In the year 1827 Robert Brown a biologist, noticed that if you looked at pollen grains in water through a microscope they jiggled in water. He didn't understand why this was happening. He then tried looking at saw dust on water to eliminate the thought that the pollen grains were 'alive'. The saw dust jiggled too. Brown could not explain why this was happening.
By 1860 physicists became very interested in Brownian Motion and were trying to explain its various characteristics such as
- a particle was equally likely to move in any direction
- future motion was totally unrelated to past motion
- the motion never stopped
An experiment in 1865 involved a sealed suspension left undisturbed for a year. The Brownian motion remained unchanged. Further experiments revealed that smaller particle size and low viscosity of the surrounding fluid resulted in faster motion. It was also observed that increase in temperature resulted in faster moving particles. In 1877 it was suggested that the cause for such a motion was due to "thermal molecular motion in the liquid environment." The theory that molecules in a liquid are constantly in motion, colliding with each other and bouncing back and forth was put forth by Maxwell, Boltzmann and Clausius in explanation of heat phenomena. According to this theory, the temperature of a substance is proportional to the average kinetic energy with which the molecules of the substance are moving or vibrating. It was natural to guess that somehow this motion might be imparted to larger particles that could be observed under the microscope. If this were true, it would be the first directly observable effect of the kinetic theory.
An explanation with a mathematical equation to describe such a motion did not come along until Albert Einstein applied the kinetic theory of fluids to explain what Brown had observed. Einstein realized that the pollen grains were jiggling because the water molecules were hitting the grains in a random fashion. The pollen grains were visible while the molecules were not. Hence it looked like the pollen grains jiggling on their own. One must remember that the concept of atoms and molecules were still under debate in 1905. Einstein also showed that one can calculate the number of water molecules hitting a single pollen grain and how fast the water molecules were moving all by looking at the pollen grains.
Using Statistical mechanics Einstein calculated the probability(P) of a particle moving a certain distance (x) in any given direction in a time interval (t) in a liquid with diffusion coefficient D as
When P is plotted against x , this gives a normal distribution curve, which would arise when the random variable is a sum of independent, statistically identical random variables, in this case many little pushes add up to the total motion.
His paper on Brownian Motion also made predictions on the properties of atoms that could be tested which were indeed useful when three years later, the French Physicist Jean Perrin worked out the size of atoms, thus removing any doubts about the existence of atoms.
By 1860 physicists became very interested in Brownian Motion and were trying to explain its various characteristics such as
- a particle was equally likely to move in any direction
- future motion was totally unrelated to past motion
- the motion never stopped
An experiment in 1865 involved a sealed suspension left undisturbed for a year. The Brownian motion remained unchanged. Further experiments revealed that smaller particle size and low viscosity of the surrounding fluid resulted in faster motion. It was also observed that increase in temperature resulted in faster moving particles. In 1877 it was suggested that the cause for such a motion was due to "thermal molecular motion in the liquid environment." The theory that molecules in a liquid are constantly in motion, colliding with each other and bouncing back and forth was put forth by Maxwell, Boltzmann and Clausius in explanation of heat phenomena. According to this theory, the temperature of a substance is proportional to the average kinetic energy with which the molecules of the substance are moving or vibrating. It was natural to guess that somehow this motion might be imparted to larger particles that could be observed under the microscope. If this were true, it would be the first directly observable effect of the kinetic theory.
An explanation with a mathematical equation to describe such a motion did not come along until Albert Einstein applied the kinetic theory of fluids to explain what Brown had observed. Einstein realized that the pollen grains were jiggling because the water molecules were hitting the grains in a random fashion. The pollen grains were visible while the molecules were not. Hence it looked like the pollen grains jiggling on their own. One must remember that the concept of atoms and molecules were still under debate in 1905. Einstein also showed that one can calculate the number of water molecules hitting a single pollen grain and how fast the water molecules were moving all by looking at the pollen grains.
Using Statistical mechanics Einstein calculated the probability(P) of a particle moving a certain distance (x) in any given direction in a time interval (t) in a liquid with diffusion coefficient D as
When P is plotted against x , this gives a normal distribution curve, which would arise when the random variable is a sum of independent, statistically identical random variables, in this case many little pushes add up to the total motion.
His paper on Brownian Motion also made predictions on the properties of atoms that could be tested which were indeed useful when three years later, the French Physicist Jean Perrin worked out the size of atoms, thus removing any doubts about the existence of atoms.
3 comments:
Hi Sowmya
Nice writeup, quite revealing. Perhaps you could take a look at thermodynamics here and fluid dyanmics here. They are both of interest to me.
Thanks for this one.
Thanks for the links. Will definitely look into it.
Sorry about that. I did type my name or did i? Thanks for the links.
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