Physics Dept. Demonstration Manual

DHT03 - Volumetric Expansion of Metal: Ball and Ring

Objective:

To illustrate thermal expansion (contraction) of a sphere and of a ring.

 
Apparatus:  
  • brass sphere and brass ring, each mounted on wands
  • aluminum cylinder and aluminum ring, each mounted on wands
  • dry ice slurry in a thermos flask (see Note 1)

DHT03 - Volumetric Expansion
of Metal: Ball and Ring

Note 1:
In past we have used a propane torch to demonstrate thermal expansion of a sphere and a ring. Following a suggestion by one of the physics instructor's we now cool the apparatus with a dry ice slurry. The new method is considered to be safer for the instructor and it also reduces the chance of damage to the apparatus through over heating. Aluminum in particular does not stand up well to the heat from the propane torch.

 

Method:

Part A: Aluminum cylinder and aluminum ring.

This pair have been machined such that at room temperature the cylinder will slip through the ring. Demonstrate this fit to the students.

Cool the ring by placing the ring in the dry ice slurry. After about 30 second or so, remove the ring from the slurry and demonstrate that the ring will no longer slip over the room temperature cylinder i.e. the "hole" shrinks. Note that as on tries to fit the ring over the cylinder, the contact rather rapidly causes the pair to adjust to nearly equal temperatures, after which the ring again fits.

Part B: Brass sphere and brass ring

This pair have been machined such that at room temperature, the sphere is too large to fit through the ring. Demonstrate this geometry to the students.

Cool the sphere in the dry ice slurry for about 30 seconds or so and again try to slip the brass sphere through the brass ring. It should now fit. Separate the pair before the sphere warms to the point where it will no longer slip through the ring. DO NOT let the room temperature ring become jammed on the partially cooled sphere (the ring could split as they come to temperature equilibrium).