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03 June 2011

Electrostatics: Wayne Mullins' incredible demonstration

 Photo credit to AP reader Teresa Walker -- Thanks!
I would contend that 3/4 of the ideas I use in my teaching have been inspired by, or sometimes directly copied from, the people I meet at the AP physics reading.  I arrived in Kansas City on Tuesday, ostensibly to grade 125,000 physics exams.  Within an hour of the opening of the physics teachers' lounge, I had heard Wayne Mullins' revolutionary thoughts about teaching electrostatics.

My own, obsolete, thoughts are detailed in this post.  Essentially, I treat the electric field as primary, teaching F = qE for days before even discussing the Coulomb's law force between charges.  This approach has worked quite well.  The enormous, gaping drawback is the lack of a quantitative demonstration.  I don't have an electric field measurer; the Vernier charge sensor doesn't really work so well, considering that static charge bleeds off an object (and the sensor itself) so easily; and I've never even gotten my Van de Graff generator to work correctly in my dungeon of a classroon.  I had given up on electrostatics demos.

Wayne brought his own electrostatics demonstration along -- see the picture above.  On the very first day of teaching electricity, Wayne sets up these two aluminum rods about a foot apart.  He connects the two rods to 25 V AC.*  In the tray he puts a thin layer of water, about a centimeter deep if that.

* Why AC?  It prevents corrosion.  And since the voltmeter set to AC reads an RMS value, the voltage readings still do what they're supposed to.

In Wayne's mind, voltage is the primary electrostatic quantity that students must understand.  He reasons that if students can get a literal feel for the measurement of voltage, then he can define electric field as related to changing voltage.  (Specifically, he defines electric field as the slope of a voltage vs. position graph.)  And THEN he can start talking about electric forces.

So Wayne takes a digital voltmeter set to ACV.  He clips the ground lead to the gounded metal plate.  He puts the tip of the other lead in the water, and moves the lead around.  The voltage reading is seen to change as the lead moves, but ONLY IF THE MOTION IS TOWARD A PLATE.  If he moves the lead parallel to the two plates, the voltage reading stays constant.  This is a beautiful, quantitative demonstration of E = V/d between two parallel plates -- and on the first day of the unit, even.  Wow.

But there's more!

Next Wayne places two finger tips (not more than two fingers!) right in the middle of the water between the plates.  (This process is shown in the picture above.)  He moves his fingers apart in a direction parallel to the plates -- nothing happens.  But when he slowly separates his fingers, one toward one plate and one toward another... he starts to feel a bit of a tingle that gets stronger with more finger separation.

What he's communicating is how electric field, and thus electric force on charges, depends not on the value of an electric potential but on the voltage DIFFERENCE between two positions.  Since the voltage is the same along a line parallel to the plates, two fingers separated on that line feel no electric field or force.  But when one finger is at a significantly higher voltage than the other, ooh, tingle!

I did ask the obvious question about safety concerns.  He's using 25 V, not a full 120 V outlet, but still, as his students say, we're told not to touch "electrified water" since we can understand English.  Wayne doesn't force any student to try the experiment, but he is sure that it is safe with the following three caveats:

(1) The fingers used should have unbroken skin.  I tried this yesterday, and all I got was a tingly sensation.  Wayne says that a papercut will hurt to high heaven.

(2) Only use TWO fingers.  Multiple fingers act as parallel paths for the flow of charge along the skin, meaning a much higher current experienced.

(3) Obviously, if a student has heart issues or a pacemaker or something like that, he shouldn't do this.

The next follow-up to the parallel-plate setup is to place an aluminum-wrapped piece of PVC in the region between the plates.  What should you discover about the voltmeter reading inside the PVC?

GCJ