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Voltage of an Isolated Conductor from an Electric Field

EANDM-MYG4ZC

An isolated conducting sphere of radius $10 \,cm$ is hooked up to a voltage source by a thin conducting wire. It is charged up until the electric field at its surface reaches the field at which dry air breaks down and creates a spark. This field is approximately $30 \, kV/cm$.

What was the voltage on the sphere when it created this field that broke down the air?

A

$30 \, V$

B

$300 \, V$

C

$300\, kV$

D

$3 \, MV$