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Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"

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 Author Topic: About particles and walls /board:23-100-  (Read 4590 times) 0 Members and 1 Guest are viewing this topic. Click to toggle author information(expand message area).
st@lk3r
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 « Embed this message on: November 01, 2009, 12:55:35 am » posted from:Athens,Attiki,Greece

In the "particles&walls" simulation, there's a parameter called "threshold" in the code that calculates force and field. What is it for?
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Fu-Kwun Hwang
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 « Embed this message Reply #1 on: November 01, 2009, 08:42:19 am » posted from:Taipei,T\'ai-pei,Taiwan

Charged Particle and Wall

Please check out the following code in the custom / force tab  :
Code:
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.

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Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"