Understanding Brownian Motion: The Theory Behind Random Particle Movement

[bobsmith76]

Is there a theory regarding why particles move in random paths. My high school physics teacher said it's energy left over from the big bang, but that doesn't explain why they move in random paths, they could just as easily obey Newton's Laws of Motion and still have energy left over from the Big Bang.

 

[zhermes]

Aaaaah! They said WHAT?!

Here is a good place to start on http://en.wikipedia.org/wiki/Brownian_motion" .

On the scale of individual molecules, particles are rapidly undergoing collisions for all of the other particles. Its like bumper cars. A particle will get bumped in one direction, then it will get bumped in another, and another... and you end up with 'brownian motion' or the 'random walk'.

On larger scales, like small dust particles, brownian motion can still be important. For particles as large as dust, TONS and TONS of particles are ALWAYS bumping into them from all sides. Every once and a while they get bumped a little extra hard in one direction, however, and then a fraction of a second later, they get bumped extra hard in another direction----thus they also can undergo brownian motion.

Hope that helps; and let me know if you still have questions.

 

[bobsmith76]

But I thought particles could change direction for no reason. Is that true?

 

[rollcast]

No they would have to have a force acting upon them to do this

 

[zhermes]

rc1102 is correct.
Note that if they did randomly change directions, that would violate conservation of momentum.

 

[rollcast]

What was conservation of momentum, I vaguely remember it but can't remember the details

 

[bobsmith76]

I'm talking about on the atomic level. Aren't things different on the atomic level? Or is it just when they crash into another atom they do not behave as billiard balls and fly at any angle they choose?

 

[Mutsi]

Billiard balls and particles do not move in a random direction they move into a fixed direction given by a collision. That is not random and so they do not "choose" a direction move in.