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Intro Physics: Essential Understandings for Energy and Rough Outline

June 20, 2013

I’ve been doing a bit more work with the Intro Physics course, and I think I’ve come away with two big ideas I want students to understand after our first unit.

  1. We ean model the flow of energy as flows between various forms within a system, and transfers via work, heat and radiation between the system and surroundings.
  2. We can build a general intuition about energy to gain intuition into the likelihood of various processes.

Let me illustrate these ideas a bit with some of the experimental work I’ve been doing. After 3 iterations, I’ve finally designed a halfway decent experiment to measure the energy in a battery. In a previous post, I proposed a prompt for starting this unit of having students compare the energy content in a peanut to the energy content in a battery.

Here’s a photo of my setup:

Screen Shot 2013 06 19 at 11 50 46 PM

Yep—I’m using a graduated cylinder as a calorimeter with about 10ml of water, and I’ve simply shorted the battery with a thin piece of foil and a rubber band to maintain electrical contact. I know this isn’t the best experimental design, and I’m not really looking for an experiment to give very precise results—I simply want an easy experiment that will help us make a rough comparison between the energy of the battery and the energy of the peanut.

My experiment produced this very nice graph.

Screen Shot 2013 06 20 at 12 11 56 AM

As you can see, the battery was able to quickly increase the temperature of 10mL the water by about 30°C in a little over 5 minutes. I haven’t done the peanut experiment in a while, but my recollection is that within this same amount of time, the peanut would have come close to boiling the water.

Hopefully, this would lead to all sorts of interesting discussions about why the energy content of the peanut is so much greater than that of the battery. Also, why is the battery so much more expensive if it contains such a tiny amount of energy?

I think we could also extend this further by devising other experiments, like connecting a genecon to a resistor immersed in the water, and seeing how much effort is required to raise the temperature of the water. Maybe we could also see how much the temperature of the water can be increased simply by holding the test tube in one’s fist.

When all of these experiments are done, I think we could could create a generalized energy scale on the board comparing the energy flows of various systems: a peanut, a battery discharging in water, 5 minutes of turning the genecon, 5 minutes of all out exertion on an exercise bike, 5 minutes of of a pot of water sitting on a stove on its maximum setting. At this point, we should be able to develop pie charts and LOL diagrams to talk specifically about the various types of energy (kinetic, thermal, gravitational interaction, and chemical interaction) in each system at various instants, and the flows of energy between the system and its surroundings between those instants.

Once we have this intuition, it might lead to a need for a more precise experiment—so we could bust out mechanical equivalent of heat experiment, or perhaps more simply by dropping shot inside a pac pipe and measuring the temperature increase. We could go back and precisely measure the energy in a battery with an ammeter and voltmeter. At this point we would probably introduce the Joule, and some specific numbers for various energy transformations e.g. it takes 4.18 J to increase the temperature of 1 g of water by 1°C. We won’t be introducing formulas for various energies at this time—we’ll save that for a later pass.

Hopefully we’ll also explore more questions along the way. Why does the temperature graph for the battery look the way it does? Why does is the rate of temperature change greatest when the temperature is greatest, and then as the water cools, the rate of temperature change is smaller? Can we develop a model of thermal energy to explain this?

By the end, I’d like students to be able to create LOL diagrams and pie charts to carefully explain the energy flows in any of the experiments we performed in the unit. I’d like them also to be able to easily debunk videos like this homemade coffee warmer I discovered previously, and make rough comparisons of the amount of energy that is wasted when you leave a window open on a cold 20° day compared to the energy wasted when leaving your lights on for the day, or calculate the mass of batteries you would need to deliver the same energy content as the gasoline in your gas tank.

So here are some questions I have:

  • Does it make sense to jump right into full blown 1st law of thermodynamics stuff in this course, rather than starting with closed, isolated systems where the total energy is always constant? Is this too much?
  • How much can we do with the matter model and other simulations to build a real understanding of thermal energy?
  • Is it a good approach to go with simple, rough mythubster type experiments at this level?
  • And maybe most importantly, are students going to be able to tease out and understand a law of conservation of energy from such a variety of experiments and demos?
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9 Comments leave one →
  1. June 20, 2013 5:17 am

    One of the things that excited me about summer is having the time to do these awesome projects that don’t fit into the regular year. This is one such example.

    Nice work!

  2. npisenti permalink
    June 20, 2013 8:12 am

    Another idea related to energy scales and the energy density of gasoline… Try converting “miles per gallon” into units of area, the interpretation being, What if instead of having a tank of gas, my car slurped up the gas it needed as it drove along? How much gas would we need to drizzle along the road? WolframAlpha even provides some nice size comparisons… Shockingly, for a 40mpg car, the cross sectional area of that line of gasoline is on the order of a single pixel, or about 7 times the cross sectional area of a human hair.

    I vaguely recall hearing this first as an example Feynman gave in some interview/lecture…

    • June 20, 2013 8:36 am

      Neal,
      This is a great point. I think we could even come up with a way to safely design an experiment to show that the energy density of gasoline is much greater than the peanut or battery. And I love the idea of thinking about the rate at which the car is transforming energy from chemical to thermal and kinetic as it moves along the road. In fact, you’ve just inspired me to transform a problem and write another blog post…

  3. Luke Baumann permalink
    June 20, 2013 10:25 am

    Why didn’t we do this last year? I want to boil water with a peanut!

    • June 20, 2013 10:51 am

      Well, I just thought of it recently and how to connect it to the physics curriculum. But I think the peanut lab is a part of honors chem, so you’ll get to do it next year. :)

  4. June 20, 2013 12:35 pm

    On the first question, I always love having students have to make a model choice as soon as possible, so not being able to auto-pilot the open/closed question is good, even if only conceptually.

  5. jsb16 permalink
    June 22, 2013 3:53 pm

    Just a minor comment: Before you do any sort of peanut-burning, be sure there are no peanut-allergic folks in the building. The smell is pervasive…

    • June 23, 2013 7:44 pm

      Good point. This has been a standard chem lab, and I’ve never thought of implications for the nut-allergic.

      • July 1, 2013 12:40 am

        I have done it with Cheezels before to avoid nut allergy problems. Google tells me that this roughly translates to Cheetos in your language. :)

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