Why doesn't the Van de Graaff nail cause a visible arc?

[JFS321]

Hi all, I've been playing with a van de graaff this week. As I'm sure you know, pointing a nail at someone with their hair raised by the machine will result in the hair immediately sticking back down. I can easily imagine electrons flying off of the tip of the nail and into the hair of the demonstrator, but this is what I don't understand well: Why would this not cause an arc, too (since electrons are moving)?

My explanation: The voltage at that distance simply too weak to initiate a concerted gas breakdown...In other words, I reason that only a "few" electrons are flying off of the nail and do not ionize enough gas to be visible in an arc as compared to a deluge of electrons if the object was placed close enough to create an arc.

Is this reasoning correct? Thanks in advance.

 

[anorlunda]

Hi all, I've been playing with a van de graaff this week. As I'm sure you know, pointing a nail at someone with their hair raised by the machine will result in the hair immediately sticking back down. I can easily imagine electrons flying off of the tip of the nail and into the hair of the demonstrator, but this is what I don't understand well: Why would this not cause an arc, too (since electrons are moving)?

My explanation: The voltage at that distance simply too weak to initiate a concerted gas breakdown...In other words, I reason that only a "few" electrons are flying off of the nail and do not ionize enough gas to be visible in an arc as compared to a deluge of electrons if the object was placed close enough to create an arc.

Is this reasoning correct? Thanks in advance.

i

I believe you are correct, but I'm not sure how to prove it. Objects can acquire and discharge very high voltage static charges with only tiny currents.

Perhaps if you could surround it with a noble gas, such as in a plasma globe, you could make the currents visible.

 

[tech99]

Hi all, I've been playing with a van de graaff this week. As I'm sure you know, pointing a nail at someone with their hair raised by the machine will result in the hair immediately sticking back down. I can easily imagine electrons flying off of the tip of the nail and into the hair of the demonstrator, but this is what I don't understand well: Why would this not cause an arc, too (since electrons are moving)?

My explanation: The voltage at that distance simply too weak to initiate a concerted gas breakdown...In other words, I reason that only a "few" electrons are flying off of the nail and do not ionize enough gas to be visible in an arc as compared to a deluge of electrons if the object was placed close enough to create an arc.

Is this reasoning correct? Thanks in advance.

The books on electrostatics describe "action at points". In these cases, the charge carriers are small particles such as dust or water, which are able to pick up electrons from the point and are then repelled, forming a wind. If the voltage gradient is high enough near the point, then a number of other discharge methods are possible - the silent discharge, the brush discharge, the spark and the arc for instance. An arc is what you see in welding, and is a sustained high current discharge, which we do not see with the V de G machine because it can only furnish a few microamps.