fix bug in composite surface form model (#4293)

Author: rtimms

pybamm/models/submodels/electrolyte_conductivity/surface_potential_form/composite_surface_form_conductivity.py

@@ -99,6 +99,10 @@ def __init__(self, param, domain, options=None):
     def set_rhs(self, variables):
         domain = self.domain
 
+        a = variables[
+            f"X-averaged {domain} electrode surface area to volume ratio [m-1]"
+        ]
+
         sum_a_j = variables[
             f"Sum of x-averaged {domain} electrode volumetric "
             "interfacial current densities [A.m-3]"
@@ -116,7 +120,7 @@ def set_rhs(self, variables):
 
         C_dl = self.domain_param.C_dl(T)
 
-        self.rhs[delta_phi] = 1 / C_dl * (sum_a_j_av - sum_a_j)
+        self.rhs[delta_phi] = 1 / (a * C_dl) * (sum_a_j_av - sum_a_j)
 
 
 class CompositeAlgebraic(BaseModel):