Iteration of Ideal Gas Law

Hi,

How can I do the iteration for the follwoing Ideal Gas Law? I would like to use the Vn and get the Pn value and compare the Ln value to the L. When Ln and L are very close, it will stop the iteration. I would really appreciate your help.

P=101325; 
R=8.314;  
r=20*10^-6;  
L=117*10^-6;  
V=pi*r^2*L;  
T=300;  
n=(P*V)/(R*T); 

% Iteration should start from here
Tn=300+273;  
Pn=(n/V)*R*Tn;
stress=Pn;
E=69.22*10^9;  
S=stress/E;  
dL=S*L;  
Ln=L+dL;
Vn=pi*r^2*Ln;

[Walter Roberson]

Please note that I am not a category of knowledge, and so should not be named in a tag.

Answer 1

[Cris LaPierre]

Iterations can be done using either for loops or while loops.

If you want to repeat until a condition is met, a while loop is probably the best option. You just need to set up a conditional statement that evaluates as false until your statement is true. For example

L=1;
Ln=5;
while abs(Ln-L)>0.1
    Ln=Ln-1
end

Ln = 4

Ln = 3

Ln = 2

Ln = 1

There is one thing to be careful of. It is possible to create what is called an infinite loop. This occurs when your contidional statement is always false.

[Sohel Rana]

Thank you for your response. How can I use the new Vn value every time? Note that Pn depends on Vn and vice-versa. The V will be replaced by Vn every time.

Tn=300+273;

Pn=(n/V)*R*Tn;

stress=Pn;

E=69.22*10^9;

S=stress/E;

dL=S*L;

Ln=L+dL;

Vn=pi*r^2*Ln;

[Cris LaPierre]

I would suggest setting Vn equal to V before the loop starts, and then only use Vn inside the loop. Each loop will use the previous value of Vn. It would be something similar to what I did with Ln in my previous example.

...
Vn = V;
while abs(Ln-L)>0.1
    Pn=(n/Vn)*R*Tn;
    Vn=pi*r^2*Ln;
end

[Sohel Rana]

Thank you so much. In the while loop, there is no value for Ln. How the loop will know about the Ln value?Should I set Ln=L before the loop?

[Cris LaPierre]

I haven't given you the whole solution. In the code you shared originally, you calculate Ln right before you use it.

Ln=L+dL;
Vn=pi*r^2*Ln;

If you need to use the current value of Ln as L in the next loop, then yes, you should set Ln=L before the loop, and update all uses of L inside the loop to be Ln.

If you don't understand how loops work, I suggest looking at Ch 13 of MATLAB Onramp. It covers for loops, not while loops, but the underlying principles are the same.

[Sohel Rana]

Thank you!