Here’s a wonderful tidbit from a book that every physics teacher should have — The Flying Circus of Physics.  My old mentor PD gave it to me with the inscription, “until I write my book of physics stories, this is the best collection of science stories in print.”  As much as I love Paul, I think even he’d have a big task to outdo the wide array of stories and strange facts in this book (though I’d love to see him try!).  Need something to spice up a lecture on sound?  How about an explanation of why we hear our upstairs neighbors more than they hear us?  Or need a story to make the idea of pressure come alive?  How about the girl who got her tongue stuck in a bottle and needed glass cutters to help her get free?

So here’s the story of how electricity helps flowers grow.  We generally think of pollination as being a sort of accidental process — the bee gets himself all covered in yellow snow at one flower, and then loses some of it at the next flower.  No, it turns out that bees get positively charged (they lose some electrons) as they fly through the air.  When the positive bee approaches the neutral flower, that induces a charge in the pollen, which jumps onto the bee.

This is the same phenomenon as when you rub a balloon on your sweater.  The balloon becomes positively charged and when you bring it to the wall, it induces a charge in the wall.  Thus, it sticks to the wall.  There’s a nice simulation of this effect here.

Anyway, the pollen sticks to the hairs on the bee.  If it stuck to the bee itself, it would lose its charge.  The hair acts as an insulator, keeping the pollen grain just far enough away to keep it charged, and thus attracted to the bee.

Now, when the bee goes to the next flower, it induces a negative charge in the stigma of that flower.  The pollen grains are more strongly affected by that concentrated negative charge (the stigma, after all, has more charge than the bee, it’s connected to the ground so has an infinite source of electrons to draw from), and the pollen grain is polarized in the opposite way and jumps to the flower.

Wow.  I wonder if pollination doesn’t work as well in moist climates, then?  Is that why Colorado wildflowers are so stunning in their concentrations?

Bee Picture from TTaylor

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From Nessman on Flickr

Here was an interesting discussion on a science teacher’s listserv, which came down to the question — can a vacuum become a conductor?  What is it that we really need in order for charge to flow between two points?  What is the physics of electron flow?  The physics teacher in question wrote:

The Paul Hewitt book has a test question that reads:  In order for charge to flow from one place to another, there must be a
A. Potential difference between the two places.
B. Conductor, such as a wire, connecting the two places.
C. both A and B.
D. none of these.

The book’s answer is C (both A and B).  I’m wondering why A isn’t the answer. In the case of the van de Graff or lightening you create a potential difference between two locations (say me and the van de Graff) and the charge will eventually flow … I guess the air acts as the conductor from van de Graff to me? But is there a situation where there is enough of a potential difference between to places and charge doesn’t flow? Would the van de Graff not work in a vacuum?

A veteran physics teacher (Al Sefl, who always knows more physics than moi) responded:

The key to the Hewlett question is that it asks about charge flow.  Current cannot flow through an insulator until the point of breakdown is reached.  If you have a sphere X with a positive charge and a sphere Y with a negative charge there will be no flow between them until a conductor connects them.  Before that conductor is there, there will be lines of electrostatic force but no flow.  If the potential were great enough the air would break down to become a conductor and you would have flow.  So, C is the correct answer, you must have a potential difference AND a conductor to conduct the charges.

Yes, a Van de Graaff generator will work in a vacuum.  High voltage capacitors used in broadcast transmitters and radar units are vacuum capacitors where the charge is stored between two plates surrounded by an evacuated space.  The electrostatic lines of force do not need air to exist.

That’s all fine and dandy, but most of the people on the listserv didn’t understand that second paragraph (and neither did I), not knowing enough about broadcast transmitters and such.  A teacher asked, do you mean that the Van de Graaff will store charge in a vacuum, but not throw off sparks?  After all, what would the charge be flowing through if the Van de Graaff is in a vacuum?  There’s no air to ionize (or “break down”).

Al responded with a clarification:

A vacuum may also be a conductor.  The old cathode ray tube of years past sends a beam of electrons from a gun through a high vacuum to impact the phosphor screen.  So, when the potential becomes high enough current will flow through a vacuum.  In a CRT it does get an assist from thermionic emission in the gun.

