Index exceeds number of array elements

I am trying to write up the code for solving the heat transfer equation for a sphere and i have receieved the following error message.

Index exceeds number of array elements

Index exceeds the number of array elements. Index must not exceed 1600.

I am new to coding and any help to fix this would be very much appreciated.

Answer 1

[KALYAN ACHARJYA]

I didnot find any coding syntax error, but code improvement is required.

r1=17.6e-2;   % inner radius of sphere, m
r2=23.6e-2;   % outer radius of sphere, m
k=0.034;   % thermal conductivity of sphere wall, W /(m C)
bc1=40;   % temperature at inner sphere wall, deg C
bc2=-81;   % temperature at outer sphere wall, deg C
n=100*2*2*2*2;   % number of unknown temperatures between the two radii
m=(r2-r1)/(n+1);   % step size: difference between two consecutive difference pointa, m
%%%%%%%%%% GENERATION OF MATRIX A5
for i=1:n
    r(i)=r1+i*m;  
    a(i,i)=-2*r(i)^2/m^2;
    if(i==1)
        a(i,i+1)=(r(i)^2/m^2 +r(i)/(m));
    end
    if((i>1)&&(i<n))
        a(i,i-1)=(r(i)^2/m^2 -r(i)/(m));
        a(i,i+1)=(r(i)^2/m^2 +r(i)/(m));
    end
    if(i==n)
        a(i,i-1)=(r(i)^2/m^2 -r(i)/(m));
    end
end
%%%% BOUNDARY CONDITION MATRIX GENERATION OF MATRIX B5
b(1,1)=-(r(1)^2/m^2 -r(i)/(m))*bc1;
b(n,1)=-(r(n)^2/m^2 +r(i)/(m))*bc2;
t=inv(a)*b;
temp(1,1)=bc1;
temp(2:n+1,1)=t(1:n);
temp(n+2,1)=bc2;
radius(1,1)=r1;
radius(2:n+1,1)=r(1:n);
radius(n+2,1)=r2;
figure,
plot(radius,temp)
xlabel('Radius, m','fontsize',20,'fontweight','b')
ylabel('Temperature, (^{o}C)','fontsize',20,'fontweight','b')

Answer 2

[Torsten]

Maybe it's interesting to compare with the analytical solution:

T(r ) = a/r + b

where

a = 121/(1/0.176 - 1/0.236)
b = 40 - a/0.176