Unable to solve for a variable

2 views (last 30 days)
Hi all,
I am trying to solve for a variable 'a' and saving it into an array, However, I am left with an inifinitely long running program. What should I do?
Heres a section of the code:
clc, clear, clear all
J_value=0.5
syms a;
for TonTc=.01:0.01:.99
sig_sigo_1=((2*J_value+1)/2*J_value)*coth((2*J_value+1)/2*J_value)*a-(1/2*J_value)*coth(a/2*J_value);
sig_on_sigo_2=((J_value+1)/3*J_value)*TonTc*a;
z=sig_sigo_1-sig_on_sigo_2;
for k=1:99
val_a(k)=vpasolve(z,a);
end
end
Cheers

Accepted Answer

Walter Roberson
Walter Roberson on 7 Oct 2021
J_value=0.5;
syms a;
TonTc_vals = .01:0.01:.99;
num_Tonc = length(TonTc_vals);
val_a = zeros(num_Tonc, 1);
for k = 1 : num_Tonc
TonTc = TonTc_vals(k);
sig_sigo_1=((2*J_value+1)/2*J_value)*coth((2*J_value+1)/2*J_value)*a-(1/2*J_value)*coth(a/2*J_value);
sig_on_sigo_2=((J_value+1)/3*J_value)*TonTc*a;
z=sig_sigo_1-sig_on_sigo_2;
sol = vpasolve(z, a, -1);
if isempty(sol)
val_a(k) = nan;
else
val_a(k) = sol(1);
end
if k == 1
fplot([z, 0], [0.5 1])
title("plot for TonTc = " + TonTc)
ylim([-2 2])
end
end
figure
plot(TonTc_vals, val_a)
title("a vs TonTc")
That first plot makes it clear that there is no root at 0.75
  5 Comments
Jonas Freiheit
Jonas Freiheit on 10 Oct 2021
Yes, Sorry How would I plug in the second solutions?
Just trying to test to see if that gives me the theoretical results. This is the equation I'm supposed to follow to obtain sigma/sigma0 which I think I've correctly used in the code.

Sign in to comment.

More Answers (1)

David Hill
David Hill on 6 Oct 2021
What are you solving? You need to set z equal to something.
z=sig_sigo_1-sig_on_sigo_2==0;%what do you want z to be?
  3 Comments
Jonas Freiheit
Jonas Freiheit on 7 Oct 2021
Hi,
For some reason when I do z==0 and do vpasolve(z,a) for TonTc=0.1 I get 0.972 when the actual correct answer is 0.75 from my calculator? Howcome is this wrong?
Thanks
Also solve doesn't work and has to be vpasolve

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!