The miniature lightning bolts we see from a Van de Graaff are really the paths of broken down insulator air that has become conductive and ionized.  You would not see that in a vacuum.  If you put a sharp point on the negative terminal then the charge concentration will push off electrons that will travel to the positive terminal.  The vacuum will become a conductor.

So, C is still the correct answer.  If charges FLOW they must do so through a conductor.  ANYTHING will become a conductor if the electrostatic charge exceeds its dielectric.  If electrons are flowing through something it *is* a conductor.

Perhaps where the Hewlett presentation becomes unclear is the definition of what a conductor is.  Most of us immediately think of a piece of copper wire *but* it can be anything if the potential is high enough.

So, a vacuum can become a conductor, even though there’s nothing to ionize (and thus you won’t see the glow from the electrons as they travel through a vacuum, as you do in the air).  But by definition, if charge is flowing, it’s flowing through a conductor!  Paul Doherty explained that when there is an electric field that is large enough (it has to be very very large), then it will produce electron/positron pairs in the vacuum.  Those electrons and positrons are what flow to conduct electric current.

On a side note — the charged particles given off by the Sun aren’t visible as they pass through the vacuum of space… but they are visible when they hit our magnetosphere as the aurora borealis.

And another teacher offered a clarifying comment:

I was taught to get over the idea of being protected by an insulator. We were told that an insulator is a bad conductor. My trade teacher felt that insulator was a weak word and preferred to talk about everything being a conductor, just good conductors (copper) or bad conductors (glass).

So, the discussion got interestingly esoteric here.  The original questioner then posited:

If any space can be considered a conductor given a high enough potential difference, then I think the answer to Hewett’s question should be we just need a potential difference to get a flow of charge.  After all, he didn’t explicitly state that we need to have charged particles, which I think would be necessary to have a flow of charge.  So why state that an omnipresent conductor is necessary?

Also, if a vacuum has charged particles moving through it, is it still a vacuum?

Paul Doherty emphasized that the correct answer to the question is still “C.”  You can have a potential difference and no flow of charge, because the voltage may not be low enough to create its own conductor out of the insulator between the two places.  With enough potential difference an insulator is turned into a conductor, but you STILL need both a potential difference and a conductor for charge to flow.

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I just got this question from a teacher on Webconnect (which lets teachers ask science questions):

“In the past when I taught electricity I always understood that it flows from the negative terminal to the positive.   The CPO books and materials have the opposite – from positive to negative.  This doesn’t make sense to me in how you generate the flow of electrons, pulling to the opposite charge.  Is the book wrong or have I forgotten stuff? 8th grade teacher”

It depends on what you define as “electricity”.  Do you mean the flow of “electrons” or the flow of “current”?  Because, due to an unfortunate quirk of history, the direction of *current* flow is opposite to the direction of *electron* flow.  Take a moment and re-read that, because it’s not what you would expect.  If electrons are flowing to the right across this screen, then we say that current is flowing to the left.

So, let’s say that the left hand side of this screen is the positive terminal and the right hand side is the negative terminal

+                  -

*Electrons* will flow towards the opposite charge, as you say.  That’s which direction?  Right to left

<—-  electrons

But *current* is the opposite direction.  Left to right.

—-> current

So *current* does flow from positive to negative, like your books say.  And electrons do get pulled towards the negative charge, like you say.  But we define electric current to be the opposite direction of electron flow.

There’s some good history on why it’s defined this way, but I’m too busy to find it right now — if someone has a good link, stick it in the comments, thanks!

UPDATE 4/27

Here’s a relevant comic from xkcd

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I posted a new podcast – “Ooh you make my motor run” on my Science Teaching Tips podcast.  One of the Exploratorium staff educators, Modesto Tamez, tells how he gets students exploring electromagnets, a great preparation for making an electric motor.

Here’s the Stripped Down Motor activity: www.exploratorium.edu/snacks/stripped_down_motor.html

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[[AAPT SESSION: TRANSFORMING UPPER-DIVISION E&M I]]

This post is primarily for college physics teachers.

Hey, if you’re ever presiding over a conference session, here’s a tip for you. If one of your presenters has technical issues, don’t give her a hard time after the talk is over, even in jest, about having gone over time. Trust me. She’s already been beating herself up for it over the last half hour.

So, technical issues aside (and I tried four times to check my presentation on the conference computer prior to the session and was thwarted in some way each time, I swear!), my talk tonight went well. Here’s what I’ve been working on.

