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1. The question is 'a Van de Graaff generator is charged with 2.5 x 10^-7 coulombs in a school lab demonstration. The spherical metal dome has a diameter of 30cm.

a) what is the electric field strength at a distance of 5cm from the surface of the charged dome?
b) what is the electric potential at a distance of 35cm from the surface of the charged dome?

I know that the answers are a) 56000 vm^-1 and b) 4500V but I don't understand how to get there!! Please help xx
2. (Original post by vicsjaggers)
The question is 'a Van de Graaff generator is charged with 2.5 x 10^-7 coulombs in a school lab demonstration. The spherical metal dome has a diameter of 30cm.

a) what is the electric field strength at a distance of 5cm from the surface of the charged dome?
b) what is the electric potential at a distance of 35cm from the surface of the charged dome?

I know that the answers are a) 56000 vm^-1 and b) 4500V but I don't understand how to get there!! Please help xx
The metallic dome of the Van de Graaff generator can be considered as uniformly charged spherical shell. For the electric field, draw a Gaussian sphere of radius 20cm (30 cm diameter so 15 cm radius and add 5 cm to get 20 cm). And then use the famous equation for electric field. KQ/r^2 to get the electric field equal to 56,250 V/m.

For electric potential use K Q/r with r = 50 cm to get 4500 V.

Hope this helps.
3. (Original post by tangotangopapa2)
The metallic dome of the Van de Graaff generator can be considered as uniformly charged spherical shell. For the electric field, draw a Gaussian sphere of radius 20cm (30 cm diameter so 15 cm radius and add 5 cm to get 20 cm). And then use the famous equation for electric field. KQ/r^2 to get the electric field equal to 56,250 V/m.

For electric potential use K Q/r with r = 50 cm to get 4500 V.

Hope this helps.
Thank you so much for your help!! For some reason it didn't click in my head that you have to add the distance from the surface of the dome to the radius... but it makes a lot more sense now haha!

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