# Why the light increases when Prof. Lewin touch the end of the tube?

1. Jun 15, 2013

### anhnha

Hi,
Can you explain why the light increases when Prof. Lewin touch the
end of the tube with his finger?
He explained it but I am not quite understand it.
The video is here at 45:40 : http://www.youtube.com/watch?v=ldJhMDuOGxY

2. Jun 15, 2013

### Staff: Mentor

he's touching the tube and so the potential where he touches is at 0 and so more current flows and the light is much brighter.

if you recall just before he did the experiment he said the potential at the close end was 100K v and at the middle 50K volts and at the far end 30k volts but when he touches the far end the 30k becomes 0k v so that the potential diff across the tube is 100K volts whereas when he didn't touch it the potential was 70K volts.

3. Jun 15, 2013

### anhnha

Thanks jedishrfu!
How can you know the potential of the Earth is zero volt?
And when he doesn't touch the tube how electron move after go through the tube?
I think it will go through the air but does it come back to the tube?

4. Jun 15, 2013

### Staff: Mentor

Its zero by definition:

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

Actually the Van degraff generator globe is positively charged so electrons are travelling from the ground thru the air to the tube endpoint.

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

Last edited: Jun 15, 2013
5. Jun 15, 2013

### anhnha

Hi,
Is it mean that in this case there is a closed loop: Van degraff generator globe - air - tube - air - ground - air - Van degraff generator globe?

6. Jun 16, 2013

### Staff: Mentor

yes although like lightning there could be multiple paths like ground - air - van degraff globe that aren't significant enough to be seen.