Electric fields and potentials demo in corn oil... and why the voltmeter didn't work.

Several years ago I shared Wayne Mullins' demonstration of electric fields and potentials.  He used two metal PASCO masses placed parallel to one another in water to produce a uniform electric field in the water.  The electrodes were connected to ~25 VAC.  The linear variation of potential with position between the plates can be demonstrated with a voltmeter; a couple of fingers spread in the water (done carefully -- read the post!) can show viscerally what a potential difference really means.

Today in my visit to TASIS American School in London, blog reader Scott Dudley showed me and his classes a similar demonstration.  He connected 2000 VDC to two small wires placed in a pool of corn oil.  A sprinkling of some grass seed between the wires showed these long particles lining up with the electric field lines, as you can see in the picture.  This demonstration provoked three thoughts from me.

(1) Why would the particles align with the electric field rather than along the equipotential lines?  Teacher Dallas Turner once suggested using goldfish in water between the electrodes to show the equipotentials.  The goldfish will align perpendicular to the electric field so that no current runs through their bodies due to a potential difference.  So what makes grass seeds different?  I expect that the seeds are slightly polarized... then they experience a torque because they're dipoles in a uniform electric field.  That torque aligns them with the field: the positive end is forces as close as possible to the negative plate, and vice-versa.  (Right?)

(2) I suggested that Scott use a voltmeter to map the equipotential lines, as I do in Wayne's demo.  So Scott gamely stuck the probe in the oil... and nothing.  No reading.  Why not?  Because, as Scott immediately pointed out to me, the meter produces a small (few milliamp) test current in order to measure a voltage.  The oil is a strong insulator, thus not allowing the meter to make the measurement.  The demonstration works fine when I do it in tap water, because tap water is quite conductive.  Of course, Greg... that's why I need water in the first place rather than just the air in between the two electrodes.  And that's why the "field mapping" lab exercise is generally done with conducting paper.

(3) The AP Physics 2 exam does not deal with traditional field lines.  Instead, field mapping is done using "vector fields" in which a multitude of arrows indicate the magnitude and direction of the electric (or magnetic or gravitational) field at various positions.  The grass seed can help develop an understanding of the vector field representation.  Each individual grass seed is pointing in the correct direction; now, draw each seed, but draw it bigger or smaller depending on the strength of the field at that position.  Nice.

Thank you to Scott for hosting me at his school.  I met a number of clearly excellent teachers; I wish I could have spent more time with everyone there.  Perhaps I can convince my school to send me to London a second time... :-)

GCJ

What the science teaching community can learn from NBC's soccer coverage

The best sporting events need no over-the-top, carnival barker-style salesmanship in order to draw a large audience; physics, or science in general, similarly needs no hype to make it interesting.  Bear with me as I give a brief tutorial of American sports coverage.  I'll get to the physics teaching connection at the end.

For decades, baseball was the only American sport that mattered.  Coverage included the dulcet voices of Vin Scully and Al Michaels, who took the game seriously, even though they didn't take themselves too seriously.  They knew that baseball, interwoven with a century of history, would sell itself -- their job was to tell the story of that days' game.

Baseball lost its title of "America's Pastime" to football not because of underpromotion, but because football is by far more suited to television and 21st century lifestyles.  When FOX took over national telecasts in the late 1990s, they tried to change baseball's downward trend in popularity with wrestling-style promotion: "NOW!!!  PUJOLS VS LESTER!!!!  LIVE!!!"  If anything, FOX has turned people off by misrepresenting their product.  Baseball is not suited to such treatment.

On the other hand, the championships at Wimbledon and the Masters golf tournament explicitly reject the typical "loud men screaming and laughing at each other" coverage that is typical for an American sporting event.  The tournament hosts insist upon a serious, nay reverent broadcast; yet they draw extraordinary television ratings, and tickets are next to impossible to come by.  Funny, that.

Then there's soccer.  For most of my life, what little soccer coverage I could see tried too hard to sell sizzle.  "Americans don't know about this game, and it's a boring game, to boot," said the producers (who also knew nothing about soccer).  So the announcers talked down to us: "Now, when I was little, my coach called this big box here the 'mixer.'  You're supposed to put the ball in the mixer to score goals."*  The pregame shows tried to explain the rules of the game again and again in excited voices, rather than to tell the story of the game's history.  The broadcast ignored everything but items deemed of direct relevance to Americans, who had no soccer history anyway.  It was all so, so condescending to even the mildly knowledgeable fan.  No wonder no one watched: those who were serious soccer fans felt talked down to, and those who weren't certainly didn't fall for the artificial sales job.

* Not kidding -- approximate quote from 1994 World Cup coverage.

Let's examine that paragraph in a science teaching context.  Rewrite, substituting science for sport.

Then there's science.  Too many science education programs try too hard to sell sizzle.  "Kids don't know about science, and science is boring, to boot" say the people providing education grants, who too often know little about science or science teaching.  So the teachers, program directors, and presenters talk down to students.  "And without science, we couldn't have iphones, and you couldn't twitter to your friends!  Isn't science great?"  Classes are taught facts and equations, without connecting those facts and equations to experiments that students can themselves perform.  Topics are ignored unless they can be made immediately "relevant to everyday life," even if said relevance is so forced as to be a camel through the eye of a needle.  It is all so, so condescending to even the moderately intelligent student.  No wonder people get turned off: smart, otherwise interested students feel talked down to, and those who aren't already interested don't fall for the artificial sales job.

