Convection and Conduction


Purpose:

To compare two methods of heat transfer in the atmosphere: convection and conduction

Equipment:

Two tall cylindrical beakers, water, food coloring, 2 salsa containers, a short piece of metal chain.

Early Preparation:

The metal chain is placed in one salsa container. Subsequently both salsa containers are filled with equal amounts of water dyed by food coloring and placed in the freezer until the water is frozen.

Procedure:

1) Two tall beakers, A and B, are filled with warm water (3/4 full only).

2) Ice chunks are taken out of the salsa containers and carefully placed in the tall beakers: The ice with chain in Beaker A; the ice without chain in Beaker B.
 
 

What happens?

 
Beaker A
Beaker B

Immediately:

  • The ice without chain floats near the surface of Beaker B
  • The ice with chain sinks to the bottom of Beaker A

After few minutes:

  • In Beaker A, the cold, colored water remains near the bottom of the beaker, and warm clear water stays near the top
  • In Beaker B, the dyed water melted from the ice quickly starts sinking down and warm water from the bottom of the beaker rises to replace it

After 30 minutes:

  • The ice is still largely unmelted at the bottom of the beaker A. Just above the surface of the ice, there is a small layer of colored water. Above that level, the water is clear and much warmer than the ice (at a temperature fairly close to it's initial temperature)
  • In Beaker B, all ice has melted, and all of the water in the beaker is of uniform temperature and color

 

Explanation:

Warm water is more buoyant than cold water. Since the warm water is below the ice and cold water in Beaker B, the warm water rises, whereas the cold water sinks.  This is convection in action!

In Beaker A, on the other hand, the warm buoyant water overlays the cold ice. This is a very stable arrangement, and convection does not occur.  Heat transfer only occurs through conduction in this case.
 

Conclusion:

In fluids, such as water and air, convection is a much more efficient method of heat transfer than conduction.  This difference in efficiency produced the dramatic difference in the time required to melt the ice. Although conduction was at work in both cases, it transferred much less heat than convection.