The discussion focuses on the reasons behind the low current output of a Van de Graaff generator, particularly in relation to its potential difference and the mechanisms of charge transfer. Participants explore various aspects including electrostatic induction, impedance, capacitance, and the effects of current on human safety.
Participants express a range of views on the mechanisms behind the low current in Van de Graaff generators, with no clear consensus reached. While some agree on the role of impedance and capacitance, others emphasize different aspects of charge transfer and safety.
Limitations in understanding include the dependence on specific definitions of impedance and capacitance, as well as the unresolved nature of how these factors interact during operation. The discussion also reflects varying interpretations of the relationship between charge, current, and safety.
This discussion may be of interest to students and educators in physics, electrical engineering, and those exploring electrostatics and safety in high-voltage applications.
It's simply because all the charges are being taken to the ball on the moving insulated belt. Those charges are formed on the belt at the bottom by electrostatic induction and the rate they are produced is very small. Current is rate of flow of charge. Few charges per second equals low current.weezy said:e. why the extremely low current between the generator and ground?
sophiecentaur said:It's simply because all the charges are being taken to the ball on the moving insulated belt. Those charges are formed on the belt at the bottom by electrostatic induction and the rate they are produced is very small. Current is rate of flow of charge. Few charges per second equals low current.
for me, and maybe the OP, that doesn't even begin to answer the question, as it is not about the charge transfer to the dome from the belt rather it is about the transfer from a dome that is now fully charged to a person/other objectanorlunda said:@sophiecentaur gave you the right answer. A belt picking up one electron at a time is no competition for a wire for conducting currents.
You can look at it as a high resistance (charges carried by the belt at a slow rate) and a small capacitance to Earth. At 250kV (on a good day) and with not many pF, the charge available is very small.davenn said:It does appear as tho it is all about impedance ...
sophiecentaur said:You can look at it as a high resistance (charges carried by the belt at a slow rate) and a small capacitance to Earth. At 250kV (on a good day) and with not many pF, the charge available is very small.
Q=CVweezy said:But isn't a lot of charge required to reach the potential in the first place?
davenn said:it is not about the charge transfer to the dome from the belt rather it is about the transfer from a dome that is now fully charged to a person/other object
. . . . and charge. In the end, it's total charge passed that governs the damage done. Initial current from a 250kV source through a human will be very high ( a substantial fraction of an Amp, even). That current is only sustained for a very short while (fraction of a millisecond) so it's harmless.anorlunda said:If it makes a flash every 3 seconds, then discha
The deceiving part is the impressive lightning flashes. Unlike natural lightning, these flashes have very low currents. That is why the humans don't get injured.
Yes I agree it's the short duration which makes the current spark sting from a V.D generator but nothing lethalsophiecentaur said:. . . . and charge. In the end, it's total charge passed that governs the damage done. Initial current from a 250kV source through a human will be very high ( a substantial fraction of an Amp, even). That current is only sustained for a very short while (fraction of a millisecond) so it's harmless.
So the potential should go down right? With C increased so much?sophiecentaur said:Capacitance is very relevant here. If you get a chain of half a dozen kids to hold hands, standing on plastic buckets, with one end attached to the Van de Graaf (switched 0ff). Then start the motor and wait a minute or so. The total capacitance of kids plus ball is much greater and will give a seventh kid much more of a belt when they touch one of the charged kids.
Leakage may get worse and limit the final volts a bit but with a higher C, it just takes longer to charge up. The input current is, as always, limited by belt speed and rate of deposited charges on the best.weezy said:So the potential should go down right? With C increased so much?