble or the total number of marbles.) Make sure to define your variables. If you are having trouble, the Feynman reading might be a good place to look for inspiration. 1.2. Now rewrite your conservation of marbles equation using values that are specific to your collection of marbles. (For example, write 12 in place of the total number of marbles.) The only variables left in your equation should be the number of marbles in the bowl, the total mass of the box, and the level of water in the cylinder. Read This: Now you’ll play the game to see if your equation works. The following instructions will refer to Partner 1 and Partner 2. If there are three of you, two of you should follow the instructions for Partner 1. Do This: First, make sure the toy box is not on the scale. Do This: Partner 1 should close their eyes or turn around so as not to see what Partner 2 is about to do. 4 Do This: While Partner 1 is not looking, Partner 2 should hide some number of marbles in the toy box and some number of marbles in the graduated cylinder. (You do not need to hide all the marbles.) Also, make sure the toy box is not on the digital balance. Do This: When Partner 2 is done hiding marbles, Partner 1 can turn around or open their eyes. Read This: Only Partner 1 will record responses to the next two steps. Partner 2 will get a turn later. 1.3. Partner 1 should use the equation from Step 1.2 to predict how much the toy box will weigh. (Don’t weigh it yet!) Show your work. Do This: Weigh the box. 1.4. Record the weight of the toy box. Were you right, Partner 1? If not figure out what went wrong and try again. Do This: Play the marble hiding game again with your roles reversed. In other words, Partner 1 should distribute the marbles and Partner 2 record responses to Steps 1.3 and 1.4. Switching roles will most likely require a trip to the sink. Dump the marbles into the strainer, emptying your graduated cylinder completely. Then pour the marbles from the strainer into your bowl. Finally, fill the 166-­‐mL bottle all the way to the top and pour it into the graduated cylinder. Done! Read This: This activity is very similar to common problems that are solved using conservation of energy. Here are two problems side-­‐by-­‐side for comparison. For simplicity, none of the marbles will be in the bowl.
Marbles Energy There are 12 marbles. Some of them are in a box, and some of them are in a graduated cylinder filled with water. If you measure the level of the water to be 𝑥 milliliters, what is the weight of the box? A projectile has 12 joules of mechanical energy. Some of that energy is kinetic energy, and some of that energy is gravitational potential energy. If you see that the projectile is 𝑥 meters above the ground, what is the speed of the projectile? 1.5. Play the game one more time as a group. Have a nearby group distribute the marbles for you. (So your lab group is like Partner 1 and the nearby group is like Partner 2.) This time, though, you will use the weight of the box to predict the level of the water. Record the experience and show your work as your response to this step. 1.6. What factors may have caused your predictions to be slightly off? Was it a failure of the law of conservation of marbles, or was it something else? Explain. 5 Part II: Energy Conversion Kits The Story As mentioned in the Bit of History, the first steps toward understanding the law of conservation of energy involved scientists looking at the way certain kinds of “effects” could produce other kinds of “effects.” For example, the chemical effect of a battery could produce an electrical effect. We would now say that energy can be transformed from one form to another. In this second part of today’s lab, you will do several mini-­‐experiments that are very much in line with those earliest energy experiments. In the end, however, you will be able to connect these same experiments with some of the latest technology in energy storage. 2. Conversion After Conversion After Conversion Do This: Connect the battery to the fan using the red and black leads. This needs to be done carefully. Connect the red terminal of the battery to the red terminal of the fan. Then connect the black terminal of the battery to the black terminal of the fan. (For the rest of the manual such a connection will be described in shorthand as “R-­‐R, B-­‐B”.) 2.1. What happens? (This is just recording an observation. No explanations necessary.) Read This: In addition to those observations, hopefully you were thinking about the different ways that energy is being transformed when you connect the battery to the fan. Several of the following steps will ask you to describe energy transformations in a given system. You will be expected to state two things. First, you should state the form of energy input and the form of energy output. Then you should detail the ways in which energy is transformed in between. Here’s an example of what will be expected. Example Step: Detail the energy transformations taking place when the battery is connected to the fan. Example Response: Input/Output: We start with chemical energy in the battery and end with kinetic energy in the air. (This should be a complete sentence.) Transformations: The format is: Mechanism {input form of energy → output form of energy} •
