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Thermal conductivity: metal v. plastic

Demonstration

This demonstration shows, against intuition, that an ice cube melts more quickly when in contact with a metal block than a plastic block.

Apparatus and materials

Metal and plastic blocks of identical dimensions
(approx 5 cm square by 1 cm thick)
Ice cubes at 0°C
Clock

2 temperature probes with displays (optional)

Health & Safety and Technical notes


There are no safety problems with this demonstration.

A suitable ‘ice melting kit’ is available from Timstar

Procedure


a Pass the metal and plastic blocks around the class; ask your students what differences they observe. They are likely to comment that the metal block feels colder to the touch than the plastic one.
b Explain that you are going to place identical ice cubes on each block. Ask for predictions as to what will happen.
c Place one ice cubes on each block. Observe the ice melting over a few minutes.
The film below shows how to carry out this demonstration, together with typical results.

Thermal conductivity

d You could use a timer to determine the time for each cube to melt completely. Alternatively, attach a temperature probe to each block and observe how their temperatures change.

 

 

Teaching notes


1 This demonstration can form the introduction to a structured development of ideas about energy transfers between objects at different temperatures. Ice cubes are placed on metal and plastic blocks; the cube placed on metal melts much more quickly than the cube placed on plastic. This is counterintuitive (for many students) because metals feel cold while plastics feel warm.

2 Energy is transferred to the ice cubes by conduction from the blocks on which they have been placed. A metal block is a better conductor and so energy is transferred more quickly to that ice cube.
Why isn’t this obvious? Metals feel cold to the touch. This is because, when you touch a piece of metal, energy conducts away from your fingers into the metal, lowering the temperature of your fingers. Plastics are good insulators so, even though the plastic is at a lower temperature than your fingers, little energy is lost to the plastic and it feels warm.
Hence it is best to start the demonstration by asking your students to feel the two blocks so that they may be misled by this experience. Then show that the ice on the metal block melts more quickly, and discuss the reasons.

3 You may then wish to take the discussion to a deeper level. Students may think that some materials (metals, water) are intrinsically cold, while others (plastic, wood) are intrinsically warm. (We talk about ‘warm clothing’). So you could use thermometers to test the temperatures of different objects and materials in the room.
Then repeat the demonstration with electronic thermometers monitoring the temperatures of the blocks as the ice cubes melt. Show that the two blocks are both at room temperature at the start, and observe the rapid drop in temperature of the metal block.

4 You could ask your students to explain why the temperature-time graphs for the two blocks are curved (they are roughly exponential). The reason for this is that the rate of transfer of energy from the block to the ice decreases as the temperature difference between them decreases.

5 Note that there is a complication to this analysis which we have avoided mentioning so far. The rate at which energy is transferred to the ice depends on both the conductivity of the block and its heat capacity. It might be that the ice on the plastic block melts very slowly because the temperature of the plastic block drops very rapidly to that of the ice. This would happen if plastic had a low specific heat capacity. This is shown not to be the case by when a temperature probe is used.

 

Related guidance


Some general notes on teaching about conduction, convection and radiation:

 

Related experiments


An experiment to  compare the thermal conductivities of different materials: