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Van de Graaff Generator - Shock vs. No Shock?

  1. Apr 17, 2013 #1
    Hello all,

    I have a quick, albeit probably ignorant (forgive me) question generated by (bu dum) observations of a van de graaff generator.

    In particular, two distinct scenarios caused me to question what was going on.

    Scenario 1 – I’m standing on a plastic stool, with one hand on the generator’s dome. I’m holding a nail in the other hand. I point the nail at a grounded discharge electrode, and hear the effects of escaping charge. I bring the nail close to the grounded electrode until (at a very close distance) visible arcs form. I then put the nail in contact with the discharge electrode. I feel no appreciable sensation.

    Scenario 2 – I’m standing on the ground, in sneakers, not in contact with the generator dome. In one hand, I’m holding a neon-filled gas tube. I being the tube near the dome and hear charge escaping as the ionizing gas in the tube emits light. I feel no real sensation. I assume this is due to small amounts of charge leaking off of the dome. I get a bit too close, and a large (~6”) arc jumps from the dome to the gas tube. I’m holding the glass envelope, and feel…an appreciable sensation in my hand and in my right foot.

    In both cases, I am serving as the conductor that connects the dome to the ground. In scenario #1, I feel nothing, while in scenario #2…definitely something. Why does this happen?

    Here’s my preliminary suspicion – please correct me where I’m wrong. In scenario #1, lots of charge is leaking off my body (hair and fingers both relatively pointy). By the time the nail contacts the discharge electrode, potential difference is low (why the arc is so short), and so the sensation produced by resulting current is minimal. In scenario #2, the dome retains almost all of the 200 kV potential (it is a sphere), and none leaks (save minimal charge prior to reaching the critical distance) until I get close enough, and then – full discharge. The resulting current produces a substantial sensation.

    Is that it? Or is there more/else to it?

    Some follow-up questions.

    1. When I got the shock sensation, why was it felt in my hand and foot, but not in between?

    2. Does touching the discharge electrode with the nail result in less sensation that if I had contacted it with my finger? Why? Is there something special about the point where the arc occurs, as it pertains to sensation? More substantial that the current flowing from me into the nail without the arc?

    3. Is there a better way to demonstrate gas ionization in this fashion (without taking the shock every time)?

    4. There are videos on youtube of chains of 5-10 people in contact with the generator dome. When the last person in the chain is touched, there appears to be a substantial sensation. Why is the charge not leaking off the people (x10) in such large amounts that contacting the final person has no result? Moreover, since all of the people are standing on the floor, why does an 11th person (also standing on the floor) cause significant discharge/sensation? When he contacts the chain, he is no more grounded than each of the other participants, no?

    Thank you in advance for any clarity you can provide! Please feel free to criticize my understanding, and please do forgive me if this is obvious to you.
  2. jcsd
  3. Apr 17, 2013 #2


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    Hi tomcat
    welcome to the forums :)

    I am no expert with this ... BUT experience is a good teacher ( if it doesnt kill ya)
    others may fill in the gaps

    cuz those are the concentrated points where the charge is entering and leaving your body

    Yes using the nail would have that result. if you used your fingertip instead, the spark would have been leaving the concentrated area of your finger tip instead.....

    Think about the times you have walked across a carpet and got a zap when touching a doorknob
    same thing ... if you had reached outto the door knob with nail instead you wouldnt have felt the zap.
    ... and also ... when you were charging up walking across the carpet, you didnt feel any different did you? even tho you had charged yourself up to 10's of 1000's of volts

    of course .... DONT be part of the circuit !! :) set the fluorescent tuble up with one end grounded and the other end coming near the generator dome ( you and the others watch from the sidelines)

    and this goes back to your prelim. comments. I think you will find that the charge leaking off you is rather slow and low. And that's why a person touching the end of the chain of people still gets a significant zap.

    I havent played with a VDG Gen. for many many years, but I suspect you will find, and you can try the experiment. Charge yourself up and remove your hand from the generator and do some timing to see how long you hold a significant charge for ...10 mins, 30 mins, maybe 1 hour ??

  4. Apr 18, 2013 #3
    Thanks Dave! Appreciate all the explanation.

    By the numbers:

    1: so the only reason I feel the shock at the hand/foot (but not side of my body) is that the charge travels along the surface of my body in distributed form, but enters/exits at a very small point - thus the area is smaller, and the sensation, more significant. Is that correct? I can't believe I missed such a simple factor - thank you.

    2: more of less the same. lots of nail-to-hand contact area, so less charge density and sensation. But if the arc exits from the finger, greater sensation due to the small area of hand-air surface through which the charge exits.

    3: Unfortunately, the tube is only ~5" long, so it won't reach a convenient ground. I will try attaching a grounded wire to it, but it doesn't have any metal electrodes or convenient attachment points. It's just a glass envelope. I've seen it held by hand during demonstrations, and am now wondering if they just dealt with the resulting shock. It was a substantial discharge, and I would prefer not to do that twice every second just to demonstrate the gas ionization. :-)

    4: I agree that charge leaking off the person should be small. That explains why after 10 people, the potential is still significant. Also explains why hair takes a long time to settle down once off the generator dome, but still insulated. However, if that's the case, how do we answer the original question?

    In the original scenario 1 vs. 2, I had substantial contact area between my hand and the item being held (in #1, a nail; in #2, a glass tube). And yet in #1 I was fine, while in #2 I got juiced. If we don't explain this difference by citing large amounts of charge leakage from my body in case #1, then what explains the substantial jolt in #2?
  5. Apr 18, 2013 #4


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    I believe that to be the reason, yes

    again, Yes

    OK would probabaly be easy enough to set it up on a stand od some sort ... even a microphone stand would be good, with its plastic clamp on the top to hold the neon tube

    OK lets break down senario #2 a bit more.....

    Yes, at this time you have stated there has been no arc/discharge from the dome to the tube you are holding.
    Consider that there will be a significant electric field surrounding the dome and any object brough into that field is likely to have a charge induced into it.
    Also remember that the tube and you have a much lower ( ummm .... trying to find the right words... ) "energy level" ... you dont have the hi charge on you that the dome has ( hope you figured what I meant)

    OK now you have gone to a point close enough so that a discharge from the dome can occur and this discharge results in a large flow of charge from it to you via the neon tube. That fast build up of charge in you is also now high enough to discharge across your rubber etc sneakers.

    This would be in a similar fashion to a lightning discharge from cloud to ground, where the charge needs to build up high enough to bridge the gap between cloud and ground. abd even before that discharge occurs there is a very hi electric field that had built up, which collapses when the discharge occurs. Then it starts to build up again till the next discharge occurs.

    did that help ? :smile:

  6. Apr 19, 2013 #5
    Yes - something you said at the end there caused me to wonder if this is the case:

    Perhaps, with the nail (a pointy object, subject to lots of charge escape) being brought near the grounded discharge electrode, the amount of charge leaking off the nail progressively increases as I bring the two in proximity. The metal is a good conductor, and it's pointy. The progressive charge escape prevents a large potential difference, which prevents the more instantaneous discharge that's seen with the tube. The tube, which is not a conductor, or as pointy, loses less charge as I near the dome. Then, when the distance is sufficiently close, the electrical resistance of the air is suddenly overcome, and the charge drains very quickly.

    I'm not convinced that the nature of the materials (conductor/insulator) explains that much difference in leaking charge...but...an idea?
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