Question

In the video Nº4 minute 42 aprox the vandergraph gives the electric field to the fluorescent tube to produce light, but ¿where does the electrons come from? Because even with electric field you need current to make de tube work ¿electrons come from the vandegraph? ¿from the air? If they come from the vandregraph ¿is the air ionized to be a good conductor to the current? ¿whould we have the same effect in the vacuum, without any ionized air?

Answer 1

Dear electron comes from the ground, as vandergraph is emerging positive charge then automatically negative comes from the ground

Answer 2

I think the electrons which are already present in flourescent tube(it contains gaseous material which has electrons), under the influence of electric field from vandergraf, move along the field. So electrons do not come from vandergraf or from earth but are already present in the flourescent tube.

Answer 3

I apologize for my English, is my second language so I don’t dominate it yet.
@pysics era: ok, but that's not my point. I mean: electrons go from earth through ionized air? And what would happen if the vandergraph was negative?
@madhusudan: that would work only for an instant, if you have a continuous flux of electrons (you have light for several seconds) then you would need to get new electrons from somewhere (in the case of the negative side) and the electrons would need to go out of the tube to somewhere (in the case of the positive side). Maybe your theory would work in a electric field that changes of direction many times per second (like AC), then you may have the same electrons going right, then left, then right then left….etc.

Answer 4

I think the light observed for few seconds, depends on the amount of charge present in the gas, inside the fluorescent tube. So its quite possible to have light for few seconds depending on the kind of gas inside. But after few seconds the tube might not glow.
Also in the experiment done, the fluorescent tube is rotated in the electric field. So the electric field acts like AC due to change in the direction of the tube. So if one rotates the tube quickly, before the previous glow disappears, one might see a continuous glow. These are the two reasons i could think of. If you have a better explanation, please let me know.

Answer 5

@madhusudan mmmh, it may be but i don’t think that he rotates the tube fast enough. Also he rotates only 180º and then returns, so the electrons would go right, then no movement of electrons, then right again (or if you talk about a positive vandergraph left-no movement-left). But most of all I don’t think he rotates fast enough.
I think that the air ionizes and that’s where the electrons come from and also is where they go to (in the other side of the tube), but I’m not sure. And even if so, I would like to know what would happen in vacuum, without ionized air.
Thanks for answering, I think I will also try in the 2010 course forum because I think is more active but please if you think something interesting, let me know.

Answer 6

There are always a few ionized atoms (thus electrons)in the gas within the tube from collisions with cosmic rays, etc which will be accelerated by the electric field. Collisions will excite other atoms which release light as their electrons transition back to lower energy levels. If you notice, the lights are turned off to view this - the number of emissions is less than you would see if it were connected in a circuit. Sufficient ionization takes place that the glow does not go out in an instant

Answer 7

@stan Are you sure? You may be right, but I still think that the ground should have something to do with the effect. And would you conclude that eventually the glow would turn of when all positive ions go from high electric field to low electric field and electrons go from low electric field to high electric field? I mean: if you only consider the electrons and ions within the tube eventually all electrons would be at one side and positive ions at the other, is like what I was answering madhusudan