What is the maximum charge that can be put on a charged aluminum sphere?

[Frogmansailor]

I am doing some conceptual design, and was hoping to be able to get some answers here before I devote too much time to this.

I would like to be able to charge a piece of aluminum so that it holds a charge (negative) of 8 Coulombs. I was hoping to achieve this with something similar to a van der graaf generator, which would charge a spherical piece of aluminum which is encased in a strong insulator so that the charge is safely contained.

So my questions are:

How big would this piece of aluminum have to be?
Would a van der graaf type generator work for this application?
How fast would this conductor bleed it's charge?
What insulator would best reduce this loss of charge?

Thanks in advance
Dave

 

[Manchot]

It's not going to happen. The field at the surface of a sphere is Q/(4*pi*epsilon_0*R^2), and air breaks down at a field strength of about 1 MV/m. To put 8 Coulombs on the sphere, you'd then need a sphere of a radius of 268 m to prevent the air around it from breaking down. 8 Coulombs is a lot of charge.

 

[Frogmansailor]

Well the dielectric breakdown of air being so low is the reason I want to surround it with a better insulator, so that isn't accurate.

I realize 8 C is a ton of charge, I was just hopeful I guess. What would the maximum charge that one could put on a sphere about the size of a basketball then?