Heat is conducted at different rates along rods of different metals. You can show this with wax and a blowtorch!
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Teachable Topics:
- thermodynamics
- heat transport
- thermal properties of materials
Molecules in a solid vibrate. If the solid is heated, the molecules in that solid have more energy, so they vibrate more vigorously.
If a solid rod is heated at one end, the molecules at that end vibrate more than those at the other end. The vigorously vibrating molecules cause neighboring molecules to vibrate more as well. Heat added to one end of the rod is thus transferred to the other end. This is called heat conduction.
The best conductors of heat are metals. Further, some metals conduct heat better than others. This can be shown with the conductometer. The conductometer is five rods of equal length attached to a handle. At the free end of each rod is a small piece of wax holding up a small flag. The rods are each made of a different metal - steel (flag #1), brass (#2), aluminum (#3), stainless steel (#4), or copper (#5).
When the handle is heated, the heat is conducted along each of the rods. This heat will eventually melt the wax and make the flags fall. Because some metals conduct heat better then others, the flags will fall one by one, starting with the flag on the rod which best conducts heat (aluminum) and ending with the flag on the poorest-conducting rod (stainless steel). The poorest-conducting rod happens to be such a poor conductor that the aluminum rod melts before the flag falls off the stainless steel rod.
Figure 1: Apparatus
Apparatus:
- lab stand
- blowtorch
- toothpicks, wax, and numbered flags
- metal rods of similar length and diameter
Procedure:
- Fill the holes in each rod with solid wax.
- After making small numbered flags from paper and toothpicks, stick one into each wax-filled hole.
- Mount the conductometer apparatus above a gas torch.
- Turn the gas torch to a medium setting, and watch as each flag falls in turn.
SAFETY WARNINGS:
- When heating the metal rod, DO NOT heat for more than 5 minutes! You may end up melting the rods off the handle.
- The blow-torch can cause serious burns if used improperly. Do not use a high flame when heating the metal.