Troubles with plotting two functions in one graph

I am having troubles with the following code:

function polarisatiecurve
P = 100
I = 0:0.0001:200;
U1 = P/I;
U2 = 800*(I+19)^2 - 2*(I+19)^3; 
plot(I,U1,'b',I,U2,'r',[0,100,0,600]);
subplot (1,2,1);
xlabel('Stroom (A)')
ylabel('Spanning (V)')
title('Polarisatiecurve')
axis([0 120 0 4000])
grid on
grid on
end

What I want to do is plot two functions in one graph. The terms 'polarisatiecurve, stroom, spanning' are Dutch and mean polarisation curve, current (stroom) and voltage (spanning). What I want is that it plots two functions in the same graph. Both functions depend on the current I (the 'stroom'). However I can't get it to work. It gave me the following error:

Error using / Matrix dimensions must agree.

Error in polarisatiecurve (line 4) U1 = P/I;

That refers to the following line of code:

I tried converting I to a symbolic expression but that didn't work either. Who knows how to fix it?

Answer 1

[Amit]

Few errors: like U1 = P/I. This is invalid operation as I is a matrix. To do this you use '.' operator. Similarly for ^, you will use .^ instead for the operations for matrixes.

function polarisatiecurve
P = 100;
I = 0:0.0001:200;
U1 = P./I;
U2 = 800*(I+19).^2 - 2*(I+19).^3; 
plot(I,U1,'b',I,U2,'r'); %,[0,100,0,600]);
%subplot (1,2,1);
xlabel('Stroom (A)')
ylabel('Spanning (V)')
title('Polarisatiecurve')
%axis([0 120 0 4000])
grid on
grid on
end

[Wouter]

Thanks but that gave me a new problem I just can't wrap my head around. It shows the the lines the wrong way.

I currently have it this way:

function polarisatiecurve
P = 100;
I = 0:1:30;
U1 = P./I;
U2 = 20*I.^2 - 2*I.^3; 
plot(I,U1); 
xlabel('Stroom (A)')
ylabel('Spanning (V)')
title('Polarisatiecurve')
grid on
end

That gives me a nice plot of a 100/I= U1. That works. I see everything I need. Now I have the function But when I want to plot U2 in the same graph, it changes the window automatically. And now I can't see anything interesting (I can't see the points where they intersect. What is going on?

So:

plot(I,U1); 
that gives me a nice view.

plot(I,U1, I, U2);

That gives me the wrong window and I can't see the interesting parts. What is going on?

[Amit]

The problem is that they intersect between 0 and 0.001 and 0 to 200 is a much bigger range for this. Try this

function polarisatiecurve
P = 100;
I = 0:0.000001:0.001;
U1 = P./I;
U2 = 800*(I+19).^2 - 2*(I+19).^3; 
plot(I,U1,'b',I,U2,'r'); %,[0,100,0,600]);
%subplot (1,2,1);
xlabel('Stroom (A)')
ylabel('Spanning (V)')
title('Polarisatiecurve')
axis([0 0.001 0 0.5e6])
grid on
grid on
end

[Wouter]

Never mind, I needed to add an axis options after the plotting function. Thank you so much for helping me!