Brownian motion

Resources

Brownian motion is the constant, but irregular, zigzag motion of small colloidal particles such as smoke, soot, dust, or pollen that can be seen quite clearly through a microscope.

In 1827, Robert Brown (1773–1858), a Scottish botanist, prepared a slide by adding a drop of water to pollen grains. As he watched the tiny particles of pollen under his microscope, Brown noticed that they were constantly jiggling about. He thought that the motion might be related to life processes within the pollen, but later he observed the same kind of zigzag motion with pollen from plants that had been dead for many years. Others found the same strange motion when they observed tiny inanimate particles of dye, dust, smoke, or soot.

Brown could offer no explanation for his observation, which became known as Brownian motion; nor could anyone else, until James Clerk Maxwell (1831–1879) and others developed the kinetic molecular theory a generation later. According to this theory, Brownian motion is the result of collisions between the small microscopic particles and the invisible but constantly moving water or air molecules surrounding them. Since particles such as pollen are thousands of times larger than water molecules, we would expect that on the average the particle would be hit as many times by water molecules on one side as it would be by molecules striking it from the opposite direction. However, since molecular motion is random, there will be moments when the particle is struck by more molecules moving in one direction than in any other. When that happens, the particle will respond to the unbalanced force and move accordingly.

Imagine yourself caught in the middle of a large crowd of people who are undecided as to which way they should go. You would find people bumping into you from all sides. Sometimes pushes from all directions would be equal and you would not move. At other times, more people would be bumping you from the right than from the left and so you would move to the left. A short time later, there might be more people pushing from behind than from in front, and you would move forward. Your motion would be similar to that of a tiny pollen particle suspended in, and constantly being struck by randomly moving molecules of water.

The fact that the jiggling movement of a particle exhibiting Brownian motion increases with temperature provided evidence that its motion could be explained by the kinetic molecular theory. Early in the twentieth century, Albert Einstein (1879–1955) published a series of papers in which he statistically analyzed the expected velocity of particles of various sizes and masses undergoing Brownian motion at various temperatures in liquids with different viscosities. In an effort to verify Einstein’s theoretical work, Jean Perrin carried out a number of experiments using small uniform particles of known size and mass. His results confirmed Einstein’s analysis and put to rest any doubts about the molecular nature of matter.

See also Molecule.

Resources

BOOKS

Hewitt, Paul. Conceptual Physics. 10th ed. Englewood Cliffs, NJ: Prentice Hall, 2005.

Perrin, Jean and F. Soddy (trans.). Brownian Movement and Molecular Reality. New York: Dover, 2005.

Serway, Raymond, et al. College Physics. 7th ed. Pacific Grove, CA: Brooks/Cole, 2005.

Brownian motion

Brownian motion is the constant but irregular zigzag motion of small colloidal particles such as smoke, soot, dust, or pollen that can be seen quite clearly through a microscope .

In 1827, Robert Brown, a Scottish botanist, prepared a slide by adding a drop of water to pollen grains. As he watched the tiny particles of pollen under his microscope, Brown noticed that they were constantly jiggling about. He thought that the motion might be related to life processes within the pollen, but later he observed the same kind of zigzag motion with pollen from plants that had been dead for many years. Others found the same strange motion when they observed tiny inanimate particles of dye, dust, smoke, or soot.

Brown could offer no explanation for his observation, which became known as Brownian motion, nor could anyone else until James Clerk Maxwell and others developed the kinetic molecular theory a generation later. According to this theory, Brownian motion was the result of collisions between the small microscopic particles and the invisible but constantly moving water or air molecules surrounding them. Since particles such as pollen are thousands of times larger than water molecules, we would expect that on the average the particle would be hit as many times by water molecules on one side as it would be by molecules striking it from the opposite direction. However, since molecular motion is random , there will be moments when the particle is struck by more molecules moving in one direction than in any other. When that happens, the particle will respond to the unbalanced force and move accordingly.

Imagine yourself caught in the middle of a large crowd of people who are undecided as to which way they should go. You would find people bumping into you from all sides. Sometimes pushes from all directions would be equal and you would not move. At other times, more people would be bumping you from the right than from the left and so you would move to the left. A short time later, there might be more people pushing from behind than from in front, and you would move forward. Your motion would be similar to that of a tiny pollen particle suspended in, and constantly being struck by randomly moving molecules of water.

The fact that the jiggling movement of a particle exhibiting Brownian motion increases with temperature provided evidence that its motion could be explained by the kinetic molecular theory. Early in the twentieth century, Albert Einstein published a series of papers in which he statistically analyzed the expected velocity of particles of various sizes and masses undergoing Brownian motion at various temperatures in liquids with different viscosities. In an effort to verify Einstein's theoretical work, Jean Perrin carried out a number of experiments using small uniform particles of known size and mass . His results confirmed Einstein's analysis and put to rest forever any doubts about the molecular nature of matter .

See also Molecule.

Resources

books

Haber-Schaim, et., al. Introductory Physical Science. 5th ed. Englewood Cliffs, NJ: Prentice-Hall, 1987, pp. 268–275.

Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Prentice Hall, 2001.

Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. College Physics. 6th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Wheeler, Gerald F., and Larry D. Kirkpatrick. Physics: Building a World View. Englewood Cliffs, NJ: Prentice-Hall, 1983, pp. 142-142.

Brownian movement The continuous random movement of microscopic solid particles (of about 1 micrometre in diameter) when suspended in a fluid medium. First observed by Robert Brown in 1827 when studying pollen grains in water, it was originally thought to be the manifestation of some vital force. It was later recognized to be a consequence of bombardment of the particles by the continually moving molecules of the liquid. The smaller the particles the more extensive is the motion. It can be observed in the particles of a colloidal solution and in the cytoplasm and nucleoplasm of dead cells.

Brown·i·an mo·tion / ˈbrounēən/ • n. Physics the erratic random movement of microscopic particles in a fluid, as a result of continuous bombardment from molecules of the surrounding medium.

Brownian movement Random, zigzag movement of particles suspended in a fluid (liquid or gas). It is caused by the unequal bombardment of larger particles, from different sides, by the smaller molecules of the fluid. The movement is named after the Scottish botanist Robert Brown (1773–1858), who in 1827 observed the movement of plant spores floating in water.