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Brownian Motion

  • Date Submitted: 10/18/2011 09:57 AM
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Brownian motion
Brownian motion (or Brownian movement) can be defined as "the random movement of microscopic particles suspended in a fluid." It is the erratic and constant movement of tiny particles when they are suspended in a fluid or gas. The name of the movement was kept after the scientist Robert Brown who first observed the phenomenon. We shall explore how.  

  i. Brownian motion in fluids :
The best evidence for the existence and movement of particles in liquid was given by Robert Brown in 1827. He suspended extremely small extremely small pollen grains in water. On looking through the microscope, it was found that the pollen grains were moving rapidly throughout the water in a very irregular or zig-zag way. It was also observed that warmer the water, faster the pollen grains move on the surface of water.
The movement of pollen grains on the surface of water can be explained as follows: Water is made up of tiny particles which are moving very fast. The pollen grains move on the surface of water because they are constantly being hit by fast moving molecules of water. So though the particles of water are too small to be seen, but their effect on the pollen grain could be seen clearly. The random motion of visible particles (pollen grains) caused by much smaller invisible particles of water is an example of Brownian motion.

Pollen grain

Invisible moving particles of water

  ii. Brownian motion in gas :
Brownian motion is also observed in gas. For example: Smoke particles or dust particles in a light beam observed   using a microscope appear as quivering specks of light, moving about unpredictably and erratically. Thus is caused by repeated and continual impacts on each smoke particles by fast moving air molecules, too small to see but nevertheless able to make the smoke particles move about noticeably. The erratic motion of each smoke particle occurs because the...


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