What potential difference or current would kill a person? I was watching this YouTube lecture, and earlier in the vid he mentions 120 volts from a power outlet would kill you. So if human resistance is $300\ \Omega$, $0.4 \rm A$ should be enough to do the job.
Then he proceeds to touch a higher voltage of $200,\!000$ V and says its because the current flowing through him is low. Is it really? It should be around $700$ A. Which should have cooked this man inside out.
There must be a mistake in my understanding, what was it?
 A: A human's skin resistance is not 300 Ohm.
But an actual 200,000V is enough to kill you if it actually reaches you, whatever the skin resistance is. However, the source's output impedance and energy behind the source might prevent it from actually reaching you at full voltage.
The 200,000V was measured when zero current was flowing.
If the source's output impedance is very high it means that as soon as current starts flowing, a huge voltage drop develops across the output impedance it limits the current flow and also drops most of the voltage leaving little remaining to actually shock you. So if you had a voltmeter on the Van de Graff generator, it would read 200,000V before he touched it, and the voltage reading would drop drastically after he touched it since it would be supplying current and a lot of voltage would be dropped across the high output resistance as current flowed through it.
If the 200,000V does not have enough energy behind it, the voltage also drops rapidly. That's why you can survive a 30,000V static shock, but not a 120V outlet. The carpet might hold a static charge of 30,000V but there is so little charge (or energy) behind it that as soon as you touch it, it redistributes through you to be a much lower level. Like a tall, but small graduated cylinder draining into a bucket. The water level in the graduated cylinder is high, but very low once it is in the bucket. Electrically, this is like taking the charge inside a very small capacitance and placing it inside a very large capacitance. The very small capacitance needs little charge to climb in voltage but the large capacitance needs a lot of charge to climb in voltage. So for the same charge being contained in both, the voltage in the large capacitor will be very low for the same stored charge compared to the small capacitor.
A 120V outlet on the other hand has both low enough output impedance and high energy behind it so it stays at 120V when it starts conducting current after you touch it.
A lightning bolt is a very large static charge stored in the capacitance between the sky and ground, and that capacitance is much bigger than your body's capacitance so it can easily charge up your body's capacitance to reach near the voltages of the lightning bolt long before it empties.
Compare a freight train and a speck of dust both moving at 60km/h. Measuring their speed when they have no obstacles is like measuring the output voltage when there is no current being supplied. But measure their speed when they have obstacles is like measuring voltage when current is being supplied. The freight train's speed doesn't change much, but the speck of dust comes to a near dead stop.
Voltage, like speed, isn't enough to determine how much damage is caused on its own.
A: Basically there are 3 things to take into consideration:

*

*In order for the shock to be dangerous the current flow must be: High enough, last for a certain amount of time and pass through the dangerous areas (heart).
I thinks we are talking about tens of milliamps for tens of milliseconds through the heart, but don't quote me on that.


*Actual body resistance: If you apply a metal plate to your palm and a metal plate to back your hand and you measure the resistance between them it would be   relatively low because the area of contact is high and the path is short. Just see how a defibrillator works. From a source online I found that the resistance between the plates of a defibrillator applied with gel is only ~50 Ohms. Even a relatively low voltage ( e.g. 12V ) can create a dangerous current this way. If you touch a multimeter probes with dry hands the measured resistance is huge ( >1MOhm tested now).


*Even if a certain source have a lot of voltage does not mean it will be able to sustain high currents, see a Van de Graaf generator: As soon as you put a load to it the voltage will drop drastically providing very little current flow. A power outlet  can push currents order of magnitudes greater than what is dangerous while barely dropping in voltage as long as the resistance is low.
