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 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.

A variable transformer, or Variac, is connected to two identical incandescent light bulbs in parallel. These bulbs are viewed behind red and blue filters respectively. As the voltage is increased by the variac, the lights glow more brightly, and more light is seen through the blue filter relative to that of the red filter. Very little or no blue is seen at low voltages, whereas red is seen to be emitted even at very low voltages.

Engagement Suggestion:

Background:

Wilhelm Wien postulated in the 1890s that the power curve of blackbody radiation from an object could be computed from its temperature. His original calculations, obviously, did not take quantization into account; in modern practice, the revised calculations are still commonly referred to as Wien's Law.

Note that this apparatus only works with incandescent lightbulbs. Fluorescent and LED bulbs do not produce their primary light through thermal excitation, and thus don't produce the same kind of blackbody spectrum.