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Old 10-14-2010, 07:11 PM
Chronos Chronos is offline
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Acrylic, wool, and static electricity

I teach college physics labs. One of the labs is on electric charge, and the students at one point rub a wool cloth against an acrylic rod to charge the rod. According to the lab manual, this produces a positive charge on the rod, and the students then use that fact to determine the sign of the charges on other objects.

Well, I recently noticed that, according to Wikipedia, wool is towards the positive end of the triboelectric series, while acrylic (AKA Lucite AKA Plexiglas) is near the middle (and in fact, a little on the negative side). That would imply that, if I were to rub a wool cloth against an acrylic rod, the rod would end up negatively charged, and the cloth positive, contrary to the manual's claim.

OK, that wouldn't be the first time that the lab manual was wrong, but I decided to double-check to make sure. I know for certain that when glass is rubbed with silk, the glass gets a positive charge (in fact, that's the original definition of positive charge). So I got one of my silk neckties and a glass bottle, and checked... And it turns out that the glass and the acrylic do, in fact, have the same sign. In other words, the lab manual was right after all, and the rod did become positive.

But why? The Wikipedia article does caution that substances close together in the series might occasionally go the "wrong" way, but wool and acrylic are pretty well separated. I suppose it's possible that there are different related plastics all called "acrylic" but with different electrical properties, but the rods we're using sure look like Lucite. I've double-checked everything in my experiments, and it doesn't look like there are any uncontrolled variables. Is the specific combination of wool and acrylic just a weird fluke in the triboelectric series?
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Old 10-15-2010, 08:14 AM
Fridgemagnet Fridgemagnet is offline
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It's difficult to get repeatable results with triboelectric effect experiments, and the properties of the materials can vary due to any number of factors. Materials like wool and silk would be affected by humidity, chemical treatment (bleaching, dyeing etc.), contaminants like grease and dirt*, and so on, so a better reference would be something neutral and stable like steel. Chronos appears to have access to a polarity indicator, so referenced against steel at least the true polarity of the second material can be given with confidence. If there's a charge meter kicking around then a custom triboelectric series of Stuff In Chronos' Lab can be drawn up. If not, a crude charge level indicator can be improvised with a small neon bulb in parallel with a capacitor (try 10 nF , 100 V rating) - hold one end and poke the other end into the charge field, and the faster it flashes the higher the charge.

*The triboelectric effect is used in dust sensors.
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Old 10-15-2010, 10:59 AM
Chronos Chronos is offline
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First of all, I should mention that my materials are extremely limited. It's a teaching lab, not a research lab, so materials are pretty much limited to whatever is in the lab manuals, plus whatever I can scrounge up myself. The "polarity indicator" I was using was a couple of pieces of tape pulled off of a countertop (a very simple experiment; you put a piece of tape on a countertop, then you put a second one on top of it, pull both off, and then separate the two-- The two tapes end up with opposite charges).

Second, from experience in teaching this lab many times, I've found that the acrylic and wool we use is remarkably consistent. I've done this experiment at all different times of the year, with windows open and closed, and with a room full of students handling and mis-handling the equipment, and it's still always the case that the acrylic rod repels the top piece of tape, and attracts the bottom one. I don't have a similar degree of experience with glass and silk, but apparently Benjamin Franklin trusted the consistency of that one enough to use it as the defining standard for charge.
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