help in runge kutta 4 2nd order with establishing the functions

% Ecuaciones Diferenciales Ordinarias dy/dx=f(x,y) a resolver --------------

f1 =@(x,y1,y2)

f2 =@(x,y1,y2)

% Condiciones iniciales -----------------------------------------------

%y1(0)=1;

%y2(0)=0;

x=0

xn=5

y1=1

y2=0

h=0.2

% Método de RK4Orden ---------------------------------------------------

while x(end)<=xn

k11= f1(x(end),y1(end),y2(end));

k12= f2(x(end),y1(end),y2(end));

k21= f1(x(end)+.5*h,y1(end)+.5*k11*h,y2(end)+.5*k12*h);

k22= f2(x(end)+.5*h,y1(end)+.5*h*k11,y2(end)+.5*h*k12);

k31= f1(x(end)+.5*h,y1(end)+.5*k21*h,y2(end)+.5*k22*h);

k32= f2(x(end)+.5*h,y2(end)+.5*k22*h,y2(end)+.5*k22*h);

k41= f1(x(end)+h,y1(end)+k31*h,y2(end)+k32*h);

k42= f2(x(end)+h,y2(end)+k32*h,y2(end)+k32*h);

x(end+1)=x(end)+h;

y1(end+1)=y1(end)+1/6*(k11+2*k21+2*k31+k41)*h;

y2(end+1)=y2(end)+1/6*(k12+2*k22+2*k32+k42)*h;

end

% Graficación de la solución ------------------------------------------

plot(x,y1)

hold on

plot(x,y2)

xlabel('tiempo')

title('Carga respecto al tiempo')

[Paul]

Hi LG,

What is the question? Hard to help if only code is posted.

Having said that, the equations for f1 and f2 don't look correct. Normally, for RK solutions the differential equation is expressed in the form

dy/dt = f(y,t)

i..e., the right hand side of the equation will not be a function of the derivatives, which it looks like the code in the Question is trying to do.

Suggest that the Question be edited to show the differential equation to be solved in mathematical form and also explaining what problem or question you actually have about the code.

[LG]

where

L=.5

R=6

C=.2

x=time and y1 and y2 are q and i

Answer 1

[Paul]

Ok. Let y1 = q and let y2 = qdot (or y2 = q and y1 = qdot if preferred)

Then the first equation we need is

y1dot = qdot = y2. This equation should be implemented as f1

f1 @(x,y1,y2) (y2).

What is the equation for y2dot? That is, can you express y2dot as:

y2dot = f2(x,y1,y2)?

Also, the solution needs to account for E(t) as well.

As an aside, why does the code use x as the independent variable fo solving a differential equation in time? Of course, any variable name is fine, but something like 't' or 'time' would be more clear.