In our department, we’ve made a lot of changes to the freshman level courses, adding things like Tutorials and clicker questions and peer instruction that have been shown to improve student learning, because they get students really thinking about the material and engaged (and thus learning) instead of sitting passively and waiting for knowledge to be imparted upon them by the instructor. At the upper-division, however, there’s this sense that we need to stop “coddling” our students with these kinds of techniques, that we learned by lecture, so why shouldn’t they? If they’re not learning, maybe they should consider changing major at this point. But, I argue, that how you learn doesn’t suddenly change between the sophomore and the junior year, and we might do better by our students to try using some techniques that have proven effective at the lower division.

So, in our course we developed

• Lists of what we concepts and skills we wanted students to learn
• Homework questions that targeted those concepts and skills
• A new assessment exam to see if we taught them those concepts and skills
• New tutorials for teaching those concepts and skills
• Interactive lecture techniques
• Clicker questions

We found that when we compared students in a Traditional course to those who took a Transformed course, even though they were similar to one another coming in to the course, at the end of the course those in the Transformed course scored significantly better than those in the Traditional course on common exam questions and a conceptual exam. So, it worked! We can teach our majors better.

We’ve got all those materials on our website for other instructors to use.

And here is the Powerpoint of my talk (PDF) I’ve also got two posters — see the website above for those.

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I just posted a new episode of my Science Teaching Tips podcast – Electrifying Ideas. My old boss, Paul Doherty, is a great storyteller, and this is one of the stories of science that he uses to explain the history of science. The ancient Greeks knew about magnets, and they knew about electricity, too. But it wasn’t until the nineteenth century that a connection between the two was discovered. Paul tells the story of how a professor made the connection . . . which led to modern motors.
Paul Doherty’s Web site

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This is a great little activity from Eric Muller’s While You’re Waiting for the Food to Come.

Get a plastic drinking straw, in its wrapper. Unwrap one end, so the straw is still wrapped in the paper, and then slide the wrapper quickly up and down over the straw, until the straw and wrapper feel a little warm.

Take off the wrapper and the straw will stick to the palm of your hand!

For extra fun, do this with a friend. Ask them first if they’re attracted to you or repulsed by you! Then both do the straw-thing, and hold the straws near one another (you’ll find that your friend is repulsed by you).

You can find a writeup of this activity on Eric Muller’s website.

Why does this work? The straw ends up negatively charged (it’s got just about 40 nanocoulombs of charge) after being rubbed with the paper. Since like charges repel, it will repel the other straw. And charged objects attract neutral objects, so it sticks to your hand.

This is the same principle as the old rub-the-balloon-on-your-head-and-stick-it-to-the-wall trick.

Here is a simulation you can play with that shows you the physics of this sort of trick.

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I’ve created a couple posters of Maxwell’s Equations (differential and integral form) and you can buy them online at Zazzle, or just ask me to send you the electronic file and you can print them yourselves. Good for the junior level physics classroom.
Here’s a link to the integral form poster and to the differential form poster.

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No post last week, as I was driving across the country from San Francisco to my new digs in Boulder CO. As I was camping in the middle of very very very dry Nevada, I noticed something a little cool. As any science teacher can tell you, any demonstration having to do with static electricity works best on a dry day (and is a terrible failure in, say, Florida). As I was rolling up my thermarest, I saw the tiny little grains of hay from the ground were sticking to it, and sticking straight out from it like little porcupine quills.

You can do this yourself with spices, like dill. Rub a plastic comb with a piece of wool, and hold it near dill and you can watch the dill dance in the electric fields. It may very well stick to the comb, too. The comb has grabbed electrons from the wool and is negatively charged. The dill has no charge, but when it’s brought near the comb, those negative charges push away the electrons on the dill, making the near end of the dill positive and the far end negative. It’s induced a charge on the dill. So, then the positive end of the dill sticks to the comb, and the negative end strains to get away, so you get the porcupine quill effect.

The same thing probably happened with my thermarest and the hay. The thermarest rubbed against the fabric of the tent, making one of them negative and one positive (I don’t know which, but if I had a tape electroscope I could have found out — I’ll write about that later). It’s easier to charge things like this on a dry day because water on the surface of things gets in the way of electrons jumping from one to the other.

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