MATLAB: A simple code gives me an error, anyone help in this issue

van der waals equation

My function file is, function [ y ] = van( v ) y = (p + a./v.^2).*(v – b) – (r *t); end My script file is, >> r = 0.082; >> a = 3.6; >> b = 0.4; >> p = 1; >> t = 300; >> v = linspace(0,50); >> y = van(v) Undefined function or variable 'p'.

Error in van (line 4) y = (p+a./v.^2).*(v – b) – (r *t);

Best Answer

p is not known to your function. You have to define the function with all arguments you need inside:

 function y = van(v, a, b, r, t)

And then call it with

y = van(v, a, b, r, t)

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MATLAB: Replace expression of variables with an equivalent variable

Your substitution can't work because you defined "theta_v" to be "omega*t + phi_v". So you're just substituting "omega*t + phi_v" for the same thing. If you want to keep "theta_v" then don't define it as "omega*t + phi_v". Keep it as a symbolic function, and substitute in at the end.

>> syms x
>> syms theta_v(t)
>> syms Vx
>> v = Vx*sin(theta_v)
v(t) =
Vx*sin(theta_v(t))
>> y = diff(v, t)
y(t) =
Vx*cos(theta_v(t))*diff(theta_v(t), t)
>> syms omega phi_v
>> subs(y,theta_v, omega*t + phi_v)
ans(t) =
Vx*omega*cos(phi_v + omega*t)

MATLAB: How can I solve non polynomial equations.

FIRST: see Getting Started. What you posted is not valid MATLAB code.

There may be many solutions, depending on the initial estimates:

R = 0.082;
a = 4.225;
b = 0.037;
T = 320;
eqn1 = @(v1,v2,P) P+(a./v1^2) - R*T/(v1-b);
eqn2 = @(v1,v2,P) P+(a./v2^2) - R*T/(v2-b);
eqn3 = @(v1,v2,P) P*(v1-v2)-R*T*log(v1-b./v2-b)-a*(1./v1 -1./v2);
[Vv,fval] = fsolve(@(b) [eqn1(b(1),b(2),b(3)); eqn2(b(1),b(2),b(3)); eqn3(b(1),b(2),b(3))], ones(3,1))

producing:

Vv =
   1.071530062473936
   1.071530062474078
  21.684426220811453
fval =
   1.0e-11 *
  -0.252597942562716
  -0.001776356839400
  -0.189449129302390

Experiment to get the result you want.

Best Answer

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MATLAB: Replace expression of variables with an equivalent variable

MATLAB: How can I solve non polynomial equations.

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