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Water-bottle capacitance - Van De Graaff generator

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LAN_403

Plasma Lover

Tue Jan 06 2009, 07:15PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

A while ago, a friend of mine built a VDG and, one day, he placed a water bottle on top of it while it was running. He gave me some details and here they are -

"I was running a fairly powerful van de graff. I don't know what the current was, I never measured it, but it would make a corona connection to you at over 2 feet and would spark up to about 15". The collector on top was about 16", so the charge on the sphere was probably in the neighborhood of 100kv. I sat the water bottle on it just because it was the closest surface around at the moment. I typically just had the van de graff on all the time, so it wasn't an intentional experiment at all. It was a standard plastic water bottle, about 1 liter, about 3/4 full. No metal anywhere on it, just a regular paper label. It sat there for about 5 minutes probably. I grabbed it, and it didn't immediately shock me, it was like there was a delay for a few seconds. I had both hands on it, apparently one in contact with one plate and the other somehow made contact with the other, possibly one hand was on the label and one just on the plastic, and perhaps the water was one of the plates and punctured right through the plastic. I remember it felt sort of like being in an earthquake, it shook me at what seemed like a very low frequency and for what seemed like a duration of at least 2 seconds. I have a lot of experience in low frequency acoustics and to me it felt like the oscillation was somewhere between 10 and 20 hz, which seems very odd. I have no logical explanation.
It jolted my whole body and left me dazed and bewildered. It was the most intense shock I had ever encountered, and I've been shocked by 110ac, by flybacks, by hv capacitors, and high powered tesla coils, but nothing ever like that. And since there were no metal plates that, in my mind, should be necessary for the creation of a Lynden jar, it really blew my mind that I had just gotten such an incredible shock from a water bottle."

Sorry for the size of this post, but I wanted to let you know all of the information. Does anybody here know what the reason for this may be?

Antonio

Tue Jan 06 2009, 07:58PM

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

A strong shock from a bottle only can happen if it is charged as a Leyden jar. And it is a simple capacitive discharge shock. Possibly with repetitions due to charge redistribution along the outer surface and as the water moves inside.
An open bottle placed over a VDG can really get charged, because the humid interior can make a connection between the water inside and the mouth of the bottle, where corona would drain charge to the air. Charge coming from the VDG terminal would then accumulate on the outer side of the bottle, while opposite charge coming from the ionized air enters the bottle trough its mouth and goes to the water. If someone tries to drink the water, will get a strong shock.
If this can really happen, it's a simple experiment to make. I would try if the air humidity here were not at 90%...

Nik

Tue Jan 06 2009, 10:05PM

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

Any insulator with a large surface area can build up A LOT of charge from a Van De Graff for example I have charged up a sheet of glass with one side positive and the other negative just by holding it next to the van de graff. This works with pretty much any large surface area object.

DrZoidberg

Tue Jan 06 2009, 10:15PM

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

Seems to me your friend already gave a good explanation for what happened.
There must have been a spark jumping from the water through the plastic into his hand.

But I don't think there really was an oscillation. That bottle can only give a strong shock if it gives off its charge within a tiny fraction of a second. It couldn't have given him a 2 sec long shock. Probably just felt like it.

rp181

Tue Jan 06 2009, 10:53PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Ive gotten pretty strong shocks several times (like the 240v that comes into the fuse box) The shock is short, but there is a long lingering affect after. It was intense for a couple of seconds, and tension in my muscles for almost a hour.

Tom540

Wed Jan 07 2009, 12:28AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

I think that maybe what he described as the oscillating was just maybe a series of pulses he got. perhaps it took a few jolts to fully discharge. I got a bad shock off a doorknob cap marx generator once.

Plasma Lover

Wed Jan 07 2009, 04:25AM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

What I was thinking is that, because he may have just left the VDG on as he placed the water bottle on top, which would result in the small frequency he was describing. What is the amperage put out by a moderate-powered Van De Graaff? What is the amount of time, at that amperage, required to charge a capacitor of the named properties? I'm not nearly qualified to use these equations, although I'd like to learn them, if you don't mind. If he had left the VDG on, it would give pulses from the capacitor and he would feel it surge through him at a low frequency, due to the fact that the VDG doesn't charge at a fast rate.

What I'm looking for is an equation illustrating the amperage input versus charge-time of a capacitor of a given farad value. Based on what I've read it seems that one ampere for one second will charge a one farad capacitor.

Antonio

Wed Jan 07 2009, 01:51PM

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

charge = capacitance x voltage
charge = current (assumed constant) x time

charging time = capacitance x voltage / current

A bottle may have 1 nF of capacitance. A regular VDG will output 10 uA of current. Assuming 100 kV:

charging time = 1e-9 x 100e3 / 10e-6 = 10 seconds

Plasma Lover

Wed Jan 07 2009, 05:21PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

Antonio said -

charge = capacitance x voltage
charge = current (assumed constant) x time

charging time = capacitance x voltage / current

A bottle may have 1 nF of capacitance. A regular VDG will output 10 uA of current. Assuming 100 kV:

charging time = 1e-9 x 100e3 / 10e-6 = 10 seconds

Doesn't the equation you provided have the incorrect voltage measurement? You said that "charging time = 1e-9 x 100e3 / 10e-6", but 100^3 equals 1,000,000. That is assuming the input voltage to the capacitor is at one-million volts. Instead, it's only a tenth of that, and, I believe, that would make the charge time one second.

Is this correct?

Antonio

Wed Jan 07 2009, 08:21PM

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

100e3 = 100 x 10^3 = 100000.
Standard exponential notation on computers, calculators, etc. AeB = A x 10^B.

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