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Q & A: seeing Brownian motion

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Most recent answer: 06/26/2013
Explain that the law of equipartition of energy demands that the motion of relatively large Brownian particles are practically unnoticeable.
- Premashis Kumar (age 18)
Kolkata,West Bengal,India

I'm a little surprised that you say that. Brown observed Brownian motion through a microscope. It's really not all that hard to see the Brownian motion of particles (say 10 μm) big enough to follow individually by looking at scattered light. The typical thermal speed is about sqrt(kT/m)  where k is Boltzmann's constant, T is absolute temperature, and m is the particle mass. For a fair-sized bead (m say 10-9 gm) at room temperature that's almost 10-2 cm/sec. The main reasons why it isn't really easy to see are that:

1. The particles are usually in some liquid. Friction with the liquid causes the particle to lose its random thermal velocity and pick up a new one. Our little 10 μm particle would do that about every 10-5 s. So the particle does a random walk rather than uniform motion. After about a day, the particle would have typically wandered about 0.1 mm, easy to see under a microscope, but not a huge distance.

2. The liquid often isn't sitting quietly. Things like thermal convection can stir it up. By eye it can be hard to separate those motions from the Brownian motion. 

There are ways (e.g. laser Doppler light-scattering) to detect the rapidly changing thermal motion accurately. The key is to pick up the fast small motions, before the gradual convection becomes more important.

Mike W.

(published on 06/26/2013)

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