Positive? or Negative charge? How to tell on a Van de Graaff generator?

[space-time]

electrons flow in the opposite direction

[Mike Johnson]

space-time wrote:
Do you have a multimeter to connect to it, I would set it up to measure current, and turn it on, there should not be much charge accumulated but you might measure a few microamps. If that does not work, use it as a voltmeter, although depending on how large the impedance of the DMM is, the voltage can get high and I am not sure if it will damage it or not. I would stick with Ammeter first

do you have some capacitors? some diodes?

We never managed to measure anything on my little digital multimeter. I could get some diodes at Radio Shack for under a buck, I'm sure, but to what purpose? We tried some LEDs but some didn't work and the others were damaged. It is generating 30,000 - 40,000 volts. What would we do with capacitors?

A CRT television has an electron gun, right? We drove the ion motor by simply sticking some foil to a television screen and running a wire to the motor. So we tried hooking the ion motor to the top lead of the VDG, then seeing if the tv lead would attract or repel the charged motor arm. It attracted it. Then we hooked the ion motor to the opposite end of the VDG. The tv lead still attracted it! I'm assuming the tv lead is strong enough to induce polarity in the wire of the ion motor arm, regardless of what the VDG is feeding it.

[freeradical]

You can determine whether a stream of particles is positive or negative based upon the amount of deflection that occurs between two charged plates. The protons, with much greater mass will deflect less than the electrons.

[Mike Johnson]

freeradical wrote:
You can determine whether a stream of particles is positive or negative based upon the amount of deflection that occurs between two charged plates. The protons, with much greater mass will deflect less than the electrons.

Huh. Okay. So how could we do that?

[btfc]

Compass. Galvinometer.

[haikuman]

I would suggest placing contacts on your "Johnson" if it stands up it is positive.
If it remains flaccid it is negative and you are still M Johnson :jest:

[haikuman]

Thanks for this "post" I actually learned something today *(>*

http://en.wikipedia.org/wiki/Van_de_Graaff_generator

[TheTominator]

Mike Johnson wrote:
We never managed to measure anything on my little digital multimeter. I could get some diodes at Radio Shack for under a buck, I'm sure, but to what purpose? We tried some LEDs but some didn't work and the others were damaged. It is generating 30,000 - 40,000 volts. What would we do with capacitors?

Hook one side of the capacitor to the end of the VDG you wish to measure. Keep track of which side of the capacitor you connected. Connect the other side of the capacitor to ground.

Charge the capacitor by running the VDG for a little while. Stand back in case you pop the capacitor.

While the VDG is running, disconnect the capacitor from the VDG carefully using insulated tools.

Measure the DC voltage across the capacitor with your volt meter. Note the polarity.

If you didn't accumulate a charge, try doing the same with the opposite side of the VDG or reversing the VDG motor.

[space-time]

Stand back in case you pop the capacitor.

and always wear safety glasses

[cbelt3]

And for pure fun, a paper based capacitor is called a 'confetti generator'. :biggrin: (when they explode... used to pull that trick in Engineering school.)