classroom Van der graaff generator

The sphere is positive and the comb at the bottom of the roller is negative.

Both? I'm not sure what the question is? Bill Beaty hangs out at SED, you can ask him directly.

George H.

Wiki and Bill Beaty describe the same device: "The standard classroom Van de Graff machine contains no power supply." In the whole world is probably milions such devices. All are the same. The question is: Is on the sphere of the standard classroom Van de Graff machine the excess of electrons or the deficyt? S*

szczepan bialek prodded the keyboard with:

Could it be either or both. Depending upon charge polarity. If lower roller is negative then the sphere must be positive and Visa versa.

The charge generation process is triboelectric. If you peel an initially uncharged piece of rubber off a metal surface, it acquires a charge that depends on its composition. Rubbers can be positive or negative--see e.g.

Column 3 of the table is the polarity and strength of the charge acquired when rubbed against metal.

Cheers

Phil Hobbs

The upper electrode is round. The lower electrode is half-round and has feet. While it is usual for the lower electrode to be grounded (so it doesn't electrify the floor or other nearby objects/persons), the two electrodes can be given any polarity you want.

In research machines, there's bunches of ancillary stuff that controls the currents. In a classroom-demo, it might be as simple as a pointy-thing versus a flat-thing type of gas rectifier. The positive ions are gas molecules, the negative include free electrons, so the point (where ions are generated by field emission) ) collects only a few electrons: they're fast moving and attracted to the flat.

But the question was: "Is on the sphere of the standard classroom Van de Graff machine the excess of electrons or the deficyt?

In the oryginal was the "+" and "-":

ers

de

Do you have one in front of you? You could try and measure the sign of the voltage.. maybe run it very slowly... just a little.. so that it doesn't ge t too high? (use a cheap DMM so if it does fry it you won't be out much.)

George H.

Which I answered, if you look at the table in the referenced page. Different kinds of rubber have positive and negative charging against metal. What kind of rubber does a "standard classroom van de Graaf" use? Are they all the same?

Cheers

Phil Hobbs

If it's a "Van de Graaf" shouldn't it follow Dr, Van de Graaf's design?

Phil Hobbs prodded the keyboard with:

Thanks for the informative link.

All your designs use CK722s and uA709s then? ;)

Cheers

Phil Hobbs

Classroom Van de Graaf generators were made for teaching the principle concepts put forth by Dr. Van de Graaf. It would be nice if the model represented that.

Why? It generates HV triboelectrically, which is the point of the demo. I think it's cool that by changing a belt you can have positive and negative HV.

For instance, positive and negative corona discharges look and act very differently.

Wiki says that his first one used a silk thread, which assuming it's electrically like human hair, should charge up positive.

Assuming that his second one used latex rubber, which is probably a good guess for 1929, that would charge negative. Which is the real authentic van de Graaf in your view?

Cheers

Phil Hobbs

If I use his name I like the demo to represent what he did. There are many ways to generate static electricity but this one is his. Not a big deal, just me.

But he did lots of them.

Cheers

Phil Hobbs

OK

"Do you have one in front of you? You could try and measure the sign of the voltage.. maybe run it very slowly... just a little.. so that it doesn't get too high? (use a cheap DMM so if it does fry it you won't be out much.)

George H."