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

[Mike Johnson]

My son and I built a small Van de Graaff generator & we've been playing around with it. We get sparks up to two inches. We've done the fluorescent light thing, and built an ion motor whirligig, and all the usual stuff.

The science fair is coming up, and he wants to test whether the charge building on the top sphere is negative. I know VDGs can be built either way, and we can use the triboelectric series to predict whether the top sphere has a positive or negative charge based on how we built it. But what's an easy way to see which is the case? All the stuff we've done (the fluorescent light, the ion motor, etc) work just as well hooked up to either end of the generator.

Any ideas?

[Jimmypoo]

The TONGUE test is hard to argue with. If you feel bad afterwards, it’s negative.

If you feel good - it’s positive.

[space-time]

can you post a picture please? a short movie?

[Jimmypoo]

space-time wrote:
can you post a picture please? a short movie?

I agree! Would ya, Mike? Please!!!????

[Mike Johnson]

It's awfully hard to get a picture of a spark. I've tried. If we get the science fair project closer to completion, I will see what I can do.

We got the instructions from Make magazine. It's some pvc pipe, a rubber band, the glass tube from a fuse, a small hobby motor, and the top sphere is actually just an old Coke can. It doesn't charge enough to make your hair stand on end, but it does charge enough that sometimes the sparks are uncomfortable.

[space-time]

Mike Johnson wrote:
It's awfully hard to get a picture of a spark. I've tried. If we get the science fair project closer to completion, I will see what I can do.
...

Try this: use low ISO, low aperture, long exposure time. Fire the FLASH so you get a picture of the generator itself when not sparking, and eventually you're gonna capture a spark or two during the 2-3 exposure. Experiment and you will find a good combination of parameters

[freeradical]

When a balloon is rubbed with wool, it develops a negative charge. This charge is high enough that it will deflect a stream of water, which is a polar molecule.

If the balloon is attracted to the generator, it has a positive charge. If the balloon is repelled from the generator, the generator is negatively charged.

[space-time]

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?

[space-time]

freeradical wrote:
When a balloon is rubbed with wool, it develops a negative charge. ...

what is the theory behind this? Thanks

[Mike Johnson]

freeradical wrote:
When a balloon is rubbed with wool, it develops a negative charge. This charge is high enough that it will deflect a stream of water, which is a polar molecule.

If the balloon is attracted to the generator, it has a positive charge. If the balloon is repelled from the generator, the generator is negatively charged.

Yes, but that's relying on the triboelectric series to say the balloon develops a negative charge; if we trust that this end of the series is positive and that end is negative, we could do the same for the generator, right? Unfortunately, we're trying to prove that electrons are negative. It's a fun bit of history, how positive and negative got defined in the Benjamin Franklin era, and sort of gets to the point. How do we prove a flow of electrons in a particular direction?