NTNUJAVA Virtual Physics Laboratory
Enjoy the fun of physics with simulations!
Backup site http://enjoy.phy.ntnu.edu.tw/ntnujava/

Easy Java Simulations (2001- ) => Questions related to EJS => Topic started by: lookang on April 27, 2010, 03:53:59 pm

Title: how to make magnet1 and magnet2 in a single applet with different rotation?
Post by: lookang on April 27, 2010, 03:53:59 pm
how to make magnet1 and magnet2 in a single applet with different rotation?

Hi everyone,

i have been successful in making magnet1 and magnet2 in a single applet if the have the same orientation.

i even made codes flexible enough to trouble shoot.

// computes magnetic field components and magnitude from dipoles
// rename this getB to getBcomputeField because need to separate the calculation of Field from Compass
public double[] getBcomputeField(double x, double y) {
  double[] b=new double[4];
  for(int i=0; i<=nd; i++) { // lookang change i//    double dx=x-dipoles[i][0]-xMagnet;
//    double dy=y-dipoles[i][1]-yMagnet;
    double dx=x-dipoles[i][0]-(xMagnet*cs+yMagnet*sc);//FKH
   double dy=y-dipoles[i][1]-(-xMagnet*sc+yMagnet*cs);//FKH
//   double dx=x-dipoles[i][0]-dipoles2[i][0]-(xMagnet*cs+yMagnet*sc)-(xMagnet2*cs2+yMagnet2*sc2);//FKH
//    double dy=y-dipoles[i][1]-dipoles2[i][1]-(-xMagnet*sc+yMagnet*cs)-(-xMagnet2*sc2+yMagnet2*cs2);//FKH
    double dx2=x-dipoles2[i][0]-(xMagnet2*cs2+yMagnet2*sc2);//FKH
    double dy2=y-dipoles2[i][1]-(-xMagnet2*sc2+yMagnet2*cs2);//FKH

double r2=dx*dx+dy*dy;
    double r=Math.sqrt(r2);
    double r3=r2*r;
    double cos=dx/r;
    double sin=dy/r;
  double r4=dx2*dx2+dy2*dy2;
    double r22=Math.sqrt(r4);
    double r33=r4*r22;
    double cos2=dx2/r22;
    double sin2=dy2/r22;  
    // taken from http://en.wikipedia.org/wiki/Magnetic_moment
 if (magnet1==true&&magnet2==false)
 b[0]+=m*(3*cos*cos-1)/r3;   //x component
 b[1]+=m*3*sin*cos/r3;   //y component
  else if (magnet2==true&&magnet1==false)
 b[0]+=m*(3*cos2*cos2-1)/r33;   //x component
  b[1]+=m*3*sin2*cos2/r33;   //y component

else if (magnet1==true&&magnet2==true)
    b[0]+=m*(3*cos*cos-1)/r3 +m*(3*cos2*cos2-1)/r33;   //x component
b[1]+=m*3*sin*cos/r3 +m*3*sin2*cos2/r33;   //y component

else {

//   b[0]+=m*((3*cos*cos-1)*cs+(3*sin*cos)*sc)/r3;   //x component
 //   b[1]+=m*((3*cos*cos-1)*(-cs)+(3*sin*cos)*sc)/r3;   //y component
    if(r2==0||r4==0) {
  b[2] = Math.sqrt(b[0]*b[0]+b[1]*b[1]);  // magnitude
  b[3] = Math.atan2(b[1],b[0]);  // angle
  return b;

Dipole field:
B=mu/4/pi/r^3[(m*rhat)rhat-m]         //vector form


public void computeField () {
  for (int i=0; i    double x=_view.vectorField2D.indexToX(i);
    for (int j=0; j      double y=_view.vectorField2D.indexToY(j);
      //double[] b=getB(x,y);//  Gets the components and magnitude

if (magnet1==false&&magnet2==false)
  double[] b=getBcomputeField(x*cs+y*sc+ x*cs2+y*sc2,-x*sc+y*cs -x*sc2+y*cs2);//
xField[i][j] = 0;
yField[i][j] = 0;
else if (magnet1==true)
double[] b=getBcomputeField(x*cs+y*sc,-x*sc+y*cs);//FKH
      //xField[i][j] = b[0];
      //yField[i][j] = b[1];

 // old codes
xField[i][j] = b[0]*cs-b[1]*sc;//FKH
yField[i][j] = b[0]*sc+b[1]*cs;//FKH

else if (magnet2==true)
double[] b=getBcomputeField(x*cs2+y*sc2,-x*sc2+y*cs2);//FKH
      //xField[i][j] = b[0];
      //yField[i][j] = b[1];

 // old codes
xField[i][j] = b[0]*cs2-b[1]*sc2;//FKH
yField[i][j] = b[0]*sc2+b[1]*cs2;//FKH

else {
double[] b=getBcomputeField(x*cs+y*sc+ x*cs2+y*sc2,-x*sc+y*cs -x*sc2+y*cs2); // likely the codes here wrong
xField[i][j] = b[0]*cs-b[1]*sc + b[0]*cs2-b[1]*sc2;//FKH
yField[i][j] = b[0]*sc+b[1]*cs + b[0]*sc2+b[1]*cs2;//FKH  



where did the codes i did wrong?,
independently, each magnet works on its own, but when both are turn on, the bug appears to be opposite the actual position of the magnet.

Title: Re: how to make magnet1 and magnet2 in a single applet with different rotation?
Post by: lookang on May 01, 2010, 01:50:08 pm
hi prof hwang,
i doubt anyone else is going to give any sample codes to refer to on this :)
chance to help me out here?  how to make magnet1 and magnet2 in a single applet with different rotation with vector sum of the magnetic field vectors?


the problem happens when cta =! cta2.