Convection The driving force

Before you can understand convection, you need to remember density! Density = Mass Volume What happens if the volume increases but the mass doesn’t change? If you go from 1/2 to 1/4 (the bottom number got bigger) the value of the fraction goes down. So, if the volume goes up without changing the mass, the density will go down. Less dense things float up in fluids!

How do you make the volume increase without adding something and changing the mass? REMEMBER - temperature actually measures the speed of molecules. If something is heated, its molecules will speed up. If you warm a balloon, the air molecules in it will speed up and push harder on the walls of the balloon causing the balloon to expand (volume increases).

Convection Lab - Do quality work! 1. Make a side view sketch of the set-up. 2. Use arrows ( , , ,  ) to show directions water is moving throughout the tank (four sets of arrows: middle, top, sides, and bottom.) 3. Make top view drawings (before & after) to show what happens to dots floating on the water. 4. Explain completely why water moves in each of the four directions. (Start at middle bottom.) 5. Could this movement occur in the Earth’s interior? Describe how. ( THINK! Are there warmer and cooler places deep underground? Can underground materials flow?) 6. (Next slide)

6. Draw a cross-section view of the Mid-Atlantic Ridge with the asthenosphere below it. Draw arrows showing convection in the asthenosphere that could explain how the plates are pulled apart. Label everything. You show after time goes by drawing.

Why the Water Goes UP: Heat causes the water molecules to speed up. The water expands. The volume increases, but the mass doesn’t change, so the density decreases. The less dense, warm water floats upward. (7 things to say about going up!)

Why the Water Goes to the side: Warm water rising from the bottom pushes the warm water at the top towards the sides

Why the Water Goes DOWN: Cold causes the water molecules to slow down. The water contracts. The volume decreases, but the mass doesn’t change, so the density increases. The more dense, cold water sinks down. (7 things to say about the water sinking down!)

Why the Water Goes to the center: Cold water sinking from the top pushes the cold water at the bottom towards the center

Reading & Writing: Pages in purple book: Or - Pages in tan book: Section Convection Structure of Sun The driving mechanism For each of these book sections, write a paragraph explaining the significance of convection to that topic.