Error using / Matrix dimensions must agree.

Script is: P1(i)=((R*T1)/(V-b))-(a/(V^2));

I am using the loop function to solve for V. Everything else besides P is a given constant.

Message is: Error using / Matrix dimensions must agree. Please help!

Answer 1

[Star Strider]

I don’t understand ‘using a loop function to solve for V’. Is ‘V’ a vector?

That ‘everything else besides P is a given constant’ would imply that they are scalars.

If you vectorise your ‘P1’ assignment, either the problem will disappear, or your will get a ‘Subscripted assignment dimension mismatch.’ error:

P1(i)=((R.*T1)./(V-b))-(a./(V.^2));

[Kevin]

To me it looks like she's trying to solve for the volume, V of a Van der Waal's gas at variable pressure.

[Star Strider]

Agreed. But we don’t have the necessary information to know what the problem is with her code.

[Karina Reynolds]

Heres the full code:

T1 = 310; %@K

T2 = 510; %@K

T3 = 810; %@K

a = 1.337;

b= 0.03200;

R=0.0821;

for i = 1:10

    V(i)=1.5*i;

end

for i = 1:10

P1(i)=((R*T1)/(V(i)-b))-(a/(V(i)^2));

P2(i)=((R*T2)/(V(i)-b))-(a/(V(i)^2));

P3(i)=((R*T3)/(V(i)-b))-(a/(V(i)^2));

end

plot(P1,V,P2,V,P3,V)

xlabel('Vm (dm^3mol^-1')

ylabel('Pressure (atm)')

title('p-Vm function/KR')

legend('VDW_1','VDW_2', 'VDW_3')

[Star Strider]

One of MATLAB’s strengths is its ability to use vectorised code.

This works:

T1 = 310; %@K
T2 = 510; %@K
T3 = 810; %@K
a = 1.337;
b= 0.03200;
R=0.0821;

i = 1:10;
V = 1.5*i;

P1 = ((R*T1)./(V-b))-(a./(V.^2));
P2 = ((R*T2)./(V-b))-(a./(V.^2));
P3 = ((R*T3)./(V-b))-(a./(V.^2));

figure(1)
plot(P1,V,P2,V,P3,V)
xlabel('Vm (dm^3mol^-1')
ylabel('Pressure (atm)')
title('p-Vm function/KR')
legend('VDW_1','VDW_2', 'VDW_3')

My variation of your code uses the dot-operator to do element-wise calculations. See Array vs. Matrix Operations for the fascinating details!