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Electrostatic balloon - result not as good as expected

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basil_brush

Fri Jan 02 2009, 07:33PM

Registered Member #903 Joined: Sun Jul 15 2007, 09:51PM
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Posts: 8

I need to charge up a capacitor for an experiment - the capacitor is concentric aluminium foil cylinders about 100 cm tall and 50 cm diameter. This capacitor is not for discharging but will hold its charge to create an electric field between the plates. The charge should be at least 50 kV, but ideally I want to test up to 400 kV (from what I've read that's easily attainable by cheap homemade Van der Graaff machines). So first I wanted to test whether I could build up charge on any metal object.

I figured the easiest way to build up charge is the accumulator method, as per Van der Graaff and earlier devices. So I made a large paper mache balloon about 50 cm diameter and covered it aluminium foil. A couple of foil tabs inside the balloon give an electrical connection to the outside skin. If I understand the process correctly, any charge applied on the inside should go to the outside of the skin and accumulate. I have the innards of a 12 V car air ioniser with -4 kV output which I placed inside the balloon and connected to the foil tabs. A 12 V wire went in through the neck opening to power it. The whole thing was hung on a plastic string.

The ioniser will give faint continous sparks about 2mm long to my fingers. The balloon give much brighter sparks, maybe about 4-5mm long. So there was definitely some magnification of the voltage, but it was a big disappointment. I must have been getting only about 10 kV off it, even when I left it for half an hour to charge up. The air must be fairly dry at the moment considering it's very cold now. Does anyone have any suggestions on how to improve it?

Mads Barnkob

Fri Jan 02 2009, 07:42PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

As aluminum foil is rather hilly in its texture when first unrolled, all the tiny tops on each side could result in alot of loss?

you would need something with a completely even surface to store high voltages, but maybe this isnt the case as it would need higher voltages for it to break out into air. But I guess there is a microscopic losses at lower voltage, or im completely wrong :)

Antonio

Fri Jan 02 2009, 07:53PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

In electrostatics, any assembly of conductors is at the same potential everywhere. So, if you are charging the globe with a 4 kV power supply, it will charge to 4 kV. To get the effect of the VDG generator, you have to use a charge source, capable of working even with its terminal at a very high potential. A mechanical charge transporter, as a belt, is one of the few possibilities. The charging from the inner side or the terminal serves only to rise the potential of the belt above the potential of the terminal. The upper end of the belt doesnÂ´t feel the electric field coming from the charges on the outer side, and observes the terminal as if it were grounded. But if the potential of the charge source is fixed, by a direct connection to a voltage source, everything is at the same potential and charging from any side results in the same effect.

Nik

Fri Jan 02 2009, 07:56PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

MadsKaizer is right about the charge needing to be collected on a very smooth surface but it also take a bit of current to keep that charge up. My van de graff is charged by ~1.2uA and it has a max voltage of about 60kv at best. Maybe your ionizer can't supply enough current to make up for corona losses.

HV Enthusiast

Fri Jan 02 2009, 08:59PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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wrote ...

The ioniser will give faint continous sparks about 2mm long to my fingers. The balloon give much brighter sparks, maybe about 4-5mm long. So there was definitely some magnification of the voltage, but it was a big disappointment. I must have been getting only about 10 kV off it, even when I left it for half an hour to charge up. The air must be fairly dry at the moment considering it's very cold now. Does anyone have any suggestions on how to improve it?

Its not really magnification of the voltage. Basically your just adding charge via your ioniser. Its an additive process.

Also, you must think of your metal sphere as a simple equation.

TotalChargeSphere = TheoreticalMaximumCharge - Losses

Where:
TotalChargeSphere = maximum charge your sphere will reach
TheoreticalMaximumCharge = Based on the geometry, the highest voltage your sphere can obtain before breaking out
Losses = Coronal losses (due to sharp points, imperfections, leakage to air, etc...)

For your given sphere, the two biggest contributors are the ionizer and losses.
If the output current of your ionizer is less than the losses, you'll never reach the theoretical maximum voltage of the sphere.

Just for comparison, I used a 14" Diameter oblate (almost a sphere) from Science First NY with my VDG and got arcs about 12-14" in length from it.

Antonio

Fri Jan 02 2009, 09:16PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

This form of charging does NOT produce any voltage magnification.

DrZoidberg

Fri Jan 02 2009, 09:52PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
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Antonio is right. It doesn't work that way. You need a belt that runs into the sphere.

But maybe you don't really need 50kV to produce a sufficient electric field in your capacitor. If you just decrease the distance between the two cylinders the field will get stronger without having to increase the voltage.
What do you need that electric field for?
There are simpler ways to create strong electric fields like e.g. taking a big piece of plastic foil and rubbing over it with your hand.

basil_brush

Fri Jan 02 2009, 10:01PM

Registered Member #903 Joined: Sun Jul 15 2007, 09:51PM
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Posts: 8

Ok, so must I use a belt? Antonio said that a direct connection doesn't work, but did comment that there are a few methods that do. What are my options for accumulating charge other than the standard belt system? I would prefer to use an electronic, rather than a moving mechanical, system where possible.

Many thanks,
Erik

basil_brush

Fri Jan 02 2009, 10:14PM

Registered Member #903 Joined: Sun Jul 15 2007, 09:51PM
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Zoidberg: The capacitor is part of an LC oscillator system. The inventor is too old and lacking resources to test it now, so I want to have a go myself, though I not a professional in this field. The best distance between the plates is not known. The inventor lives near me and I spoke with him, and he suggested a minimum of 50 kV. It interacts with the quantum background, much like Naudin's lifters and other HV phenomena. So 50 kV is where I'll start, if no effect is measured then I'll go higher. Thanks for reminding me about electrode spacing, yes I don't want them too far apart, but equally I probably need a fair amount of physical spacing in order to get enough of the quantum world between the plates to get a measurable effect. I'll try about a one inch spacing to begin with.

DrZoidberg

Fri Jan 02 2009, 10:16PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
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Posts: 106

You can produce 50kV with a flyback transformer from an old TV. Another option is a high voltage cascade connected to a transformer that produces a few thousand volts ac.
Such a cascade consists of diodes and capacitors. There are cascades available that can convert 5000V AC into 30,000V DC.

edit:
If you want to build some kind of lifter a Van de Graaff generator is not powerful enough. You will need at least a flyback transformer.

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