HANDOUT B (with answers)
Powering the Movement of the Plates: A Model of a Convection Cell
PURPOSE:
To produce convection and to model plate motion
MATERIALS for groups of four:
Tap water; One 1000mL beaker; hot plate; food coloring gel; wood shavings; sheet of white paper; wood block same height as hot plate; bucket for collecting waste water material.
PROCEDURE:
1. Fill beaker approximately half full with tap water.
2. Place the hot plate and wood block adjacent to each other so that 1/3 of the beaker diameter is located on the hot plate and the opposite 1/3 is on the wood block.
3. After heating the water for a few minutes, gently drop the food coloring gel on the COOL side of the beaker (Try very hard NOT to disturb the water; DO NOT LET THE GEL TOUCH THE SIDE OF THE BEAKER).
4. Watch and wait for the food coloring gel to drop to the bottom of the beaker and observe the movements that occur. Draw an arrow on the sketch above to represent the motion of the food coloring as it moves in the beaker (approximately 2 to 3 minutes). In order to follow the movement of the food coloring gel more easily, place a piece of white paper behind the beaker.
5. What do you think is causing the food coloring to travel in circular paths?
As you increase the temperature of the water, it becomes less dense and starts to rise carrying the food coloring gel. When it reaches the top, the water cools becoming denser and begins to sink carrying the food coloring gel back down. The water will warm again and become less dense and continue to cycle in these up and down paths carrying the food coloring gel.
NOTE: You may have to change the water several times in order to clearly observe the path of the food coloring. If the water becomes a uniform color, this indicates the food coloring has completely diffused throughout and the convection cells can no longer be observed. Remember: Each time you must wait a few minutes for the water to reheat!!!
6. Why do you think the food coloring is rising on one side of the beaker while the food coloring on the other side is sinking? What could be the difference between the two sections of water containing the food coloring?
The food coloring gel is less dense on the side of the beaker that is rising and denser on the side of the beaker that is sinking. The water on the side that is rising is warmer and less dense. The water on the side that is sinking is cooler and denser.
7. When a substance rises in the beaker, what has happened to its density? What factors can cause a decrease in density?
The density decreases. Increasing the temperature; increasing pressure; and increasing molecular concentration.
8. What is the main difference between the two sections of the water in the beaker?
a. where the food coloring gel is rising: the water is warmer and less dense
b. where the food coloring gel is sinking: the water is cooler and denser
9. What are the currents called that are created by the rising and sinking of the water?
Convection currents or cells
10. Pour the water out of the beaker; refill the beaker ½ full with water. Place it on the hot plate and wood block as before. Reheat. Wait two or three minutes. Place wood shavings in the beaker. Observe. Record your findings. Is convection strong enough to move the wood shavings? Explain your answer.
The shavings will float on top of the water and the convection currents may move them around but they will not rise and sink.
11. Based on this model and your knowledge of the earth’s interior, how does this experiment give a possible explanation for the theory of plate movement and the cause of drifting of continents?
The heat from the Earth’s core heats up rocks. The rocks become less dense and move up into the asthenosphere of the mantle. These rocks come in contact with cooler rocks. During this contact, heat energy is transferred from the warmer to the cooler material. Once cooled, the material is denser causing it to sink again. It then comes in contact with heat from the Earth’s interior and repeats the cycle of rising again. This results in convection cells that transfer heat and create movement inside the earth which provides the energy for movement of the Earth’s plates. This is the prevailing theory for the energy source of Plate Tectonics.
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