Uses of Cloud Chambers
From MariachiWiki
The following is list of possible applications/activities for the use of the cloud chamber in the classroom setting:
Parameters for Success
- Care of construction
- Closed system
- Maintain temperature gradient
- Use pure alcohol
Experimentation
- Use different alcohols (type and concentrations)
- Dry ice with acetone (drops freezing point)
- Peltier heat transfer
- Illumination sources and diffusion patterns
- Heat upper surface to promote alcohol vaporization
Super saturated environments/solutions
The cloud in a working cloud chamber is a supersaturated alcohol vapor. Charged particles that fly through ionize atoms in the vapor, and these ions provide nucleation sites for condensation. It is important to have a source of alcohol at a high temperature (room temperature or warmer) and a cold side of the chamber (alcohol and dry ice slurry). The alcohol evaporates at the warm side and then the saturated alcohol vapor cools down on the cold side and becomes supersaturated at the colder temperature. Things to explore could include:
- What conditions are needed to make a supersaturated vapor?
- Does the temperature gradient have any effect?
- Is it important to have a closed system (no leaks or holes)?
- Does the size of the droplets change as the vapor becomes supersaturated?
- Does anything affect the thickness of the cloud of condensed vapor?
- Can other liquids create a supersaturated vapor?
- Compare and contrast this supersaturated vapor to a more commonly observed supersaturated liquid solution – how it was made, how it behaves.
Magnetometer uses
- Map various magnetic fields
- Bar magnets
- Horseshoe magnets
- Current carrying wire
- Hall effect
- Compass
Analysis of magnetic field
Build magnetometer:
- Measure and map the magnetic field around the permanent magnet
- Measure field around a current carrying wire
- Measure the magnetic field of the Earth
Evidence of atomic particles
The wonderful thing about cloud chambers is that you can really see, with your eyes, trails left by sub-atomic particles. These trails are left by particles that fly through the air all the time. So that means that sub-atomic particles must be flying through the air all the time, only we don’t normally see them or think of them. Cloud chambers have been used for half a century to detect particles. They show direct evidence that these sub-atomic particles that we read about or hear about or teach about really exist. To the student, they might be the first hard evidence that anything the teacher has been saying about the atomic structure could possibly be true.
First, there are atoms. Start a section on sub-microscopic particles with a demonstration of Brownian Motion. Brownian Motion shows, visually, that there really are sub-microscopic things called atoms. Einstein’s explanation for the random motion of the larger, visible, particles was that smaller particles, called atoms, continually bombard the larger particles, bumping them in the “random walk” patterns that are visible.
- For a demonstration idea see Brownian Motion
- For a Java applet see Brownian motion applet
Next, there are sub-atomic particles. After a demonstration of Brownian Motion, use the cloud chamber to show that there are sub-atomic particles. This page shows other things you can do with a cloud chamber.
Analysis of cloud chamber events:
Observe and classify different interactions based on the shape and curvature of their paths
Have the cloud chamber set up and running. Then:
- Students observe and record (mentally/on paper/digitally) images of vapor trails
- Students develop criteria for categories of vapor trails
- Students categorize their observed vapor trails according to their criteria
- Students hypothesize reasons for variations in vapor trail geometries
- Students present their findings to class. Include description of criteria and images of vapor trails
- Extension 1: Introduce nomenclature to students. Student groups compare and contrast their terminology and categories with accepted terminology and categories
- Extension 2: Students quantify occurrence of similar events, determine relative frequency and probability of occurrence
- Extension 3: From quantitative information, students graph frequency of events vs. time for each category, and compare frequency charts for different categories
Penetrating Ability & Shielding
With the cloud chamber setup use an emitting source to create the particle you are interested in observing. With the source held up against the glass of the chamber students should notice the increase in tracks. After observation of intial tracks students can place materials between emitting source and glass such as paper, aluminum foil, aluminum shield, and/or lead. A variety of materials can be used and a correlation between tracks observed and type of shield/thickness can be constructed.
