Thermal Expansion

 

Equipment:

  • Ball and ring
  • Bimetallic strip
  • Bunsen Burner
  • Striker
  • Gas hose
  • Beaker with cold water
  • Liquid nitrogen (optional)

Demo:

  1. Expanding Ball and Ring: At room temperature, the ball fits through the ring. When heated, the ball will no longer pass through.
  2. Bimetallic Strip: Made by welding together thin strips of brass and steel. The strip curls when heated and re-straightens when cooled.

Explanation:

When heat is transferred to an object it has a tendency to change shape, area and volume. This phenomenon is called thermal expansion.

  1. The ball can pass through the ring at room temperature. However, after it is heated it cannot due to thermal expansion.
  2. The two different metals on the bimetallic strip have different coefficients of thermal expansion. The metal will curve to the side that has a lower expansion rate. This strip is similar to the one that is used in thermostats. If it is a cold winter day in Santa Cruz and you have the heater on, it will eventually turn off when it is heated to the temperature you want and the strip bends enough to connect the switch to turn it off.