NTNUJAVA Virtual Physics LaboratoryEnjoy the fun of physics with simulations! Backup site http://enjoy.phy.ntnu.edu.tw/ntnujava/
September 29, 2020, 09:29:23 am

Progress is impossible without change; and those who cannot change their minds cannot change anything. ..."George Bernard Shaw(1865-1950, Irish dramatist, essayist and critic, Nobel Prize for Literature 1925)"

 Pages: [1]   Go Down
 Author Topic: About particles and walls /board:23-100-  (Read 5457 times) 0 Members and 1 Guest are viewing this topic. Click to toggle author information(expand message area).
st@lk3r
Newbie

Offline

Posts: 1

 « 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?
 Logged
Fu-Kwun Hwang
Hero Member

Offline

Posts: 3085

 « 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.

 Logged
 Pages: [1]   Go Up
Progress is impossible without change; and those who cannot change their minds cannot change anything. ..."George Bernard Shaw(1865-1950, Irish dramatist, essayist and critic, Nobel Prize for Literature 1925)"