# Homework Help: Equipotentials around a charge Q

1. May 19, 2010

### ghostbuster25

Hi there,

im having trouble finding the right equation for a problem ive got.

I have a point charge with an equipotential of 40v at 0.4cm. I am suppossed to work out the value of the charge.

Ive tried

q/4$$\pi$$$$\epsilon$$r

but it doesnt work and i think its because im not putting in a value for $$\epsilon$$
as i dont know what it is.

cols anyone tell me if a, im using the right equation and b, hot to find or put in the value of $$\epsilon$$

many thanks

2. May 19, 2010

### kuruman

You can use the value 1/4πε0 (the whole thing) = 9x109 N m2/C2. Don't forget to convert centimeters to meters.