Re: [Phys-L] measure voltage of van de Graaf

[brian whatcott <betwys1@sbcglobal.net>]

On 8/13/2013 8:41 PM, Carl Mungan wrote:
> I have a large capacitor (two Al disks, roughly 50 cm in diameter and
> spaced by 12 cm) connected to a van de Graaf (the usual classroom style
> model, top dome about 25 cm in diameter). I'd estimate the voltage to be
> about 100 kV, implying a charge on the capacitor plates of about +/- 1 uC.
> I'd like to measure more precisely. Obviously either a measurement of V or
> Q would work.
>
> Simplest idea I can think of is to progressively reduce the plate spacing
> until I get arcing and then assume V = (3 kV/mm) * (arcing plate spacing).
>
> But is there a better way? I'm looking for something simple (no fancy
> equipment beyond standard undergrad lab stuff) and safe, but accurate.
I don't suppose that run of the mill labs keep electrostatic voltmeters 
for the voltage range in question - but thinking out side the box - if 
an electrostatic voltmeter would use spring resisted repulsion between a 
fixed and movable plate to indicate the voltage, placing the lower 
grounded plate of your capacitor in mechanical contact with a reasonably 
sensitive lab scale should indicate an apparent loss of weight of that 
charged plate.

Brian Whatcott    Altus OK