- #1

m84uily

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## Homework Statement

A spark occurs at the tip of a metal needle if the electric field strength exceeds 3.0x10^6 N/C.

What is the minimum surface charge density for producing a spark?

## Homework Equations

int(EdA) = (Qin) / e0

## The Attempt at a Solution

I imagine the surface as a cylinder with the electric field going parallel to it.

int(EA) = backface + side + frontface = EA + EA = 2EA

E = 3.0x10^6 N/C

A = (pi)r^2

(2)(3.0x10^6 N/C)(pi)r^2 = (Qin) / e0

(e0)(2)(3.0x10^6 N/C)(pi)r^2 = Qin

Then, I don't want r is! I've probably gone about solving this all wrong, thanks in advance for your help.