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Intro Physics Redesign: Peanut vs Battery

June 12, 2013

Next year, I’m going to be returning to full time physics teaching. I’m sad that I won’t be able to teach math, but I’m also very excited by the getting back into teaching physics more, and most importantly, getting to collaborate with Mark Hammond on rethinking what our introductory physics course should be.

A bit of  background—our intro physics course is taken mostly by juniors and seniors coming from an introductory chemistry course. Their math preparation spans the gamut from students who are are concurrently enrolled in Calculus AB and quite successful to students who are struggling in an introductory precalc course and find math a real challenge. In the past, we’ve used the same modeling approach and most of the materials we used with honors physics, but the pace of the course has been much slower.

While I really like modeling and its emphasis on helping students to tease out relationships from experiments and build up problem solving skills, I am also  interested in helping all of my students to move beyond just solving problems on paper, and see that they can use physics as a tool to interrogate the world around them. One of my other big goals is to have my students leave the classroom with more than an sense of accomplishment at having learned a bunch of stuff; I’d like them to create something of lasting significance that clearly demonstrates their physics understanding and is seen by an audience larger than our class.

I am also really intrigued by the idea of starting with energy, but I don’t want to limit our focus so simple closed systems that seem to predominate much of the problems they see in modeling packet. Instead,I’d like my students to work toward being able to solve a problem like making the one of the 50+ year old faculty houses around campus more energy efficient. Could we do an energy audit of the house to characterize the efficiency of the heating and cooling system, locate air leaks in the house and ductwork, and then propose specific improvements that would lead to a measurable reduction in energy costs for the house?

I’ve also realized that my students have very little intuition for energy, particularly when it comes to quantity. When they finish a calculation, they see very little difference a final answer of a hundred Joules, ten thousand Joules, or ten million Joules, and this is my fault for not giving them more experiences that help them to develop their understanding of energy units and put it to some use.

All this has me looking for a questions that would serve as a launching point for an investigation of energy. I remembered a great question that Richard Muller uses in his Physics for Future Presidents Course:

Which has more energy, a chocolate chip cookie, or a stick of TNT?

I still find that the answer (the cookie) confronts my own intuitions about energy and brings up so many more questions. Mark and I were thinking about how one might experimentally investigate a question like this, and we quickly sized upon a slight variation:

Which has more energy, a peanut or an AA battery?

Unlike the TNT question, I think students could design a simple calorimetry experiment to test this out.

I also stumbled upon this very interesting YouTube video purporting to demonstrate how to create a coffee warmer using a single AA battery, a corn holder and wires from your headphones.

The results in the video were astonishing to me—I’m very skeptical that a single battery can heat a full cup of coffee by more almost 20 degrees Fahrenheit. I tried it out myself, with a smaller beaker of water and an AA battery wrapped in aluminum foil. I measured almost no change in temperature.

After some searching, I realized Rhett had already measured the energy content of a AA battery, and his results are in reasonable agreement with my estimate ( 1.5\; \textrm{Amp}\cdot \textrm{hours} \cdot 1.5\;\textrm{Volts}=2.25\;\textrm{Watt}\cdot\textrm{hours}=8100\;\textrm{J}).

8100 Joules should be enough energy to raise the temperature of 100 g of water by

8100\;\textrm{J}\cdot\frac{\textrm{g}\cdot^\circ\textrm{C}}{4.184 \;\textrm{J}}\cdot\frac{1}{100\textrm{g}}=19^\circ \textrm{C}

My experiment didn’t produce anything like this change—I think it was because I was using more than 100g of water, which would have greatly reduced the temperature increase, and I was doing this in a simple beaker, not an insulated calorimeter, which might have allowed the thermal energy from added by the battery to the water to flow out and prevented any significant rise.

I think this might be the start of a great investigation, assuming I can get the battery to make some measurable change in the temperature of the water. We’ve also got a great video that we critique as part of our work—this might be the first in a series of Mythbusters experiments.

If we can nail down this question I think it will lead to all sorts of interesting conversations, like why are batteries so much more expensive than peanuts, why we can’t run iPods and cameras off of peanuts, and lots of experimentation about how we might be able to refine our experiment to make a better measurement of the energy content of a battery or peanut.

That’s were I’m at for now. I’d love your thoughts on this idea. Do you think this is an engaging question? Do you have suggestions on how to design the experiment to come up for a measurable temperature rise from heating by shorting the battery?

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3 Comments leave one →
  1. June 12, 2013 1:12 am

    Short-circuiting a battery will release the heat mainly in the battery, so you would have to immerse the battery. If you want to maximize the heat released in the load, you should make it be a resistance equal to the internal resistance of the battery. Of course, that maximizes the power, but what you want to maximize is the energy, which would happen with a slower release into a larger resistance. To get maximum temperature rise, you have to trade off the energy efficiency of the heater with the thermal resistance of your container.

  2. June 16, 2013 12:27 am

    I’m awfully interested by the temperature-raise claim. I admit my first reaction was to think that maybe someone had an heat-calorie versus food-Calorie conversion glitch (this is the thing that spoils that “ice cube diet” where you might lose weight by the energy it takes to melt ice cubes and warm the water to body temperature), but obviously, if you start out in Joules and stay there there’s no room for that problem to arise.

  3. June 16, 2013 1:13 pm

    You might want to look at http://www.batteryshowdown.com/results-hi.html for energy output results for different batteries at high discharge currents (1Amp). The batteries varied enormously from 3.8Wh down to 11mWh. Note that at 1 Amp, the good batteries took 3 hours to discharge, which would require a very well insulated calorimeter to measure. At low discharge rates (200mA) the variation was less (4.6Wh down to 240mWh).

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