added introduction to q_k

tex/methods.tex

@@ -247,7 +247,7 @@ \subsubsection{Analyte-sensor electrostatic potential under an electric field}
 \epsilon_3\frac{\partial\phi_{3s}}{\partial \mathbf{n}} - \epsilon_2\frac{\partial\phi_{2s}}{\partial\mathbf{n}} &= (\epsilon_2-\epsilon_3)\frac{\partial\phi_i}{\partial\mathbf{n}} \quad \phi_{3s} = \phi_{2s} \quad \text{on $\Gamma_2$}.
 \end{align}
 %
-
+where $q_k$ are the point charges of the atoms inside the protein.
 
 \paragraph{Integral formulation}