dipole in electric field, capacitor

[burak_ilhan]

a parallel plate capacitor is filled with a layer of distilled water of 0.3cm tihck. The dipole moment of a water molecule is 6.1*10^(-30) C.M. Assume that the dipole moments of the water molecules are all perfectly aligned with the electric field. What is the surface charge density of bound charges on the surface of the layer of water.(dielectric constant of water:80).

Well, I solved lots of problems like that except with a dipole moment. I dont know where to use that. I tried to use some energy relations but no results. Thanks for any help.

[himanshu121]

surface charge density \sigma
\sigma _{bounded} = \sigma _{free} [ 1-\frac{1}{\epsilon _{r}}]

[burak_ilhan]

Thanks but I know that formula, and I bet everybody knows... I need a way to solve the problem.:)