Hello !
I've got a quick question. I've seen a video about how voltages effect our body etc.

And I've found this moment where the author says :The tower has 200kV but when we touch it, it will drop to 50kV (which it should kill him because it's 50kV), and isn't when we touch the ground then we let this 200kV go through us so it is dangerous ? Why the author says that when we touch the ground the voltage drops to 0 ?

(I don't know if I can post a video but there are also some images)







I've see this video that shows that we current will flow if we touch the ground :

 

he tower has 200kV but when we touch it, it will drop to 50kV (which it should kill him because it's 50kV),

50kV will give you a good jolt, but there's not enough current from a small Van de Graff generator to kill.
It's the amount of current through your body that determines whether it's lethal or not.

 

50kV will give you a good jolt, but there's not enough current from a small Van de Graff generator to kill.
It's the amount of current through your body that determines whether it's lethal or not.

But usually 40kV in some electric boxes says it is leathal for us. So why 50kV is not dangerous. Let's say our body has 1k resistance so the current will be 50 Ampers with 50kV. Why electric box that are outsidewith 40kV can harm me but 50kV from generator doesn't

And why the voltage dropped from 200kV to 50kV while he is standing on a wooden chair ? It is an isolator so the current isn't flowing.
And why the voltage drops to zero while he touches the ground. The tower has 200kV potential, ground 0V potential so when I touch the tower I should have 200kV voltage and not 0 ....

I don't get it why is it like that.

 

But usually 40kV in some electric boxes says it is leathal for us. So why 50kV is not dangerous. Let's say our body has 1k resistance so the current will be 50 Ampers with 50kV. Why electric box that are outsidewith 40kV can harm me but 50kV from generator doesn't

And why the voltage dropped from 200kV to 50kV while he is standing on a wooden chair ? It is an isolator so the current isn't flowing.
And why the voltage drops to zero while he touches the ground. The tower has 200kV potential, ground 0V potential so when I touch the tower I should have 200kV voltage and not 0 ....

I don't get it why is it like that.

Its all about the current
the power in the box, what current can it supply for how long ?
the van de graph, what current can it supply for how long ?

The mains, has a great big current supply, putting 1K to ground across it, the current will be huge , and keep on coming as the generator can keep on supply current,

The Van de graph, has a very small current source ( the belt ) to supply new current
so when you touch it, the current is almost immediately going to drop
also your resistance is not 1K more like 50 K
and if your not touching a good earth, or wearing shoes, then its M ohms.

regarding , standing on wodden chair
think of the Van de graph as being a capacitor
and the man as being a capacitor
If you have one capacitor with a charge on it, and put another across it, then the charge will be shared ,
lower charge in a capacitor is a lower voltage,

 

me but 50kV from generator doesn't

As I stated, that small Van de Graff generator cannot generate enough current to be lethal.

 

Its all about the current
the power in the box, what current can it supply for how long ?
the van de graph, what current can it supply for how long ?

The mains, has a great big current supply, putting 1K to ground across it, the current will be huge , and keep on coming as the generator can keep on supply current,

The Van de graph, has a very small current source ( the belt ) to supply new current
so when you touch it, the current is almost immediately going to drop
also your resistance is not 1K more like 50 K
and if your not touching a good earth, or wearing shoes, then its M ohms.

regarding , standing on wodden chair
think of the Van de graph as being a capacitor
and the man as being a capacitor
If you have one capacitor with a charge on it, and put another across it, then the charge will be shared ,
lower charge in a capacitor is a lower voltage,

Correct me if I'm wrong let's say the Van de Graaf generator provided 50 kV and I'm 1 k Ohm resistor that means the current that runes through me is 50 Ampers but the time that current is running though me is so much low that it is not dangerous even though it is 50 ampers ?

Another thing is that because human body is something like capacitor, because we collect some charges we lower the voltage form 200kV to 50 kV ? Because the capacitor is a like a small voltage supply (because it is charged).

So I I have 2 more questions.

I have to charge all the time the generator for it to create 200kV. But when I touch the generator while touching the ground it will discharge but it will not be charging to 200kV because I'm still touching it and it can't charge to this high voltage ?

My second question is, if I touch the generator while standing on the ground can be dangerous ? Because I'm discharging 200kV or 50 kV ?

Thank you.

 

The answer is insufficient sustained current.

Let is say the voltage is 200kV and your body resistance is 100kΩ.
A sustained voltage of 200kV would deliver 2A. This is enough to kill you.
However the capacitance of the Van der Graaf generator is very low. The output voltage and current will fall very quickly. Sure, you will receive an unpleasant shock but that’s about all you would receive.

 

The Van de Graaf machine is a capacitor, and the one in the video is a very small one, maybe 1pF. Discharging at 200A, the current would flow for 0.2 usec. The energy would be 0.02 Joules. That is way too little energy to harm you.

