Plotting a simple coverage equation

I am by no means an experienced Matlab user and simply want to simulate the predictions of an expression 1/Rtot=(1-theta)/Ra + (theta/Rb) where theta is an array of coverages and Ra and Rb specified resistors. I am using Rtot=(Ra./(1-theta))+(Rb./(theta)) but this is giving wildly wrong Rtot values so the expression must be formulated incorrectly. Any advice hugely well one - have stared at this for way too long and cant see the fault

Answer 1

[Jason]

OK, thats embarrassing....comes from staring at the screen too long...I am very grateful

[Ameer Hamza]

You are welcome.

Answer 2

[Ameer Hamza]

This is not an error with MATLAB syntax. You are using the mathematics wrong. What you are trying to do means

if 1/c = 1/a + 1/b => c = a + b

does this equation look correct? The correct is

c = a*b/(a+b);

So to correct your equation in MATLAB use,

Rtot= 1./((1-theta)./Ra + (theta./Rb));

Answer 3

[Jason]

Hi - I am using this:

clear all theta=linspace(1e-10,1e-2,5e3) %an array of fractional coverage data points Rn=300 % resistance per unit surface area for the normal film hundreds kOhm cm2 Rd=3e-6 % resistance per unit area for defects 3 Ohms/cm2 Rct= 1./((1-theta)./Rn+(theta./Rd)); %for low theta Rct=Rn plot(theta,Rct,':r*') xlim([1e-8 1e-4]) ylim([0 400])

This is working fine in the sense that I think everything is correct mathematically BUT this should be displaying as a sigmoid and it is not. I cant seem to get the output I expect by changing either the xlim, ylim or linespace options.