Electrostatic potentials

Suppose that we are given the electrical potential in the boundaries of some region, and we want to find the potential $V(\mathbf{r})$ inside. Once we know $V$ we can use the relation $\mathbf{E}=-\nabla V(\mathbf{r})$ to determine $\mathbf{E}$. This is a “boundary problem”. We know from classical electrodinamics (Gauss’ Law) that the potential $V(x,y,z)$ satisfies Poisson’s equation:

$$\nabla ^2V(x,y,z)=-\frac{\rho(x,y,z)}{\epsilon 0} $$

with $$\nabla ^2V = \frac{\partial ^2V}{\partial x^2} + \frac{\partial ^2V}{\partial y^2} + \frac{\partial ^2V}{\partial z^2},$$ and $\rho$, the charge density inside the region; the constant $\epsilon _0$ is the electrical permittivity of the vaccum. Besides satisfying, $V$ has also to satify the boundary conditions. Since the shape of the region can be arbitrary, we need to use the computer to solve the problem. In the particular situation that there are no charges inside the region, the problem reduces to solving “Laplace’s equation”: $$\nabla ^2V(x,y,z)=0. $$ As we did in the previous examples of PDE’s, we discretize the derivatives and we transform the equation to a finite difgferences problem. From now on we worry only about the two-dimensional case. We discretize the region in a grid with points $(x,y)$ separated by intervals $(\Delta x, \Delta y)$. The second derivatives are approximated by:

$$\begin{aligned} \frac{\partial^2 V(x,y)}{\partial x^2} &\simeq& \frac{V(x+\Delta x,y)+V(x-\Delta x,y)-2V(x,y)}{(\Delta x)^2}, \\ \frac{\partial^2 V(x,y)}{\partial y^2} &\simeq& \frac{V(x,y+\Delta y)+V(x,y-\Delta y)-2V(x,y)}{(\Delta y)^2}. \end{aligned}$$

Replacing these expressions in ([laplace]) and choosing $\Delta x = \Delta y = \Delta$ we obtain $$V(x,y) \simeq \frac{1}{4}[V(x+\Delta, y)+V(x-\Delta,y)+V(x,y+\Delta)+ V(x,y-\Delta)]. $$ That is, the value of $V(x,y)$ is given by the average of the values in the neighboring nodes in the grid.

Units

Notice that the charge of the electron is $e=1.6 \times 10^{-19} C$. From a microscopic point of view, the Coulomb is a very large unit of charge. Hence, we expect that for many simulatioons we will have to choose different units.

The finite differences algorithm

Suppose that we are given the values of $V(x,y)$ in the boundaries of a rectangular area. If we divide the rectangle in a grid, we can the apply [laplace_diff] to calculate the values of $V$ inside. However, we notice right away that if we apply this equation in a point in the middle, we get zero as a result. So the first obvious observation is that the initial estimate for the method can not be a constant value. It is evident now that the procedure has to be iterative. We start from the bottom-left corner, and we sweep across the grid. As we obtain the first estimate for $V$, we repeat the sweep until the values of $V$ do not change considerably, and we can say that we have converged with certain accuracy. Here we have started with the initial values $V(x,y)$ inside the grid that in principle can be any initial guess (exept a constant, as we have seen). The rate of convergence will be determined by the proximity of this guess to the actual solution.

The algorithm can be decribed as follows:

1. Divide the rgion of interest into a grid. The region must be enclosed by boundaries with known values of $V$.

2. Assign to all points of the boundary the given values, and all interior cells an arbitrary potential (preferably a reasonable guess)

3. sweep the internal cells by rows or columns and calculate the new values of $V$ as the average in the four neighboring points.

4. Repeat the sweep using the values obtained in the previous iteration, until the potential does not change within certain level of accuracy.

Exercise 7.1: Verification

1. Assume that the source of the potential is a point charge of $1\mu C$ at the origin. We know that the potential is going to be given by: $$V(x,y)=\frac{1\mu C}{\sqrt{x^2+y^2}} $$ Choose a reasonable value of $\Delta$ and compare the values of $V(x,y)$ given by ([testV]) in the center of one, and the average of $V$ in the four neighboring points (obtained also with Eq.([testV])). Repeat for 5 different cells at different distances from the origin. Does the relative agreement depend on the distance of the cell to the origin?

2. Repeat part 1 with smaller values of $\Delta$. Are the results in better agreement?

Exercise 7.2: Numerical solution inside a rectangular region

1. Determine the potential $V(x,y)$ inside a square region with linear dimension $L=10cm$. The boundary of the square is at a fixed potential $V=10V$. Before the computation, guess the exact form of the potential and set the values in the interior within the area of each cell to be 1cm$^2$. Iterate until you achieve a 1% accuracy.

2. Consider the same situation, but with $V(5,5)=4$. Describe the evolution of the solution with the iterations. Does the potential distribution evolve o the correct solution? Are the final results independent of your initial guess? What is the effect of a poor initial guess? Note that if you pick a constant values as the intial guess, the result does not evolve toward a solution.

3. Set each side of the rectangle at a different potential 5,10,5,10, repectively. Do a contour plot and sketch the equipotential surfaces. What happens if the potential is 10V on three sides and 0 on the fourth? Start with a reasonable guess for the initial values of the potential of the interior points, and iterate until you obtain a 1% accuracy.

4. Repeat the previous item with a twice the number of cells, and compare the results.

Exercise 7.3: Capacitance of concentric squares

1. Modify your code to treat a concentric square boundary, *i. e.*, a square centered inside a bigger square. The potential of the outer square is 10V and the inner square is set at 5V. The linear dimensions of the two squares are $L_1=5$cm and $L_2=25$cm respectively. Choose a convenient grid. Plot the equipotential lines.

2. A system of two conductors with a charge $Q$ and $-Q$ respectively has a capacitance $C$ which is defined by the formula: $$C=\frac{Q}{\Delta V},$$ where $\Delta V$ is the potential difference between the two conductors. The charge can be determined by the expression: $$\sigma = \frac{{\bf E \cdot n}}{\epsilon _0} = \frac{E_n}{\epsilon _0}$$ where $\sigma$ is the surface charge density, and $E_n$ is the component of the electric field normal to the surface and can be approximated by $$E_n = \frac{-\Delta V}{\Delta r},$$ where $\Delta V$ is the potential difference between a boundary cell, and the adjacent interior at a distance $\Delta r$. Use the results of the previous part to calculate $\Delta V$ for each point next to the square surfaces. Use this information to obtain an estimate of $E_n$ for the two surfaces, and the charge per unit of length on each electrode. are the two charges equal and opposite in sign? Calculate the capacitance in Farads.

3. Move the inner square 1cm off center and repeat the calculations of parts 1. and 2. How do the equipotential surfaces change? Is there any qualitative difference if we set the center conductor potential to -5V instead of +5V?

Challenge 7.1:

Exercise 7.3, parts 1) and 3)

Exercise 7.4: Poisson’s equation

Laplace’s equation holds for charge free regions. If there is a charge distribution inside the region we must use Poisson’s equation [poisson] instead.

1. Generalize the difference equation for the Poisson’s equation, and modify your program accordingly.

2. Consider a square of linear dimension $L=25cm$ fixed at 10V. assume that each interior cell has a uniform charge density $\rho$ such that $\frac{1}{4}A\rho/\epsilon _0 = 3V/cm^2$. Compute the potential distribution for this case.

3. Find the potential distribution if the charge distribution of part 1) is restricted to a square of side $L=5cm$ in the center.

Challenge 7.2:

Exercise 7.4