I Let current from a Van de Graff generator flow through you

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Hi.

I have two questions about the following video with a 200'000 V Van de Graaff generator:

  1. At 5:57 he touches ground, apparently feeling nothing. He explains this with a very low current and proceeds that the current from a 120 V outlet is high and might kill you. But with ##I=U/R## and assuming the resistance of the body doesn't massively increase with voltage, shouldn't the current from the VdG generator still be more than a 1000 times higher?
    Or is there just not enough charge that accumulates on the sphere? If so, what happens with the voltage when he touches ground? Does it drop quickly such that ##I=U/R## can be satisfied with a very low ##I##?

  2. At 4:44 and 7:45 he says that a lightning rod works by bleeding of the charge from a sharp point and doesn't actually attract lightning. This isn't what I read: Most sources argue that the sharp point creates a high electric field that partly ionizies the air around it and therefore make a lightning strike the rod more probable (which one could call "attract lightning"). Which one is true?
 

davenn

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CWatters

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The current through a load (eg the man) depends on the nature of the source as well as the load. To calculate it you would need to make an accurate model of the source. For a van den graff generator that would look like a capacitor in parallel with a high voltage source. However the high voltage source has a high impedance.

The high impedance means the high voltage source takes a while to charge up the capacitor. Then when it's discharged through the man the voltage, and consequently the current, falls quite rapidly.
 
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1. That's an important point to understand with high voltage: does the generator have the power to maintain all or a part of its voltage when it is short circuited by the human body, or does the voltage disappear or fall very quickly to ridiculous levels ? In this case, the electric "shock" that may be received by the human body is more or less the same as if the generator were charged at the same voltage by some electrostatic mean, e.g. by triboelectric rubbing. In other words, what imports in this case is the self capacitance of the generator, which is very low for Van de Graaf generators. And that's probably why you've never heard that cranks playing with high voltage DIY devices, powered at 5-24 V, were injured: fortunately for them, they don't know how to build really efficient devices; most of the time, the output voltage of these devices (that does not excesses 50000V at the best) falls very quickly whenever it is shorted by the human body, so, they may feel an unpleasant discharge, but are not killed with that.
Now, things are very different if big capacitors are connected in parallel to the generator (and 1-2 nanofarad is already very "big" at these voltages). In this case, don't even approach them! the capacitance is now so large that the shock can severely injure a person, or even kill him.
Here is some useful data:
  • The fur of a cat may reach a potential of 50000V with respect to the earth.
  • electrostatic potential of 20-30kV are often experienced by everyone of us, for example whenever we touch some conductive material connected to the earth after walking on a synthetic carpet;
  • the heart begins to fibrillate at 10 milli-Ampere;
  • high voltage (normally inoffensive) generators may be dangerous for persons with some heart pathology.
  • Sparks from high voltage generators in the air generate ozone, that is toxic for humans (personally, I simply feel bad with this horrible odor). That's why that shouldn't be done in an insufficiently ventilated place.
  • In old CRT televisions, there is a big capacitance between the aquadag coating the body of the CRT and the inner face of the CRT. That's why opening a CRT television is dangerous (I mean even if it is disconnected from the main). Also, the television flyback, powered at 128V and built by professionals is more efficient and dangerous than a DIY tinkered flyback at 5-24V (useless to say, if the television is connected to the main, everything here is very dangerous).
2. Well, I may be wrong but I don't see too much difference between these two statements. For me, "bleeding" and "attracting" are two terms that describe the same electrostatic effect.
 

tech99

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Hi.

I have two questions about the following video with a 200'000 V Van de Graaff generator:

  1. At 5:57 he touches ground, apparently feeling nothing. He explains this with a very low current and proceeds that the current from a 120 V outlet is high and might kill you. But with ##I=U/R## and assuming the resistance of the body doesn't massively increase with voltage, shouldn't the current from the VdG generator still be more than a 1000 times higher?
    Or is there just not enough charge that accumulates on the sphere? If so, what happens with the voltage when he touches ground? Does it drop quickly such that ##I=U/R## can be satisfied with a very low ##I##?

  2. At 4:44 and 7:45 he says that a lightning rod works by bleeding of the charge from a sharp point and doesn't actually attract lightning. This isn't what I read: Most sources argue that the sharp point creates a high electric field that partly ionizies the air around it and therefore make a lightning strike the rod more probable (which one could call "attract lightning"). Which one is true?
I think the criterion for heart safety is the number of Joules of energy delivered in the shock. It seems that defibrillators use about 200J (https://www.zoll.com/-/media/uploadedfiles/public_site/supporting_pages/biphasic/acls_defibrillation_protocols_with_zoll_rbw-pdf.ashx)
If the capacitance of the sphere is 20pF and the voltage is 200kV, the stored energy is only 0.5 * CV^2 = 0.4 J.
Regarding lightning conductors, a lightning stroke follows a random zig zag path, following low resistance cells through the atmosphere, and searching for upward coming leaders. Leaders originate from all points, such as tree branches. The lightning conductor will send up a leader which intercepts any stroke passing with a few tens of metres of it. So it is just diverting a stroke locally to a safe point. It is believed that the shape of the rod has no effect on efficacy. It is usually assumed that a lightning conductor will divert a lightning stroke to itself over a radius equal to its height. A lightning conductor cannot discharge a cloud.
 
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I think the criterion for heart safety is the number of Joules of energy delivered in the shock. It seems that defibrillators use about 200J (https://www.zoll.com/-/media/uploadedfiles/public_site/supporting_pages/biphasic/acls_defibrillation_protocols_with_zoll_rbw-pdf.ashx)
This is another criterion, but the 10 mA criterion is not bad too.

Regarding lightning conductors, a lightning stroke follows a random zig zag path, following low resistance cells through the atmosphere, and searching for upward coming leaders. Leaders originate from all points, such as tree branches. The lightning conductor will send up a leader which intercepts any stroke passing with a few tens of metres of it. So it is just diverting a stroke locally to a safe point. It is believed that the shape of the rod has no effect on efficacy. It is usually assumed that a lightning conductor will divert a lightning stroke to itself over a radius equal to its height. A lightning conductor cannot discharge a cloud.
So, it seems that you have the answer to your question.
 

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