Two strips of different metals, invar steel and brass, are welded together to form a bimetal strip. Since each metal has a different coefficient of thermal expansion, heating the bimetal strip will result in the metals expanding at different rates, causing it to bend.

When heating, always wear goggles and handle the flame with care, ensuring that it is not pointed near students or flammable materials. Use in a well ventilated classroom.

Engagement Suggestion

Background

The amount a metal expands or contracts with temperature is governed by its coefficient of thermal expansion, a property which varies between different metals depending on their molecular structure. Invar steel is an alloy designed to have an exceptionally low coefficient, about one-tenth that of most steel, while brass has a higher coefficient than even ordinary steel. So the brass expands much more rapidly than the steel does when heated.

Bimetallic strips like this are used in some types of thermometers and thermostatic controllers (including many older window thermometers and household thermostats). Check out demonstrations I1-17 and I1-18 for examples and to see how this works.

A quartz flask (very small thermal expansion) holds water at 0 degrees centigrade (in an ice bath). The water fills the entire volume under the stopper and extends up into a capillary tube. When the flask is removed from the ice bath its temperature rises, as can be read on the thermometer. The water level in the capillary tube initially goes down, but reaches a minimum level at about 4 degrees and then rises as the temperature continues to increase. The photograph at the right is a close-up that permits both reading of the temperature and observation of the water volume.

This property of water is anomolous in that it is one of only a very few substances that behave in this manner.

A calibrated thin spherical metal shell with air and shot inside sinks in water at approximately 115-120 degrees F. The water is then cooled by a fan, whereupon the sphere rises to the top of the water when a temperature of about 100-110 degrees F is reached (this cooling can take up to 15 minutes depending on room temperature and humidity).

The water can be stirred continually to keep the temperature uniform using the digital thermometer probe, which simultaneously reads the temperature, which is displayed on a large scale. If the sphere starts out floating and is sunk by heating the water, the demonstration requires more time due to surface tension.