Prepare the strips for the activity:
Add a small smear of vaseline to the end of each strip. (If you are using spoons, try and find ones with flat handles, and add the vaseline to the handle end.) Make sure you use the same amount each time and add it in the same spot. (Using an applicator, such as a coffee stir stick, will help to make the amounts more consistent - see photo.)
Push a penny, or a nickel, into the vaseline on each strip.
Each table group can have a set of strips with pennies, and a coffee cup.
Add just-boiled water to fill the coffee cup until quite full, then simultaneously add the metal, plastic and wooden strips to the water with the pennies pointing upwards. (If you are using metal and plastic spoons, place the wider scoop end into the hot water). Make sure the strips are sloped outwards by the same degree, so that this is not a variable in the penny falling off. For most students, it is best if the teacher does this step, to make it as fast and consistent as possible.
Students record on worksheet (see attachment) which penny falls off first, second and third. (See photos for my usual order of pennies falling off.) Some pennies may stay stuck for longer than you want to run the activity, but make sure at least one has fallen off before stopping. If the water has cooled before some pennies have fallen, take out the strips, replace the water with new just-boiled water, and return the strips to the cup - students will become quite engaged in the "race" as the last pennies in the class fall.
Metal strips are expected to release the penny first, but some experiments may differ. Plastic and wood release the penny later.
Record the class results, to find out what happens most of the time, and to use for discussion.
Discuss the mechanism:
Heat moves up the strip by conduction. Once the heat energy reaches the vaseline it melts it and causes the penny to fall off. The different materials conduct heat at different rates: metals conduct heat the fastest, wood and plastic much slower.
Discuss the molecular mechanism with older students:
The molecules of the water are moving around fast. As they bump into the end of the strip that is immersed in the water, they transfer their energy to molecules in the strip, which also start to move around faster. The molecules at the bottom of the strip bump the molecules higher up the strip and their heat energy is transferred too, so spreading the heat energy up the strip. In different materials, the molecules are more or less able to transfer heat energy to the neighbouring molecules, so the rate of heat transfer varies. When the heat energy reaches the vaseline it melts it (the molecules of the vaseline move faster as they change from solid to liquid). The melted vaseline can no longer hold onto the penny, so the penny drops.
The movement of heat when molecules transfer energy between each other by colliding with each other is called “conduction”.
Metals are better heat conductors than plastic and wood. A material that is not a good conductor is an "insulator".
