Brownian motion

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Demonstrating Brownian Motion in smoke molecules using a smoke cell

This is a ‘classic’ experiment that gives good evidence for the particulate nature of air. It has been adapted from an existing experiment which you can find here. We acknowledge with thanks the nuffield foundation for granting us permission to use this article.

  • Smoke Cell incorporating a light source and lens (Whitley Bay pattern)
  • Optical Microscope low power (e.g x10 objective, x10 eyepiece) and large aperture
  • Power supply, 0 to 12 V d.c
  • Microscope cover-slip
  • Smoke source (e.g. paper drinking straw)
Smcell.jpg

Technical notes

If a visiview is available for your microscope, you can set this practical up quickly to show to the whole class. However, ‘seeing for yourself’ has much to commend itself in this case.

The smoke can come from a piece of burning paper straw, a wooden split using a dropping pipette. The straw should burn at the top and then be extinguished. The bottom end of the straw should poke into the plastic smoke container.

The cell may need to be cleaned if a waxy or plastic straw is used.

Remove the glass cell from the smaloe cell assembly to clean it. Afterward, push it fully back into the assembly. It may help to wet the outside of the glass tube. You will find it helpful to clean the glass cell after every five to ten fillings to obtain the best results; otherwise the light intensity is reduced.

The cell is illuminated from one side to make the smoke particles visible under the microscope. A small piece of black card prevents stray light from the lamp reaching the eye. The lamp is placed below the level of the glass rod in order to minimise convection.

There are no significant hazards involved in this experiment. It should be conducted in a darkened room and the students advised to allow sufficient time for their eyes to be adapted to the dark before trying to view the smoke cell image.

Procedure

  • Fill the cell with smoke using a dropping pipette and cover it with a glass cover-slip. This will reduce the rate of loss of smoke from the cell.
  • Place the cell on the microscope stage, fit the mask and connect to a 12 V power supply.
  • Start with the objective lens of the microscope near the cover-slip. While looking through the microscope, slowly adjust the focus, moving the objective lens away from the cover-slip, until you see tiny dots of light.
  • Watch the particles carefully.


Teaching notes - (Reproduced from "practicalphysics.org")

1 As this is such an important experiment - one of the few to show the 'graininess' of nature and to give strong support to the idea that gas molecules are in constant motion - students should be given plenty of time to set it up and see it clearly.

2 We know what the students are supposed to see. They may not. Consequently, the students may not ‘see’ what we expect. We expect them to observe jiggling points of light. The vertical component of the motion causes the bright points to go out of focus and to disappear. This will not be obvious to every student. The points of light may also have a drift velocity but we know that this observation is unimportant. The students don’t know that the drift (due to large scale convection effects amongst other causes) is unimportant, and so this may become their major observation. A 'prepared' mind helps the scientist to see.

3 Once the students ‘know’ what to look for, it is useful to repeat the experiment – they’ll see the expected effect second time around!

4 The bright specks of light do not bounce into each other before changing direction. Why?

5 Ask students to note down an explanation of the observations. Discuss what everyone thinks is going on and then describe (or elicit through questioning) the kinetic explanation – that smoke particles are observed as points of light, and their jiggling is due to collisions with much smaller and invisible air particles. Students should realise that an invisible movement explains an observed movement. It becomes somewhat circular when Brownian motion is (incorrectly) given as evidence of the particulate nature of air – it is circumstantial at best.

"How big are air molecules?" "Well, smaller than the smallest specks of ash that make up smoke!" This may lead to guesstimating.

1 mm across?, 0.1 mm?, 0.01 mm? Or even smaller? How many smoke particles could you park side by side along the edge of a postcard (15 cm long)? This is a chance to make an order of magnitude guess on whether they think that there would be 100, 1000, 10000, 100000, working in powers of 10. And now guess how many molecules you could park side by side on each smoke speck. How many is that along the edge of the postcard? Molecules must be very small indeed and atoms even smaller. Common sense tells us that.

Other websites of interest

The use of a "flexi-cam" or similar device can be recommended to show what the class should be looking for. It is often so sucessful that, when coupled to a dataprojector, only one microscope needs to be set up. Thus saving time and coverslips! -David Ferguson.

See also;


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