Ice Melting Blocks

Equipment:

  • Ice (located on the second story of Thimann Labs)
  • Metal Block (both blocks are black and come in a kit together)
  • Plastic Block
  • Two Rubber Rings (to keep water from running)
  • Infrared Thermometer

Demo:

  1. Place the rubber rings on top of each block.
  2. Have a student feel each block and ask them which block feels colder.
  3. Have the students guess if the ice will melt faster on the plastic or metal block.
  4. Place ice on top of the blocks and observe which ice cube melts faster

Explanation:

Before the ice is even placed on top of the two blocks, the instructor can use the infrared thermometer to show that both blocks are at the same temperature. During testing, both blocks were measured to be at 17.5°C. The reason the aluminum block feels much colder is because it transfers heat from the skin  at a faster rate. Aluminum is an excellent conductor of heat, like most metals.

Generally, materials that are good conductors of electricity(metals) are good conductors of heat. On the other hand, good insulators of electricity( plastic, wood, rubber) are generally bad conductors of heat.

If we have two sets of molecules that are at different temperatures, the one that is “hot” will have a higher average kinetic energy. If the two sets collide there will be an energy transfer from the hot to the cold molecule. This transfer of heat between two objects is called thermal conduction. The equation for the rate of thermal conduction is given below:

Where, Q is the amount of heat transferred in a time t, k is the thermal conductivity constant of the material, A is the cross sectional area of the material transferring heat, d is the thickness of the material, and ΔT is the change in temperature.

For aluminum k=205(W/m K)

For plastic foam insulation k=.03 (W/m K)

Eventually, the two systems will come to thermal equilibrium and the ice will melt onto both blocks. The material with a greater heat conductivity constant will transfer heat much faster. Ultimately, the aluminum block will melt the ice much faster since it has a higher rate of heat transfer.

Written by Nick McCabe