Confusion over Van de Graaff vs. Electric Fence

Question

It is said "It is not the volts that kill you, but rather the current".

However, volts is directly related to current as V= IR (current x resistance). Since the resistance of one particular human body shouldn't vary by too much (well, maybe a little from hydration, but not significantly), then surely for a particular person voltage and current are directly proportional?

I'm confused, as a Van de Graaff is 200,000 volts and doesn't shock you badly or at all, whereas you can get a nasty shock from a 8,000 volt electric fence, yet the resistance of your body would be the same in both cases

Is it something related to the Van de Graaff being static rather than dynamic electricity? However, if the Van de Graaff is continuously on, then once it is grounded through a person, won't the electricity become continuous current electricity since the Van de Graff is continuously providing electrons?

And do you get a shock from a Van de Graaff (when grounded rather than standing on plastic)? Some places on the internet say you do, and others say you don't

Answer 1

(a) It is said "It is not the volts that kill you, but rather the current". This is basically true, but it must be realised that: (i) it's the current through you that's being referred to; (ii) The routes that the current takes through your body greatly influence the danger to your life: current through the chest or head being particularly dangerous, though no route is safe; (iii) even for the same current taking almost the same route, the outcome may be different on different occasions.

(b) "Since the resistance of one particular human body shouldn't vary by too much [...]" The resistance is strongly dependent on the contacts between the conductors and your body: their surface area, the pressure of one on the other, and dampness of skin, mainly.

(c) "a Van de Graff is 200,000 volts and doesn't shock you badly or at all" I'm not that keen on the shocks I get when a spark leaps on to (say) my hand from the dome of a working educational Van de Graaff. On the other hand, if the machine is off and discharged when I touch it, and if I keep holding it while I turn it on, I'm less likely to find it uncomfortable – though I'm sure one shouldn't do it. The reason is that the Van De Graaff has a high internal resistance. As soon as current travels from the dome through a 'load resistance' (and thence back to the bottom of the belt) the Van de Graaff output voltage drops dramatically. The same thing happens with an electric fence, but the internal resistance of its voltage source is probably much lower, so the drop in the voltage between fence and ground may not be as dramatic. This effect can be modelled by the equation $$\text{Output voltage = Output voltage if no current}\ – \text{Current} \times \text{internal resistance}.$$

Answer 2

Total electric power is what matters, it is : $$ P = VI$$

Based on this answer some "standard" Van de Graaff machine produces $2~\mu A $ current, and $100~kV$ voltage. This gives about $200~mW$ of electric power. So Van de Graff device transfers to you only $200~mJ$ of energy each second.

While say, electric fence operating at $8~kV$ voltage and $120~mA$ current, gives $\approx 1~kW$ of electric power. So electric fence transfers to your body about $1~kJ$ of energy each second. This is what makes animals to shake.