# Doubt in MATLAB coding.

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Joy Salomi on 4 Mar 2020
Edited: Joy Salomi on 6 Mar 2020
Hello. I'm trying to plot a differntial equation using MATLAB. When I run my program, I'm getting different graph.
I've attached the differential equation and my MATLAB coding below.
Can anyone tell me how to use the boundary conditions correctly?
function abcd
ex4init=bvpinit(linspace(0.00001,100,21),[0,0]);
sol=bvp4c(@ex4ode,@ex4bc,ex4init)
plot(sol.x,sol.y(1,:),'blue');
end
function f=ex4ode(r,y)
a=0.001391304348;
b=19.31459475;
f=[y(2)
-(2/r)*y(2)-a*(1-exp(b*y(1)))
];
end
function res=ex4bc(ya,yb)
res=[
ya(1)+1
yb(2)
]
end
This is the graph which I got.
But this is the exact graph.
##### 2 CommentsShowHide 1 older comment
Joy Salomi on 4 Mar 2020
Hello Sir,
If I change x axis range to 1e-9 m, I didn't get any graph. Plot is empty.
What shall I do?

Robert U on 5 Mar 2020
Edited: Robert U on 6 Mar 2020
Hi Joy Salomi,
Thanks for the paper. The differential equation should be written in spherical coordinates (edit: I think there is a mistake to assume R to be constant). Since R is assumed to be constant there is no singularity. I adjusted the code and tested it in Matlab 2019b. Depending on chosen xmesh results might differ. You have to try a bit to get a stable solution. I attached an example for the D = 200 nm graph.
abcd.m
function abcd(ah,D,phi_s)
ex4init=bvpinit(linspace(0,D/2,3e3),[phi_s 0]);
sol=bvp4c(@ex4ode,@ex4bc,ex4init);
plot(ah,sol.x*1e9,sol.y(1,:));
ah.XLabel.String = 'r [nm]';
ah.YLabel.String = 'phi [V]';
function dphidr=ex4ode(r,phi)
a = -1.602176634e-19*1e17*(1e-2)^(-3)/(8.8541878125e-12*11.5);
b = 1.602176634e-19/(physconst('Boltzmann')*600);
dphidr = [ phi(2)
a * ( 1-exp( b*phi(1) ) ) ];
end
function res=ex4bc(ya,yb)
res=[ ya(1)+ phi_s
yb(2) ];
end
end
Example call:
fh = figure;
ah = axes(fh);
hold(ah,'on')
arrayfun(@(dIn) abcd(ah,200e-9,dIn),[0.1,0.3,0.7,1])
Result:
Kind regards,
Robert
Joy Salomi on 6 Mar 2020
Hello Sir,
The code you sent is working.
Thank you so much for your help Sir. :)

This is a bit hard to solve without knowing more of the problem. What I would do is to break down the problem and analyse each step. For example, look at the values you have in the solution "sol". I plot separately:
>> plot(sol.x);
>> plot(sol.y);
And also
>> plot(sol.y');
Is anything here remotely connected to what you are expecting? My guess is that no, sol.y has values between 0 and 12e4 whilst the graph you want is between -1 and 0. So the problem is not really about Matlab but about the equations that you are using.
Hope this helps.
Joy Salomi on 4 Mar 2020
Thank you for your reply Sir. But my doubt is somewhat different. I'm trying to get the exact curve which is already available.
This is the curve I want.

Robert U on 4 Mar 2020
Hi Joy Salomi,
the implementation of a boundary value problem with singular term has to be done a bit different than you did.
function abcd
ex4init=bvpinit(linspace(0,100,5),[-1 0]);
S = [0 0; 0 -2];
options = bvpset('SingularTerm',S);
sol=bvp4c(@ex4ode,@ex4bc,ex4init,options);
plot(sol.x,sol.y(1,:),'blue');
end
function f=ex4ode(r,y)
a=0.001391304348;
b=19.31459475;
f = [ y(2)
-a*(1-exp(b*y(1))) ];
end
function res=ex4bc(ya,yb)
res=[ ya(1)+1
yb(2) ];
end
Using the values "a" and "b" you supplied still leads to a singular Jacobian matrix which provoces an error. Changing both these values to one leads to a solution that qualitatively corresponds to the curves drawn.
There are two more points that hinder me to test my solution:
1. I don't know how "a" and "b" translate to "phi_s".
2. I still assume that r is given in SI units. If r is in unit [m] the interval needs to be adjusted to [nm] as shown in your graph.
Kind regards,
Robert
Joy Salomi on 5 Mar 2020
Hello Sir,
I have attached the PDF for your reference.
a and b are combination of some parametrs.
By subtituting the values of the parameters, I have found the values of a and b.