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18 May 2016

Eight different approaches to laboratory work

In preparation for my AP Summer Institutes for 2016, I've redesigned how I present experimental work.

When I began teaching AP Physics B, I did lecture and problem solving four days a week, with lab work on the fifth day.  I integrated more and more experimental work into my daily classes, especially as I amassed equipment for each topic area.

Nowadays, experimental work is part of virtually every class all year.  My minimum goal is to get student hands on equipment three of five days each week; my students will tell you that the reality is more like four or five out of five.

Okay, many of you share this lofty goal.  But how to accomplish it?  Upperclassmen don't like routine; they will not be comfortable doing the same styles of activities every day all year, even if the topics change.

So, for Summer Institute and for reference purposes, I've categorized eight styles of lab work that I've been using for my classes.  All are applicable to teaching physics at any level, but are optimized for AP Physics 1 students.

The styles are listed roughly in the order I introduce them to my classes.  Remember, at the beginning of the year, students certainly aren't ready to do AP Physics 1 test problems in the lab with little guidance!  We can talk 'til we're blue about "open inquiry" and ideals of "student-centered learning", but it is incumbent upon us to teach fundamental skills before opening up the lab for the students to play.  That doesn't mean we TALK at students about lab skills; that means that each style of experiment builds skills in context that are taken for granted at the next style.

Read on... at my AP Summer Institutes (I'm doing four in 2016, listed in the sidebar -- please sign up!) I'll be doing experiments in each of these styles with all the participants.  And please bring your own ideas to share with us.


1. The Quantitative Demonstration

Instead of showing how to solve textbook problems in the abstract, try setting up the actual physical situation presented in the problem – do the problem, treat the answer as a prediction, and then verify the prediction experimentally.  Take a look at this post about my first day of AP Physics, or just search "quantitative demonstration" on this blog for more ideas.  


2. The whole class as a lab group for live data collection

For example… to get data for voltage vs. current to show the ohm’s law relationship:

·             *  I put a blank set of axes on the screen; I give everyone a hard copy of blank axes.
·             * I bring the class to the front of the room to see the setup – they see the voltmeter, ammeter, and how I vary the voltage by turning the dial on the power supply.
·             *  We discuss how to scale the axes such that the data will fill the page.
·             * Each student in turn is called to the front of the room to adjust the voltage, and to read and record current and voltage data.
·             * Before going back to his seat, the student writes his data in a chart on the board; and he graphs his data point on the screen.
·             * Meanwhile, each student is responsible for making his own personal graph.
·             * I move quickly – the next student is ready to go while the first student is still writing and graphing his data.
·             *  As the experiment goes on, students begin to suggest how to fill in such that the entire parameter space is explored.
·             * When we have plenty of data (usually meaning everyone in the class has had a turn), everyone draws a best-fit and calculates the slope.

·              * We estimate an average resistance with uncertainty from the class’s slopes – this always matches the resistance of the resistor nicely.

This is an excellent technique early in the year, when you’re introducing and modeling lab skills; whenever you need quick data – this takes maybe 1/4 of the time it would take for the students to do it independently;  and anytime you have only one set of equipment.

Here is a description of how I use this same technique on the first day of my conceptual physics class.

3. Quick data collection to verify prediction of a qualitative trend, or to determine the trend


Students must be able to describe the shape of a graph given the relevant equation; and students must be able to suggest the form of an equation given a graph of experimental data.

By scaling the axes ahead of time for the students (and being sure that the scale represents an appropriate range of values), you can save time in lab; more importantly, you focus the students on just this particular skill of translating equations to graphs and vice-versa.



4. Create a linear graph, use the slope to determine a physical quantity

I believe in putting data directly on a graph; I believe in hand graphing; I believe in taking slopes by hand.

If your students graph asthey go they understand intuitively what it means to “explore a parameter space.”  (And it’s easier to convince them to take more data if they haven’t put their stuff away and expected to be all finished.)

Your students are not skilled at graphing by hand; yet they are likely to have to graph data as part of an AP question.  You can teach them how to use excel to make a graph at year’s end.  And they’ll actually understand what excel is doing if they’ve been graphing by hand all year.

Similarly with taking slopes.  Make them write out (y2y1) / (x2x1).  Make them circle the points on the best-fit line (not data points) used to calculate slope.  Make them write the units of the slope.

Then make the students explain how to determine the physical meaning of a slope using equations, not just guessing based on the units of the vertical and horizontal axes.


5. Linearize a graph, use slope to determine a physical quantity

The AP physics exams expect students to be fluent in linearizing graphs.  See the 2009 Physics B problem 1 for the canonical example of an experiment requiring graph linearization.

This is one of the first linearization lab exercises I do.  We hold a cart on an inclined track with a string attached to a spring scale, varying the angle of the track.  Initially, we graph tension vs. angle – this graph is curved.  By writing out the relevant equation T = mgsinq, we recognize that a graph of tension vs. sin q will be linear with slope mg.


6. Open-ended determinations – are you hired?


Students aren’t usually aware of the intended audience for lab write-ups.  “Mr. Jacobs has done this experiment a million times, he knows how it works, and he saw us do it.  So answering these questions is just a formality.  He knows what I know and what I mean.”


So I make the audience someone OTHER than me, and put the writeup in a context they understand:

Imagine that you and your partner have been asked to make this determination for a Fortune 500 company as part of the competitive bidding process for an engineering contract. 

You will submit your marked pipes and an explanation of your methodology to the company.  From that writeup alone, they will decide which partnership to hire.

Therefore, I will have someone – not me – rank the submissions from strongest to weakest.  They’ll be placed in piles:

·        Hired (1 submission)
·        Not hired, but recommended to other companies
·        No action
·        Blacklist

7. Independent prediction exercises 


These are like quantitative demonstrations, but with the students doing all the work.  Other teachers do similar activities, calling them "stations".


I have students work independently, at their own pace.  They are welcome to collaborate; since everyone has something slightly different on their sheet, their collaboration is authentic.

I've posted about two of these:  One with energy, and one with the direction of force and motion.



8. Experiments taken (nearly) straight from the AP Physics exam


Virtually every AP Physics 1 problem can be set up in the laboratory.  I modify the problem so that it scales to the equipment in my lab; for example, using 500 g carts rather than 500 kg cars.  I often try to set up the experiments such that we can produce a graph, perhaps even a linear graph with a meaningful slope, even if that graph wasn’t part of the original AP problem.  I can't post these online, because they are based on College Board questions.  However, come to my AP Summer Institutes, and these exercises will be on the CD that you get.


2 comments:

  1. Great info and ideas. Thanks! I use one other approach. I mix simulation lab with hands on. For example construct DC circuits on the PHET simulator and build similar circuits and take measurements with a multimeter. I might have 6 students building real circuits while another 6 do it virtually.

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  2. I wonder how difficult THIS {my blog, below, describes} would be for as a lab project/experiment? The graph formed is simple enough. However, there are a couple of different principles to understand, and that may make it unacceptable as Hooke's Law and hydrostatic pressure are topics that are usually separated by several months at least.
    https://jimkadel.blogspot.com/

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