Wet and Dry bulb thermometers

Relevant Resource questions :

Concepts demonstrated :

• Phase transformations
• evaporation (Maxwellian distribution)
• kinetic nature of particles
• evaporative cooling

• ether, alcohol, acetone
• thermometer or thermocouple
• CCD camera

• Place some ether in a watch glass and use the CCD camera to show the experiment to the whole class.

Physics involved :

At the molecular level the particles in the ether (or other liquids) have a maxwellian distribution of energies. This makes it possible for some of the particles to "boil" off even though the ether is not at its boiling temperature. Since the high energy molecules escape this lowers the average kinetic energy of the rest of the ether and therefore lowers the temperature. The ether gets cooler!

How to use the Demonstration :

• POE - ask the students to explain what will happen if some ehter is left in a watch glass for a few minutes.
• Perform the experiment showing that the ether evaporates.
• Ask the students to explain why the ether evaporates.
• Ask them what other effects are present while the ether is evaporating. ie the ethers temperature drops.
• Ask them to explain how the cooling works at the molecular level.

• Cloth water bottle hung in front of a car radiator as you drive.

• Repeat the experiment using dry ice (solid CO2). In this case sublimation occurs (solid to gas). Compare this experiment to the evaporation above.
  POE - do the students expect sublimation to occur rather than melting. Compare the experiment to the melting of normal ice.

• There may be a need to explore whether there are second order effects in the experiments above which complicate the true explanation.