Color swirls

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Color Swirls

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NGSSStudents use a model to investigate how temperature-driven density changes can cause convection in a fluidStudents use evidence to construct an explanation of how temperature-driven density changes can cause convection in a fluid

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Which square has more dots?Which square is bigger?

square 2

square 1

Put the image in Figure 4 on the board for students to see. Ask them to think about and write down in their student worksheets their answers to these two questions: a. Which square has more dots?; and b. Which square is bigger? Ask students to share their answer to their first question. Explain that these dots represent molecules or pieces of matter, so when they count the number of dots, they are determining how much mass the squares have. Ask students to share their answer to their second question. Point out that the square represents a given amount of space, or area in which the dots are held.

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square 2

square 1Both squares have the same number of dots in a given amount of space

Using the diagram, show students that, if they cut the second square into four equal pieces, each fourth would have the same number of dots and amount of space as the first square.Explain to students that the reason why you can do this is that both of these objects have the same densityor amount of matter in a given amount of space. Depending on your students content background, you may choose to introduce the formula for density (density = mass/volume) and explain that it is an intensive property of objects (a property that does not change if the amount of material changes).

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Which square has more dots?Which square is bigger?

square 2

square 1

Now put the image in Figure 5 on the board for students to see. Once again, ask them to think and write down in their student worksheet their answers to the same two questions: a. Which square has more dots (or mass)?; and b. Which square is bigger (or has more space)?Ask students to share their answers. Students should mention that both squares have the same amount of matter, but square 2 is bigger.

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square 2

square 1Both squares have the same number of dots in a given amount of space

Remind students that density is a measure of the amount of matter in a given amount of space, and ask them if the squares have the same density. Encourage students to explain their answers by using the same method as step 4. Students should point out that, if we compare an equal amount of space from squares 1 and 2, there is more matter (more dots) in square 1, and therefore square 1 has a higher density.

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Which ball has a higher density?

golf ballping pong ball

Ask students why it might be useful to know an objects density. After sharing some of their answers, show students the golf ball and the ping pong ball, and explain to them that, while both balls take up the same amount of space (or have the same volume), the golf ball has more mass than the ping pong ball.Ask students to use their understanding of density to determine which ball has a higher density. Students should indicate that, with the greater mass and equal volume, the golf ball has a higher density.Show students a beaker or large clear cup filled with water and ask them what they think will happen when you place the golf ball in the waterwill it sink or float? After students share their predictions, place the golf ball in the water. Repeat the process with the ping pong ball. Explain to students that the golf ball sank and the ping pong ball floated because of their densities. Take the ping pong ball, push it into the water, and hold it at the bottom of the container. Ask students what they think will happen when you let go of the ballwill it stay at the bottom or will it rise to the surface? Ask them to justify their predictions using the balls density in their reasoning.Explain that, when materials with different densities are put together, those with higher densities will tend to sink, while those with lower densities will tend to rise. This movement of matter driven by density differences is a key process in many of the Earth systems, such as the formation of storms and the movement of tectonic plates.

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Convection

COOLINGHEATINGWikimedia CommonsConvection cellConvection cell

as water is heated from below, its density decreases and it begins to rise, while the cooler denser water sinks. The motion of water that results is known as a convection current.

Convection is heating through the mass movement of matter

Lets examine the example of the boiling pot of water as we break down the steps of convection. When the water is first added to the pot of water, all of the water molecules in that pot have more or less the same amount of thermal energy. The water is then in thermal equilibriumsince it all has the same amount of thermal energy, there is no large-scale heat transfer within the system. When the flame below the pot is turned on, it begins heating the pot through conduction to the metal. The metal, in turn, heats the water directly above it in the pot, and so the water begins gaining thermal energy. As a result, the molecules in this water begin moving faster, bumping into each other more and so spreading out over more space. In other words, the warmer water becomes less dense than the cooler molecules around it. Because the less dense warm water is below the more dense cool water there is an instability in the system, and the less dense material begins to rise as the more dense material sinks to take its place. This sinking and rising motion sets up a convection current that becomes the mechanism of heat transfer (Figure 1).

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Convection drives plate tectonics

plates at the surface of the Earth move because of convection cells within the Earths mantle. The heat source for these convection currents is the decay of radioactive isotopes.

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Convection creates wind

the Sun heats the Earths surface, and the surface in turn heats the air above, creating convective movement of air in the lower atmosphere that we experience as wind.

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CER TableCLAIMWhat do you know?EVIDENCEHow do you know that?REASONINGWhy does your evidence support your claim?Warm water rises.The red food coloring moved up after being warmed by the candle.The red food coloring sat on the bottom of the bowl until it was heated by the candle.Warm water is less dense.The blue food coloring on the sides of the bowl sank to the bottom.There is an instability in this system.The water in the bowl eventually turned purple.

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