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Easy Java Simulations (2001- ) => Questions related to EJS => Topic started by: st@lk3r on November 01, 2009, 12:55:35 am

Title: About particles and walls /board:23-100-
Post by: st@lk3r on November 01, 2009, 12:55:35 am
In the "particles&walls" simulation, there's a parameter called "threshold" in the code that calculates force and field. What is it for?

Title: Re: About particles and walls /board:23-100-
Post by: Fu-Kwun Hwang on November 01, 2009, 08:42:19 am
Charged Particle and Wall (http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=864.0)

Please check out the following code in the custom / force tab  :
private double force(int particle, double a[], double b[], boolean xAxis) {
 double force = 0.0, r2;
 if (particle>=numParticles) return 0.0;
 for (int i=0; i  if (i==particle) continue;
  r2 = (a[i]-a[particle])*(a[i]-a[particle]) + (b[i]-b[particle])*(b[i]-b[particle]);
 [b] if (r2  if (xAxis) force += K*charge[i]*charge[particle]*(a[particle]-a[i])/r2;
  else force += K*charge[i]*charge[particle]*(b[particle]-b[i])/r2;
 return force;
The above function was used in evolution to calculate vx(t+dt) and vy(t+dt) (dvx/dt=force(i,x,y,true)/mass)

In the simulation when two
The threshold were added to prevent numerical error.
The force is inverse propose to r2. The force will become very arge when r is very small.
So the force were ignored when two particles are too close to each other.
And this also allow the simulation to have a positive and a negative charge to stick together -- the major reason why it is ignored.

Please post your question at the same topic which contain the simulation you want to discuss.