When the potential on the surfaces are different then we can calculate the point where potential is minimum by differentiation.  So we would like to find out xm at which dψ/dx = 0. On differentiating equation (9.39) and put it equal to 0. We get:

From this we get a relationship for x_m as

(9.40)

Then ψ_m can be calculated as:

(9.41)

(9.42)

Now if we recall from equation (9.33) we have the expression for force as



so we need to calculate ψ_m2 to get the value of FR.