 

I understand but the statement here is correct ?

Correct me if I'm wrong let's say the Van de Graaf generator provided 50 kV and I'm 1 k Ohm resistor that means the current that runes through me is 50 Ampers but the time that current is running though me is so much low that it is not dangerous even though it is 50 ampers ?

Another thing is that because human body is something like capacitor, because we collect some charges we lower the voltage form 200kV to 50 kV ? Because the capacitor is a like a small voltage supply (because it is charged).

So I I have 2 more questions.

I have to charge all the time the generator for it to create 200kV. But when I touch the generator while touching the ground it will discharge but it will not be charging to 200kV because I'm still touching it and it can't charge to this high voltage ?

My second question is, if I touch the generator while standing on the ground can be dangerous ? Because I'm discharging 200kV or 50 kV ?

Thank you.

By the way the reduction from 200kV to 50 kV looks like that ?

 

I understand but the statement here is correct ?

No

 

No

Why not ?

and how from 200kV I drop to 50kV.

 

Why not ?

So you don't understand what the rest of us have been saying about the limited current the Van de Graff generator can provide?

Think of it as the generator having a very large internal resistance is series the the 200kV voltage that severely limits the current.

 

So you don't understand what the rest of us have been saying about the limited current the Van de Graff generator can provide?

Think of it as the generator having a very large internal resistance is series the the 200kV voltage that severely limits the current.

To be honest from this :

The Van de Graaf machine is a capacitor, and the one in the video is a very small one, maybe 1pF. Discharging at 200A, the current would flow for 0.2 usec. The energy would be 0.02 Joules. That is way too little energy to harm you.

The answer is insufficient sustained current.

Let is say the voltage is 200kV and your body resistance is 100kΩ.
A sustained voltage of 200kV would deliver 2A. This is enough to kill you.
However the capacitance of the Van de Graff generator is very low. The output voltage and current will fall very quickly. Sure, you will receive an unpleasant shock but that’s about all you would receive.

Its all about the current
the power in the box, what current can it supply for how long ?
the van de graph, what current can it supply for how long ?

The mains, has a great big current supply, putting 1K to ground across it, the current will be huge , and keep on coming as the generator can keep on supply current,

The Van de graph, has a very small current source ( the belt ) to supply new current
so when you touch it, the current is almost immediately going to drop
also your resistance is not 1K more like 50 K
and if your not touching a good earth, or wearing shoes, then its M ohms.

regarding , standing on wodden chair
think of the Van de graph as being a capacitor
and the man as being a capacitor
If you have one capacitor with a charge on it, and put another across it, then the charge will be shared ,
lower charge in a capacitor is a lower voltage,

They mention the current is high, but the time we have this current is really small. It's like 2 nanoseconds.

In the video I've send at the beggining the author of the video also mentions about high current but the time is so small that it decreases really fast to 0 ampers.

So I've assumed that the current is really high but the time it lasts is really short.

I'm really sorry if I've angered you in someway, but this is how I've understood the last 3 posts from 3 different users.

 

Let's do some quick "back of the envelope" calculations.

Suppose the capacitance of the Van der Graaf dome is 1000pF.
The total charge Q = C x V = 1000pF x 200kV = 200μC

Current I = Q / t
or Q = I x t

If you could sustain 2mA for 0.1s then all 200μC would be dissipated.
2mA for 1/10 second is not going to kill you.

 

Let's do some quick "back of the envelope" calculations.

Suppose the capacitance of the Van de Graff dome is 1000pF.
The total charge Q = C x V = 1000pF x 200kV = 200μC

Current I = Q / t
or Q = I x t

If you could sustain 2mA for 0.1s then all 200μC would be dissipated.
2mA for 1/10 second is not going to kill you.

I mean as I said before, the current can be high but it won't last long it will last only a small part of the time. So this current even high won't do any harm.

 

The discharge voltage of the Van der Graaf will follow an exponential curve.
Thus, the initial 200kV will fall very rapidly within the first 100μs, and would be very low after 500μs.

Given C = 1000pF, R = 100kΩ
the time constant t = R x C = 100μs

 

The discharge voltage of the Van de Graff will follow an exponential curve.
Thus, the initial 200kV will fall very rapidly within the first 100μs, and would be very low after 500μs.

Given C = 1000pF, R = 100kΩ
the time constant t = R x C = 100μs

View attachment 270751

Does it mean what I said here

I mean as I said before, the current can be high but it won't last long it will last only a small part of the time. So this current even high won't do any harm.

Is wrong ?
I just simplified it that the current will drop imidiatelly

 

I don’t think the capacitance is anywhere near 1000pF. If it is, it has 20 Joules of energy, which is enough to kill you.

 

By my calculation, the capacitance of a 1 ft diameter conductive sphere is 17pF, and is proportional to the diameter of the sphere..

 

Ok, I'm off by a factor of 50. Better still.