Equipotentials around a charge Q

[ghostbuster25]

Hi there,

im having trouble finding the right equation for a problem I've 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 doesn't work and i think its because I am not putting in a value for $$\epsilon$$
as i don't know what it is.

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

many thanks

 

[kuruman]

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