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Battery {chemical → electrical} Motor {electrical → mechanical} or {electrical → rotational kinetic} Fan blades {mechanical → mechanical} or {rotational kinetic → kinetic} Read This: During all of these steps, things heat up due to friction and electrical effects. That means various forms of energy are being transformed into thermal energy. As long as we 6 understand that this is happening all the time, we don’t have to worry about writing that down at each step. You may also have heard the motor spinning around. Sound carries energy, so a more complete answer would include something about that. The example response only mentions the desired energy transformations at each stage. The fan wasn’t built to heat up or make noise. It was built to spin and push air. Let’s just worry about the desired energy transformations in these descriptions. Do This: Disconnect the battery from the fan. Now connect the fan to the red LED. This requires some odd wiring: R-­‐B, B-­‐R. Do This: Create a ferocious breeze that spins the fan blade. A really big gust should get the red LED to light up. 2.2. Detail the energy transformations taking place when you create a ferocious breeze that lights the red LED. (Remember to format your response according to the example given above.) 2.3. These two experiments with the fan should remind you about something from the Pre-­‐Lab. Relate your recent activities with the fan to something from the Pre-­‐Lab. Do This: Connect the solar cell to the red LED (R-­‐R, B-­‐B). Don’t light anything up yet! 2.4. You are about to shine the flashlight on the solar cell. Hopefully, this will light up the red LED. But first, predict how the brightness of the red LED will compare to the brightness of the flashlight. Explain. Do This: Shine the flashlight onto the solar cell. 2.5. Was your prediction correct? If not, explain why your observation differed from your prediction. 2.6. Detail the energy transformations taking place when you light the red LED with the flashlight. Do This: Get the hand crank generator and connect it to the incandescent bulb (R-­‐R, B-­‐B). Turn the hand crank at a constant rate to light the bulb. 2.7. Detail the energy transformations that are taking place as you light the bulb with the hand crank generator. Do This: Turn the hand crank at a constant rate. While you are still turning the crank, have your partner disconnect one of the wires. Repeat this a few times. Both partners should try this. 2.8. By considering energy conservation, explain why the hand crank is harder to turn when it’s connected to the light bulb. 2.9. Predict what will happen if you connect the battery to the hand crank generator. Explain your reasoning. 7 Do This: Test the prediction you made in Step 2.9. 2.10. Was your prediction correct? If not, explain why your observation differed from your prediction. 3. Small-­‐Scale Energy Storage Chemical energy was the only form in which energy was being stored in the experiments that you just completed. (Chemical energy was stored in the batteries and in your body.) In the following set of experiments, you will see how energy can be stored as kinetic energy, gravitational potential energy, and electrical energy. Do This: Connect the flywheel to the hand crank generator (R-­‐R, B-­‐B). Turn the crank to start the flywheel spinning. 3.1. Detail the energy transformations taking place. Do This: Get the flywheel spinning at a good clip. Then disconnect the flywheel from the generator and instead connect it to the fan. 3.2. What happens? Relate this to the Pre-­‐Lab. Do This: Go to the In-­‐Lab Links page on the lab website and watch the video about the mass STOP lifter. This will prepare you to use the mass lifter in the upcoming steps. Don’t operate the mass lifter yet. Do This: Connect the mass lifter to the red LED. Make sure to do this R-­‐B, B-­‐R. Then remove the nail from the top of the mass lifter. 3.3. What happens? (Just relay your observations.) 3.4. Detail all of the energy transformations taking place. Do This: Connect the battery to the mass lifter (R-­‐R, B-­‐B) to raise the mass back up. Be ready to disconnect the battery when the mass reaches the top. Gently pushing down on the axle of the motor with your index finger should keep the mass in place after you have disconnected the battery. Finally, insert the nail into one of the holes in the top of the motor. If the nail will not go in, turn the axle slightly and try again. 3.5. Relate this device to one of the energy storage techniques that you learned about in the Pre-­‐Lab. 3.6. We might want to store energy for a long time. With that in mind, can you see any advantages the mass lifter has over the flywheel? Explain. 8 Do This: Connect the battery to the capacitor (R-­‐R, B-­‐B) for about ten seconds. Then disconnect the battery from the capacitor. Try to power something with the capacitor. If it doesn’t work, try again with something else or try switching the wiring. 3.7. Describe your experiences with the capacitor. Mention something about energy. Read This: Have you ever noticed that when you unplug some electronics, they don’t turn off immediately? That’s because of capacitors! You’ll learn more about capacitors next semester. Head-­‐Scratchers Don’t forget to complete the following problems. They should be at the end of your lab report. If you want to work on them during lab, start a new page in your lab notebook. •
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1.6 2.8 3.6 9