Soccer coverage has changed.  In 2008, ESPN tried something different.  They put on Europe's premier soccer tournament, one that did not involve a single American.  They named Bob Ley, perhaps the only prominent American broadcaster with a bona fide soccer background, as the studio host.  They gave up trying to force the use of American-accented commentators, and instead hired the best, most experienced soccer commentators in the world -- even if that meant hiring foreigners.  They told the story of the tournament on its own terms, not attempting to adapt to an American audience or an ignorant audience.  Point was, if soccer was so great, this major tournament which drew hundreds of millions of watchers in Europe would sell itself.

And it did.  People watched, and talked about the games and the stories.  The drama was authentic, the audience was captivated.  

Now, NBC broadcasts the English Premier League in the US using the same principles.  They tell the story of the league from a true fan's perspective, trusting the audience to keep up.  Just like Apple doesn't have to oversell the iphone, just like google doesn't need to hype its search service, NBC recognizes that the Premier League is a product that needs no enhancement, as long as the commentary is smart and authentic.  NBC's ratings are through the roof, despite the lack of on-air shouty salesmanship.

Science sells itself, as long as the teacher is good.  There's a reason that so many of you reading this are interested in science -- and it's not because someone screamed at you that science is FUN!  While many of us do some crazy-arse things in our classrooms, it's not the craziness that wins our students' hearts and minds.  It's the subject we teach, it's the way we communicate our deep knowledge of the subject, and it's the way we relate to our students about our subject.  Problems come when teachers *don't* know their subject or can't build relationships with the class.  Feigned enthusiastic salesmanship doesn't make those problems go away.

So please, folks... let's encourage science teaching in which the teacher takes science seriously.  Let's encourage expert teachers, both experts in subject and experts in relating to students, to do their thing the way they see fit.  Let's encourage more folks who are experts in one of these skills to become expert in the other.  

But let's not oversell science as a discipline.  There's no need.  We have an amazing product that a lot of people want.  We just have to manage the queue and provide outstanding customer service.

A lesson in percentages

I'm hardly the first writer to kvetch about how the dang kids these days -- or any day, really -- don't have any sort of number sense.  My kid is working on his summer math assignment, which includes a page of percentage problems.  The questions themselves are not just reasonable, but important.  "What is 31% of 75" or "28 is 25% of what number" are to mathematical literacy what the offside rule is to soccer -- not everyone understands, but you'd dang well better understand if you want to be considered fluent.

My complaint, therefore, is not that Milo's class is studying the wrong thing.  It's how they approach the problems.  He is required to do the problems the same way I was taught 30-odd years ago:  set up a proportion, translating English to mathematics.  In this parlance, "of" means to multiply, "is" is an equals sign, "percent" means to make a fraction over 100.  No calculator is allowed.  And thusly, Milo and his classmates usually get the right answer.  They often don't notice when they do a routine backwards and say that 31% of 75 is 220, but they usually get the right answer.

I've no doubt that there is some sort of validity to this pedagogy, especially if some sort of national exam is going to require precise answers to such questions with no calculator.  But consider: beyond the test, what do we really want functional high school students and adults to be able to do with percentages?  I personally would prefer my class to be skilled estimators.  What's 31% of 75?  It's about 25, or maybe 24, because 31% is just about a third.  And I would prefer that no one in my class or family* rejoin "well, actually, one-third is 33.3333 repeating percent, so you're wrong."

* For their own sake, so they don't get thrown in the scorpion pit

Me, I'd teach this topic like a video game.  

Start with obvious reference percentages: 50% is a half, 25% is a fourth, 33% is a third.  And use them intuitively to solve problems quickly.  For example, I'd set up a competition: everyone gets 30 seconds to do, say, five no-calculator problems with just these obvious percentages.  Score something like one point for getting "close" in a way defined by the teacher, and an additional point for being right-on.  Guessing is encouraged, and essentially required by the time limit.  Students are practicing making intelligent guesses, and refining their guesses.

Once the class is getting bored with the obviousness, do tricksier problems.  Now the additional point would be awarded to the student closest to the right answer.  Don't demand any formal work or method, but discuss and share methods.  After doing, say, "What is 66% of 210," one student might suggest they knew that the answer had to be more than 105, because 66% is more than half.  But perhaps someone else noticed that 66% is twice 33%, and so is two-thirds -- and perhaps someone else explains how they estimated 2/3 of 210 without painstakingly dividing by three and multiplying by two.  

What does this have to do with physics?  I use essentially this same method when teaching circuits to freshmen in conceptual physics.  They learn to estimate, not calculate, voltages across series resistors and currents through parallel resistors.  And, by unit's end, they have a better sense for the answers than do seniors who have been taught to calculate.

I understand math teachers' obsession with routine and algorithm.  When weak students -- students without any innate number sense, and without any serious interest in the subject -- simply need to get exact answers, well, algorithm can be a friend.  I'm telling you, though, an estimating approach can work wonders.  Even weak students can make progress by guessing and checking.  I've seen it happen.  If that culminating test is multiple choice, even the weak students will be able to pick out correct answers from a lineup.  

And, perhaps if a page of problems didn't represent a multi-hour sentence to proportions, cross-multiplication, and hand arithmetic, such students might develop an interest in the subject.  Or at least a competence with it.