Solve Electrical Work Problem: 15V to 5V

[punjabi_monster]

Hi there. I was attempting this problem but my answer is not agreeing with my friends.
He got 1.4 *10^-7
Any help will be appreciated.

Homework Statement

Calculate the work done on a system if its voltage drops from 15 V to 5 V.
edge length= 1 cm
water permittivity e=78.54e

Homework Equations

dw_elec = (elec pot.)dQ
Q=(elec pot)2ed
e=78.54 * e_o

The Attempt at a Solution

w = (elec pot.)f (Qf - Qi)
=(elec pot.)_f (2ed)[(elec pot.)_f - (elec pot.)_i]
=(5V)(2(78.54)(8.854*10^-12)(0.001)(5V-15V)
= -6.954 * 10^-11 J

 

[Borek]

What is the system in question?

 

[punjabi_monster]

its an hypothetical electrochemical cell.

 

[Borek]

Geometry? Edge length of 1 cm can be appliead to anything.