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Maximum negative voltage of a sphere?

  1. Jan 8, 2016 #1
    I would like to know the maximum voltage that a metal sphere can hold from applying negative charge to it.

    My thoughts are that the maximum voltage is the breakdown voltage of the surrounding medium (air in this case). I assume that at that voltage, electrons would arc from the sphere to the nearest object. I found that air actually has a breakdown electric field strength of 3 MV/m, which I assume to mean that if there is an object one meter away from a sphere charged to 3 MV, breakdown will occur (likewise, if there is an object three meters away, I can go up to 9 MV before breakdown occurs, etc.).

    Since the work required to apply another charge increases with each charge applied, I imagine that perhaps special equipment would be required to actually achieve such a large voltage. What equipment is capable of this?

    I would also like to know the best practical (household or within a reasonable purchase) way of applying a specific amount of negative charge to a metal sphere. I imagine that I could charge a capacitor and discharge half of the voltage into the sphere since the system would achieve equilibrium once the voltage of the capacitor matches the voltage of the sphere. I imagine a coulombmeter would otherwise be of great service.
    Last edited: Jan 8, 2016
  2. jcsd
  3. Jan 8, 2016 #2


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    are you familiar with Van De Graff generators ?

    you have given no indication of what electronics/electrical experience you have
    you can be dealing with VERY high voltages, in this sort of experimenting, that can kill
    Be very careful

  4. Jan 8, 2016 #3
    Oh yes, I forgot about Van de Graaff generators. We used one for demonstration in Physics II.
    Thanks for the warning. I'm not as concerned with high voltage as I am with being able to apply a specific, chosen voltage. I just wanted to know the maximum so that I would know how easy it would be to accomplish my experiment. So far, I think the best way to do this would be to charge a capacitor and discharge half of its voltage into the sphere, but I'm interested in what you and others think.

    It is important that the voltage be negative, so I assume by simply touching the negative side of the capacitor to the sphere, I can achieve this.
    Last edited: Jan 8, 2016
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