The Van De Graff generator can generate voltages in the kV and MV range.
I have read that if I touch the Van De Graff generator producing, say 50kV, with one hand (myself being insulated from ground by plastic stool) and the other hand to the grounding rod, I don't get shocked because the current passing through me is small.
But if I grab live and neutral wires in each hand from a home AC electric service, which carry 120V or 220V, I might get fried because even though it has low voltage, it delivers high current.
My questions:
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What factor determines the current flow through the human body in the case of the Van De Graff generator and of the AC wall socket?
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Why is the current less in the generator case and why is it high in the AC wall socket case? Doesn't high voltage mean high current? And low voltage mean low current? What am I missing here? Is the missing thing the internal resistance?
Best Answer
When you are stood on the insulating plastic stool, you are preventing a continuous DC current flow and your body quickly acquires the same potential as the sphere of the generator. Your scalp hairs might start to separate because of charge repulsion (see gold-leaf electroscope) but there is no more current flowing - your body has become charged to (maybe) 1 million volts. You body has roughly 100 pF capacitance to ground (see human body model) and 1 million volts and 100 pF is a charge of 0.1 mC. If your body acquires a voltage of 1 MV in 0.1 seconds then the current is: -
$$i = C\dfrac{dv}{dt} = 1 \text{ mA}$$
You might feel this little surge but only for a fraction of a second.
On the other hand if you connected your body across 220 V, it's the resistance of your body that determines the current and, this current will therefore continue to flow. If the voltage is big enough to breakdown the surface resistance of the skin, then your body resistance might be in the region of 1 kohm. 220 volts / 1 kohm = 220 mA (continual and rapidly lethal).
Do you see the difference?