Objective:
– To illustrate differential expansion of steel and brass and the principle of a bimetal thermostat.
Apparatus:
- Bimetallic wand
- dry ice slurry in a thermos flask (see Note)
- overhead projector display board
Note:
In past we have used a torch to heat the strip. Following a suggestion from one of the physics instructor’s we now cool the strip with a dry ice slurry. The cooling method is considered safer for the instructor and also reduces the chance of damaging the apparatus via over enthusiastic application of heat.
Method:
With the display board placed on the stage of the overhead projector, turn on the overhead projector and adjust the focus until the scale of the display board is sharply projected onto the screen. Place the bimetal strip in the display board clip to show the strip’s shape at room temperature.
Remove the bimetal strip from the board clip and put the strip in the dry ice slurry. Wait about 30 seconds for the strip to cool then remove the strip from the ice and place the strip back into the display board clip (brass side, i.e. concave, side to the left.
Observe that the strip returns to a straight shape as the two metals warm up to room temperature. The scale suggests that one could, after suitable calibration, use the amount of bend as a measure of temperature. The “0” on the scale would correspond the room temperature for this design.
Thermal Expansion Demo: Bimetallic Strip
– Video Created at Utah State University by Professor Boyd F. Edwards and assistants.