Electrostatic levitation of hair

  • #1
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Main Question or Discussion Point

If a person with long thin hair touches a Van de Graaff generator, his hair will begin to levitate by electrostatic repulsion. The body is a conductor. Hair is an isolator, so the charge cannot flow through the shaft from root to end. To me it is unclear whether the hair is charged triboelectrically, as the hair is not forcefully rubbed against the skin. A person's hair only partially touches his head.

I made this video last winter, using woolen thread as a model of single hair, on a piece of cardboard. Initially, the hair is lying flat on a piece of cardboard. Cardboard is a conductor for static electricity, and it is lying on a plastic bucket for isolation from ground. The cardboard is charged by a "fun fly stick" (a toy version of the Van de Graaff generator). No force is applied. The thread begins to levitate.


Questions:
1) In what way is the charge transferred to the thread
2) how is the charge distributed over the thread: like in drawing A or B?
3) how much charge will be transferred to the thread, after charging for a few minutes. (I am interested in predicting the charge from the electric properties, not in using gravity for a reverse calculation)
Haar2.png

Brown: conducting surface at voltage V. Blue: dielectric cylinder
 

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Answers and Replies

  • #2
Vanadium 50
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Hair is an isolator
I think you mean "insulator" but every material will conduct with some, perhaps high, resistance.
 
  • #3
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I think you mean "insulator"
I see, it looks like a confusing translation problem. English has two words: isolation and insulation. My language (as well as German and French) has only one word: isolation, which translates in English to insulation. The English word isolation has no single word translation in other languages, it is equivalent to a multi word description like 'separating by a large air gap or vacuum', I guess.

... but every material will conduct with some, perhaps high, resistance.
I suppose you are not implying that my original questions are solved by considering hair as a conductor instead of an insulator?
 
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  • #4
Delta2
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Interesting Video, I have a fourth question
4) Why only the edge of the thread is levitated and not the whole thread levitate. Why not the other edge of the thread?

I got no clue about the electrostatics for insulators and what exactly sort of insulator is wool.
 
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  • #5
Vanadium 50
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I suppose you are not implying that my original questions are solved by considering hair as a conductor instead of an insulator?
Sure. Your question boils down to "how does the charge get on hair"? The answer is that there is no such thing as a perfect insulator and hair is a good enough conductor to eventually move the charge. Note that it takes seconds to build up the charge, not nanoseconds like it would with metals.
 
  • #6
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I agree conductance of the thread would explain the levitation of the thread in the video. However when I made the video, I tested the conductance of the thread by trying to discharge a charged electroscope through the thread. As the deflection of the electroscope did not change, I considered the thread to be an insulator on the timescale of the experiment (1 minute).

On the other hand, I repeated that test today, and now the electroscope was discharged in about a minute. Presumably the increased conductance is caused by the moisture content of the fibers being higher on this warm summer day. The conductance of a thread is inversely proportional to its length. In the video the thread had many points of contact with the cardboard, so the segments between points of contact would be short, and flowing charge would experience a higher conductance than in the entire length of the thread.

"No deflection" of the electroscope can be translated into a resistance value of the entire thread. The electroscope has a capacitance of 5 pF . RC >> T means R >> 60 s / 5 pF = 1013 Ω.
 
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  • #7
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The static generator produces charged ions in the air. These ions stick to the hair. Some static generators produce static electricity by rubbing two insulaters together like wool and glass. For an experiment try lighting a cigarette lighter next to the charged hair. The new ions in the air will discharge the hair.
 
  • #8
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Unlike a flame, the fun fly stick (or a Van de Graaff generator) does not release a large amount of ions into the air. When holding the fun fly stick close to the thread, no significant quantity of charge is transferred through the air. However, when the fun fly stick actually touches the thread, the transfer of charge occurs.

1-2: polyester thread; 3-4 pith ball (just for comparison )
 

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