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Thread: Static electricity - Quantity of charge induced.

  1. #1 Static electricity - Quantity of charge induced. 
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    Hi all,
    Let us say I have rubbed cat fur with nylon. Is there any relation for determining the amount of charge (in Coulombs) induced on either of the materials?
    Thanks
    Girish


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  3. #2  
    Brassica oleracea Strange's Avatar
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    They will be equal and opposite. Apart from that, I would imagine that there are too many variables to predict anything.


    ei incumbit probatio qui dicit, non qui negat
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  4. #3  
    exchemist
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    Quote Originally Posted by Strange View Post
    They will be equal and opposite. Apart from that, I would imagine that there are too many variables to predict anything.
    I do not think I have ever encountered a quantitative relation for the rate of charge separation caused by rubbing insulators together. It seems to be something inherently too difficult to model. I can imagine variables such as air humidity and surface finish may make it a hopeless task, though I do not know this for a fact.

    Or does anyone know of anything that does this?
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  5. #4  
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    Results done in a vacuum are fairly predictable, but still, variables exist with regard to surface qualities. Aside from the materials asked about, smooth, hard surfaces, such as glass, produce the most (I think) predictable and repeatable results. Try searching "Wimshurst" machine, or "static Machines". Here is a reference quote:


    "XXV. A New Form of Influence-Machine.
    By JAMES WIMSHURST*

    In April 1891 I had the honour to submit to this Meeting a very useful form of experimental Influence-machine, by means of which I was able to show that almost every combination of glass and metal, and also that plain glass disks, when moved and suitably touched, were capable of producing a flow of electricity. It is one of those combinations, somewhat modified and extended, which I have now the further pleasure of bringing to your notice. The machine consists of two disks of plate-glass, each of 3 ft. 5 in. diameter and inch thickness, mounted about inch apart on one boss and spindle. This spindle is driven by means of a handle, and the disks rotate in one direction. In the space between the disks are fitted four vertical slips of glass, two being situated on the right-hand side of the machine and two on the left-hand. The vertical edges of the slips which come nearest the spindle are cut to an angle, leaving a rather wider opening at the circumference than at the centre of the disk. Upon each slip is a brush, A, and also an inductor, the brush and the inductor being metallically connected; the brushes are made of fine wire, and touch very lightly on the inner surfaces of the disks. The glass slips slide into place by means of suitable grooves at their top and bottom ends; they may be removed and replaced readily at pleasure. The essential parts are fitted together within a glass case, and in all respects the machine is so constructed as to be useful for experimental work. The limited size of the case causes the insulating distances between part and part to be small, hence the length of the sparks is reduced. All the surfaces are coated with shellac, and when the brushes are new and in proper condition the machine is self-exciting. The charges are not subject to reversal when the terminals are opened beyond the striking distance, for then the whole of the induced charges pass by way of the neutralizing brushes C, C. Another feature is that the neutralizing cur*rent may also be broken without reducing the excitement, but then the charges alternate from positive to negative with each half revolution of the disks."
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