Van de graff generators maximum voltage

[abdo799]

Why do van de graff generators have a maximum voltage( charge on the sphere ) .
Why can't they just accumulate charge on the sphere as large as we desire?

 

[Bobbywhy]

Hi abdo799! I am surprised to find you asking such a question here! What have you done to find the answer so far? No member here on Physics Forums wants to "spoon feed" any questioner.

Here on Physics Forums your question has been asked several times recently. The empty white rectangular box at the upper right of this page with the little magnifying glass symbol on the right allows one to enter key search words to find relevant posts.

https://www.physicsforums.com/showthread.php?t=468730
https://www.physicsforums.com/showthread.php?t=696707

I used Google to search and found many clear answers for you in approximately two minutes seventeen seconds. Please learn to use Google, it's your friend. Once you get the basics, if you have doubts or need some explanations, then return here and post your questions.

"The maximum achievable potential is approximately equal to the sphere's radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV."
http://en.wikipedia.org/wiki/Van_de_Graaff_generator