Electrostatic energy of two opposite charges in water and in a vacuum

[JordanGo]

Homework Statement

Compare the electrostatic energy of two opposite charges e and -e, a distance 7 angstroms apart in water at room temperature and that in vacuum (express the energy in terms of Bjerrum length)

Homework Equations

E = 1/(4(p\pi\epsilonD)*(-e^2)/r^2 ?

The Attempt at a Solution

First of all is this the right equation to use? If so, is the Bjerrum length the distance between the charges? And for a vacuum is the dialectic constant (D) just 1?

 

[szynkasz]

Electrostatic energy is:E=-\frac{e^2}{4\pi \varepsilon D}
Bjerrum length:
\lambda_B=\frac{e^2}{4\pi \varepsilon k_BT}

 

[JordanGo]

So, for water:
E=\lambdaKT/80?

And for a vacuum:
\lambdaKT?

 

[szynkasz]

No, we have:
\lambda_B k_BT=\frac{e^2}{4\pi\varepsilon}
so put it into formula for energy

 

[JordanGo]

I did do that, D=80 for water and D=1 for a vacuum

 

[szynkasz]

In my formula "D" denotes distance

 

[JordanGo]

Ok that makes sense, so D is dialectric constant and let's say r now is the distance, which is missing in the equations.