Static electricity

  1. How do I measure static electricity or is it even possible?
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    It's done routinely in the electronics assembly industry, to help prevent static discharge damage to products being assembled. Try some Google searching with those search terms, and let us know what you find. There are hand-held measuring devices that you should be able to find. :smile:
     
  4. I found this here: wikipedia.org/wiki/Electrometer

    "An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid-state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges."
     
  5. sophiecentaur

    sophiecentaur 14,170
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    There are different aspects of Static Electricity that are more, or less relevant, depending on the application. The actual Voltage can be relevant and so can the Charge. A Van de Graaf ball may be charged to several hundred kV but the charge is so small that it will do no more than give you a tickle. A big fat capacitor (100uF, for instance), charged to just a few hundred Volts could kill you. It's the total Energy stored that counts - along with the level of the Volts, which govern how much current (rate of discharge) will pass through you (or your delicate electronic equipment).
     
  6. Could you transform this explanation from electric capacitor to water reservoir and Van de Graaf ball to XXXXXXX ( I don't know what)?
     
  7. sophiecentaur

    sophiecentaur 14,170
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    I'd rather not go into Water at this stage because the water analogy is not really useful except in terms of buckets with leaky holes in them and taps filling them up.
    Suffice to say that a big object and in particular, a large plate, over Earth, will carry a large charge with a small Potential on it. If you have a small ball of, say 20cm diameter (Van de Graaf Ball), putting just a very few micro coulombs on it will raise its voltage to a very high value. That will spark across to you very easily but the spark will have very little charge - which corresponds to little energy in total. A capacitor with much less than 1mm between its plates and a total area of, dunno- say 1m squared, will store a lot of charge (maybe 1/100 Coulomb), even with just 400V across its terminals. So the Energy stored will be much much greater and, with the 400V will push that charge through you and give you a nasty shock.
    But we would really need to know a bit more about the context of your question - or else there's no end to how much information we could be giving you and you will go into overload. Let's have some feedback.

    I though of a (rather naff) water analogy - it's like comparing a single raindrop hitting you from a great height and a large tank of water landing on you, from just above your head.
     
  8. jim hardy

    jim hardy 5,359
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    okay, now search on "electrostatic voltmeter" and you should find lots of them for sale. Better companies will have a page explaining how they work.

    http://www.trekinc.com/library/Tech_Notes.asp
     
  9. sophiecentaur

    sophiecentaur 14,170
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    But what does he actually want to do with this information? Is he measuring static fields when he combs his hair or is it about lightning bolts?
     
  10. jim hardy

    jim hardy 5,359
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    I've never run across a need to measure static electricity.
    But there are people who do.
    When selecting carpet for our control room i ran across the standard for testing it which said, to effect (can't recall verbatim this was in 1980's) :
    ' Don shoes with chrome tanned leather soles and walk ten steps across the carpet lifting your feet ~ten inches each step. Touch an electrostatic voltmeter.' How practical !

    In South Florida static wasn't a problem. Even with the plain carpet that doesn't have conductive fibers woven into it, which we were replacing, it was only on very unusually cold and dry days(for S Fla ) that you could notice static effects. Rubbing a meter with some types of cloth would make the needle move an inch or two then creep back where it belonged.

    Perhaps Mr Steve will describe his need further.
     
  11. The last para is good enough.

    But I saw the Van de Graaf ball and capcitor in a different way.

    In the former the whole charge lies on the outer surface while the inner surface is free of any charge. Charge separation is over a very small thickness of the ball. Therefore, even for a small charge the potential rises to a very high value. But, as you rightly observed the discharge (Shock) pumps in only a little charge that cannot cause much of a harm. While in the case of capacitor, the charge separation is over a larger distance as a result it needs a lot of charge to raise the potential to high values.

    A cylindrical water reservoir with radious tending to zero is similar to Van de Graaf ball, while a cylindrical water reservoir with radious tending to very large values is similar to a capacitor. When they discharge, the first water reservior doesn't cause much harm just as Van de Graaf ball.
     
  12. sophiecentaur

    sophiecentaur 14,170
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    It is risky to try to pursue a 'possibly dodgy' model, such as the water reservoir too far. You will fall over when you go for one 'prediction too many'. The fact is that Electricity, in general, is very well behaved and its behaviour is often much easier to predict than any mechanically based analogy. One of the problems with the water analogy is that water has a very considerable density and it carries a lot of Kinetic Energy when it's on the move. KE is not involved in Electrical Current (electrons move very very slowly and have a very small mass).
    If you just allow yourself to accept the concept of Capacitance (followed by the formula Q = CV) then you can have a good idea of what will happen in many simple circumstances. Yes, the same sums often apply to Capacitors and water reservoirs but where and how reliably? Analogies can be false friends. Algebra will seldom let you down.
     
  13. jim hardy

    jim hardy 5,359
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    i was a fan of water analogies in the 1960's
    but they will cause trouble if one is not very rigorous.
    So i only rarely indulge them anymore.

    If you're thinking of coulombs as analogous to moles
    then it's more the elasticity of your reservoir than its size.

    Are electrons compressible? Not something i'd care to speculate about.
    The energy of a capacitor is stored in the dielectric, not on the surface of its plates.
    Alignment of the polar molecules in the dielectric is an elastic deformation. I'm not sure why free space has a dielectric constant at all, probably something to do with relativity i guess.

    Water is so incompressible that forcing "just a little more" into a rigid sided vessel raises pressure to point it's apt to split the inelastic vessel wall with very little energy input.
    But a compliant vessel , or a rigid one being filled with something that is compressible, will store enough energy to be quite dangerous - eg scuba tanks.

    Vandegraff ball is electrically fairly 'rigid', having free space for a dielectric and a LOT of distance to its 'other plate'. Its εA/D has a large denominator.
    Sophie's 1m2 air capacitor with plates close together has a small denominator in its εA/D so more capacitance. It's electrically less rigid.

    Something with polar molecules makes a better dielectric than free space.
    Pure water is ~80X better but it's impractical to keep it pure..

    hope i didn't just muddy the water.
     
  14. sophiecentaur

    sophiecentaur 14,170
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    Standard practice is to use deionised water for cooling high power radio transmitter anodes with voltages of many kV, aamof. Scary, when you think that a pinch of salt in the coolant will knacker the whole power supply. (Definitely not for cooling!)

    And Vacuum Capacitors are also standard - I don't know why they are chosen but they are only suitable for low(isn) frequencies because they are physically pretty large. Lovely shiny copper bits inside the glass envelope.
     
  15. Well, small Van de Graaff generator can deliver a slight shock too, but it is harmless.
    Very big machine, like this one , can even kill!
     
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