Students are responsible for constructing a data table of observed tracks per time period.
Questions that students should be capable of answering upon completion of lab activity are:
- What thickness of aluminum would be equivalent to one sheet of lead?
- What type of emissions are blocked by aluminum?
- What type of emitters are blocked by lead? Which are not?
- What type of emitter has the greatest penetrating ability?
Possible Questions for Further Study:
- Do watches that glow in the dark emit radiation?
- Do computers emit radiation?
- Do portable telephones/electronic devices emit radiation?
Emitters (sources)
The cloud chamber detects particles. Almost all the particles that leave trails in the cloud chamber come from space. But what happens when you bring a particle emitter (source) near the cloud chamber?
For this activity you need a particle emitter (source).
- Some Physics or Chemistry teachers have, or have access to, Beta emitters / sources. These are small pieces of material that are known to emit beta particles.
- Rumor has it that smoke detectors have an alpha source hidden inside them. However, you might have to take one completely apart to get to the alpha source.
After viewing the cloud chamber in the lab, ask students some open-ended questions:
- Where do the particles come from?
- What would happen if there were a source of particles beside the cloud chamber?
Bring the particle emitter (source) near the cloud chamber and observe the different behavior.
- Compare the number of events with and without a particle emitter.
- Are new events all similar, or are they different kinds of events (see entry under Analysis of cloud chamber events)?
Paths of charged vs. uncharged particles
The tracks that you see in a cloud chamber are lines of droplets that have condensed from a supersaturated alcohol vapor. Questions that students ask, or that a teacher could ask as open-ended questions include:
- What kind of particles pass through the cloud chamber?
- Where do the particles come from?
- Are the particles charged or uncharged, and how would we know the difference?
Most particles that we see in a cloud chamber originate from cosmic rays, but have scattered off other particles before getting to us. It turns out that charged particles, passing through the cloud chamber, ionize vapor creating nucleation sites for the condensation to start. Thus, charged particles leave trails in cloud chambers. Uncharged particles do not.
The cloud chambers described here have a strong magnet under the cloud chamber’s base plate. The strong magnet affects the direction of motion of charged particles. More questions come to mind:
- How does the magnet affect the motion of charged particles?
- Do all the trajectories have the same radius of curvature?
- What parameters affect the radius of curvature of each trajectory?
- How do we know if a given particle is moving from left to right and curving clockwise, or moving right to left and curving counterclockwise?
The magnet exerts a force on moving charged particles. The force is perpendicular to the motion of the particle, so, in a magnetic field, moving charged particles will curve in a circular or spiral path. The particles whose tracks appear to be almost straight lines have very high energies. The particles all move so close to the speed of light that we can not tell whether they are moving from left to right or from right to left. An electron might move from left to right in a clockwise path, but, under the same conditions, a positron might move from right to left in a counterclockwise path; the trails left by both trajectories would appear identical. It is, however, possible to tell the difference between electrons and, say, positron in pair production events. Sometimes you can see – or catch on video – an event where two trails appear to start from the same location. One trail that spins off in a clockwise direction and one trail that spins off in a counterclockwise direction, from the same point in space and time, is evidence that an electron-positron pair has just been created.
Quantification of observations:
- Measure the curvature of the path of a given event.
- Use the strength of the magnetic field and the charge of the particle to calculate the energy of the cosmic ray.
Further information about cloud chambers and bubble chamber path analysis:
Inquiry Type Activity
Many students might construct false explanations for what they see. It will be important to get the students to discuss why they think things are happening and then let them test (if possible) ways to change what they see. Here's some possible things to have students try:
- Does the amount of sunlight affect how many tracks or what kind are seen?
- Is there a better light source that could be used?
- Do windows (or the lack of windows) have any effect on the number or types of tracks seen?
- Are the tracks still there, even when the light is not on them and you can't see them?
-to be continued-
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