For loop and plotting trouble

I need to plot the results of a physics function called the Kohler equation... I tried setting up a loop for a given range of the variable r as such:

m=10^-15;

for r=0.1:100;
S(r)=(exp((2*s)/(n*k*T*r))*((1+((a*m*Mw)/(Ms*(4/3)*pi*(r^3))*p-m)))^-1);
end

SS(S)=100*(S-1);
semilogr (SS)
xlim([0.1 100])
ylim([-1.5 0.5])

> So, the first issue I run into is an error in the function itself:

Attempted to access (0.1); index must be a positive integer or logical.

I seem to remember that there's a simple fix for this, like adding a period somewhere, but I'm not sure where.

> The second issue is that I need to plot this for 5 different values of m: 10^-15, 10^-16, 10^-17, 10^-18, and 10^-19. Would it be practical to set up an additional loop for this? The intervals are different for each value, so I wasn't sure how to do this.

> And then the final issue is that I need to plot r on the x-axis, scaled logarithmically, vs. SS on the y axis, with a linear scale. I'm not sure if I have that set up right...

I'm relatively inexperienced (and also out of practice) with Matlab, so I apologize if some of my questions seem elementary.

Answer 1

[Image Analyst]

If you want to iterate over r, then you'll need a counter as the index for S, like this

sIndex = 1;
for r=0.1:100;
  S(sIndex )=(exp((2*s)/(n*k*T*r))*((1+((a*m*Mw)/(Ms*(4/3)*pi*(r^3))*p-m)))^-1);
  sIndex  = sIndex  + 1;
end

For the second issue, you could have a second, outer loop to loop over the various values of m. You'd want to plot S before the "end" of this outer m loop, unless you want to make S a 2D array.

For the third issue, you can plot r vs. S and it should look right.

plot(0.1:100, S, 'b-');

Answer 2

[Mike]

Thanks for the help. I updated my code, but now I'm running into more trouble. My output vectors for S and Su are empty...Which may be why my plot is now not even loading.

Here's what I have now:

for m=10^-15:10^-1:10^-19;
sIndex=1;

for r=0.1:100;
S(sIndex)=(exp((2*s)/(n*k*T*r)))*((1+((a*m*Mw)/(Ms*((4/3)*pi*(r^3))*p-m))).^-1);
sIndex=sIndex+1;
Su=100*(S-1);
end

semilogx(0.1:100, Su);
ylim([-1.5 2.5])
legend('1','2','3','4','5')
xlabel('Droplet Radius (µm)')
ylabel('Supersaturation (percent)')
title('Kohler Curves for Solution Droplets Containing (NH4)2SO4')
end

[Image Analyst]

Please edit the code to supply typical values for n, k, T, etc. - whatever we need to make this code snippet run.

Su should be a single scalar value since you didn't make it an array like you did for S. Use Su(sIndex) if you want it to be an array.

[Mike]

I forgot to mention that I did define values for all the other variables in my code, I just didn't post it here to save space.

Su does need to be an array, because what I have to plot is that value and not S. I will update my code and see if it works.

[Mike]

After making the changes, I'm not even getting the S and Su matrices to output now, and yet I'm not getting any errors.

[Image Analyst]

So you don't want to make it easy for us to help you by posting typical values for n, k, T, etc. like I asked? Strange. It would be quicker if you helped.

Define what you mean by "output" - does that mean plot, like your call to semilogx() doesn't do anything whatsoever??? If you set a breakpoint right after the loop, are S and Su there?

Answer 3

[Mike]

% Define values and constants
s=75.64*10^-3; % Sigma (Interfacial surface energy) [J m^-2]
n=3.348*10^28; % # H2O molecules per unit volume of liquid [molecules/m^3]
k=1.38*10^-23; % Boltzmann constant [J K^-1 kg^-1]
Ms=0.13214; % Molecular weight of (NH4)2SO4 [kg/mol]
Mw=0.018; % Molecular weight of H2O [kg/mol}
T=293; % Temperature [K]
p=1769; % Density of (NH4)2SO4 [kg m^-3]
a=3; % Number of dissociations of (NH4)2SO4

The rest of the code is what I have above, just with Su changed to Su(sIndex). When I run the entire code, the vectors S and Su do not appear in the workspace. If I just run the inner loop, I get the following error:

 In an assignment  A(I) = B, the number of elements in B and
 I must be the same.

Answer 4

[Nur Fabien Idrissa]

hello Mike!

how did you solve the question. may you share with me the process.

thanks!