Several Issues with Van der waals function used to plot P against V
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Primary issue- With the for function- Illegal use of reserved keyword "for".
Matlab Script:
function [P,V]=ISOTHERMS_VDWTest(T,Pc,Tc,vmax)
R=8.314;
b=(R*Tc)/(8*Pc);
a=(27/64)*(R^2*Tc^2)/Pc;
v(i+1)=v(i)+v(i)*0.01
v(1)=b+b*0.01
i=1
while v(i)<vmax
v(i+1)=v(i)+v(i)*0.01
i=i+1
end
v=[v(1),v(2),.....v(i)]
for J=i=1
P(J)=(R*T/V(J)-b)-(a/V(J)^2)
end
P=P/1000000
v=v*1000
plot(v,P)
end
********************
Any other issues or easier way to do it is also appreciated.
1 Commento
dpb
il 9 Ago 2018
Modificato: dpb
il 9 Ago 2018
for J=i=1
has two "=" signs instead of a colon for a range expression. Not sure what you had in mind???
Also not sure what your intent/thought is here; you already have a vector v that was made by the while loop.
It'll be a real nuisance that you have left of the trailing ; on so many lines to have all that output every time...
v=[v(1),v(2),.....v(i)
Risposte (1)
Pieter Hamming
il 9 Ago 2018
Modificato: Pieter Hamming
il 9 Ago 2018
Part one: problems with code in the current form
v=[v(1),v(2),.....v(i)]
is incorrect syntax, as it is interpreted as
v=[v(1),v(2),
..v(i)]
v is already a row vector, so this row is redundant. Deleting it brings us to the second problem:
for J=i=1
As dpb pointed out, "|for J is i to 1|" has syntax
for J=i:-1:1
Also, i is used before it is defined. Row 5-7 need to be reversed
i=1 %define i
v(1)=b+b*0.01 %define v(1)
v(i+1)=v(i)+v(i)*0.01 %define v(2) based on v(1) and i
At row 14 you use capital V instead of lowercase v.
Fixing all of these gives me a bug-free code.
Part two: easier ways to do the same
please end your lines with a semicolon to suppress output
v can be rewritten:
v(i+1) = 1.01 * v(i); %original
v(i+1) = 1.01^i * v(1); %rewritten
Now, instead of a while loop over i, you can do it in a single calculation:
imax=5; %example
v(1)=b+b*0.01; %define the first entry
v=1.01.^(0:imax) * v(1); %single calculation to do all remaining values of v
I used imax in the above example; but you only have a vmax. Arithmatic to the rescue: we need to find the highest value imax for which v(imax)<vmax holds true.
v(imax) = vmax
1.01.^imax * v(1) = vmax
vmax / v(1) = 1.01.^imax
imax=log(vmax / v(1)) / log(1.01)
So knowing vmax and v(1), you can just calculate imax; no pesky while loops required. The imax provided by this will not be an integer, so use
imax=ceil(imax) % round-up to next integer
Next we have the calculation of P. This can be simplified using matrix-dot-multiplication (or element-wise multiplication).
P=(R*T./v-b)-(a./v.^2);
This does the same thing as the for loop in your code.
Combining all, we get this example code:
function [P,V]=ISOTHERMS_VDWTest(T,Pc,Tc,vmax)
%original code
R=8.314;
b=(R*Tc)/(8*Pc);
a=(27/64)*(R^2*Tc^2)/Pc;
v(1)=b+b*0.01;
%new code
imax=log(vmax / v(1)) / log(1.01);
imax=ceil(imax);
v=1.01.^(0:imax) * v(1);
P=(R*T./v-b)-(a./v.^2);
%original code
P=P/1000000;
v=v*1000;
plot(v,P)
end
Done. Sorry for the long post, but I hope this helps
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