Most physicists know that if you want to demonstrate conservation of energy, you choose an experiment that doesn’t waste time. Each iota of time that you spend during the experiment allows energy to escape or invade your (necessarily) imperfectly isolated system. So, if you’re showing potential energy converting to kinetic energy, you choose a short and steep hill and you measure the kinetic energy as close to the base of the hill as possible. Similarly, a projectile must not travel too far through the friction-fraught air, and thermal conduction should happen fast. Aluminum is a fast conductor.
This week’s lab tested conservation of thermal energy in a closed system. Two aluminum blocks were presented at two different temperatures (273 K and 373 K). The thermal energy of each block was calculated and added to result in the total thermal energy of the system of two blocks. The blocks were then brought into thermal contact with each other and we watched as the temperatures equalized in less than 10 seconds (very fast indeed). We used the equalization temperature to again calculate the thermal energy of the system and compared it to the previous value.
When there is limited class time, it is often important to come to a class prepared to discuss why an experiment did not work. And so I had; but this experiment occurred so quickly that the largest disparity in five trials between the two calculated values was less than 1.5%.
We should all be so lucky in our own research.
Ideally, a gas will expand when it is heated. That’s the law… at least under constant pressure. Such was the investigation of our lab today. A small, inflated balloon held a constant quantity of gas. Since the balloon was barely inflated, we approximated it as being under constant pressure. All we needed to do then was to measure the volume of the balloon at different temperatures. I asked my students how we should do that; aided by some lab materials on the bench, eventually they gave me the right answer. We submerged the balloon in water of different temperatures and measured the displacement. We graphed the result and checked its linearity. It worked well.
But with this lab, we are finally working toward topics that are relevant to the research that I presented to them at the beginning of the year. I charged my self, then, to find a way to show them how thermally expanding gas can be used to push a cantilever. This I accomplished by taking a full balloon and bathing it in liquid nitrogen. The balloon shrunk to a fraction of its original size. Then I laid a large cantilever on top. The warming balloon expanded and pushed the cantilever up. A fruitful discussion on the effect followed, but not until after I was compelled to show the effect again.
Liquid nitrogen will get anyone’s attention.
Calorimeters are devices for measuring how much heat energy is added to water and comparing that quantity to how much electrical or chemical energy is used to cause the heating. Small calorimeters are notoriously lossy. Because of their high surface to volume ratio, heat loss is inevitable. Light loss (in the electric calorimeter case) is even more of an issue. So in general, you cannot make a direct comparison of energy input to energy of heat. This means they are no good for measuring the heat capacity of a material. On the other hand, as a learning tool, they make great anecdotal conservation of energy tools. After carefully calculating the energy of heat and the input of electrical energy, my classes had fruitful discussion about where the rest of the energy went.
Today I gave a presentation to the kids about research in general: what it is, who does it, what they study, how they become researchers; the whole gamut. I prefaced the presentation and discussion with some impromptu research I had undertaken over the previous week about how much money people make in this country with various careers and how those careers and incomes relate to the degree of education those people achieved. A more captivated and enthralled audience I have not yet had. For this presentation, I encouraged questions of any and all kinds, and those are the kinds of questions I got. I spent a lot of time over the previous week collecting information beyond what I expected to present just so I could answer their possible questions. The questions ranged from ‘how much money can you make as a researcher’ to ‘what is it like two hold down three jobs’ to ‘can you survive on the income you can earn without going to college’. Even personal questions like ‘was high school hard for you’, and ‘how about college’ held tremendous weight for these kids. We did move on to talk about research in general, and my agenda for the day was fulfilled, but I think the kids may have learned quite as much from the opportunity to ask questions as from any structured part of my presentation.
Force equals mass times acceleration. This is taught in a number if different ways, but the most straightforward way may be the fan cart. Here a friction-less cart is placed on a flat track, a battery operated fan is attached to the cart. The fan provides a constant force and when it is turned on the cart undergoes constant acceleration. Best of all, fascinated by the gadgetry, the kids pay close attention. I wasn’t able to make a bunch of fan-powered carts, so the lab was a demonstration. These are sometimes difficult to manage, but a graded worksheet for everyone to fill out, and student-helpers to catch the cart, record measurements and perform calculations go a long way to keeping people involved. If handled with proper respect, sometimes you can even get some of the most disruptive kids to help out. That is often a very effective way of keeping everyone involved.
This past week the physics students at Lowell Freshmen Academy learned, among other things, how to make various types of graphs. They learned that graphs are devices for storing or modeling data. If an experiment is done, the data gleaned from that experiment can be arranged and stored in a graphical representation. Similarly, if a physical relationship between two variables is known, that relationship can be modeled by graphing the known relationship. These are important skills to know, but just as important is the ability to interpret data that is presented as a graph.
Interpreting graphed relationships was the focus of this week’s labs. The students, in groups of four, were given distance vs. time graphs of various different types and were asked to interpret those graphs by recreating the data shown there. They were required to build a metric along which to walk or run, and then practice recreating the data shown on the graph. They were given about 10 minutes to ask questions and practice the walking before being required to present for the rest of the class their work.
What was surprising and uplifting was not the skill or accuracy shown in recreating the graphs, but instead it was how involved almost everyone became. Some groups required a little bit of prodding, it is true; but what I found was that most of the slow starters simply didn’t know how to start. They were looking for specific instructions about what to do now. After answering a question or two for a few groups, they were up and running. Although they all had sufficient opportunity to become disconnected from the lab, very few people did. Everyone got involved, and as a consequence, experienced a small example of how to interpret graphically represented data.
It was a good lab.
The Scientific Method: a conceptual cornerstone of the way we, as scientists, view and study the world.
It is dreadfully important that students lean about the Scientific Method as a cornerstone to science in general. They will use this method explicitly and deliberately for years to come; checking off each step as they go. Following these steps one by one and checking off progress from a list as they go is tedious and time-consuming. The idea, of course, behind learning the Scientific Method in this way is to ingrain it into the mind as the way that science is practiced and to turn these steps into habits that are practiced automatically.
It is a lesson that sometimes we would do well to relearn. How many hours have I seen wasted, or wasted myself simply by skipping a step? Usually the step that is skipped is one that is implicit to the Scientific Method, a cornerstone of the cornerstone, if you will: isolation of variables. It often occurs during the course of intense experimental scientific research, that the next few steps of a project seem perfectly simple and obvious. It is a strong temptation at those times to take more than one step at a time. To save time, more than one variable is changed; a new device is created, or a procedure or method is changed in multiple ways. This saves a lot of time, and often it goes unnoticed even by the researchers themselves. From these changes, positive or negative results are realized.
In the best-case scenario, the mistake is noticed immediately. In that case, often the researcher must stop, go back, and rework the experiments that have just been done. In the worst-case scenario, the experiment works or doesn’t (the hypothesis is right or wrong) and the outcome is labeled as results. From those results, papers are published or future work is planned so that months or years can be wasted on such an oversight. There are perhaps many scientists, including this one, who could benefit from 40 minutes spent near the beginning of the year in a 9th grade science classroom.
Uncategorized | joshuamason | 
February 10, 2011 3:01 pm | 
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