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# How are convection currents related to density?

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Convection currents occur due to the density difference between hot and cold fluids. A fluid that is heated from below expands and rises. When this fluid becomes cool, it contracts and sinks. This continuous cycle of expansion and contraction causes convection cells.

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Convection is the only one of the three heat transfer methods to involve the movement of matter. The other two methods, conduction and radiation, transfer heat between relatively stationary medium particles or without the need for a medium at all, respectively.

The decrease in density that accompanies heating is explained by the ideal gas law. At constant pressure, the temperature of a gas is directly proportional to its volume. As the temperature increases, the kinetic energy of the gas molecules or atoms also increases. The more energetic these gas molecules are, the faster they move. Assuming a constant average time for collisions between gas molecules, an increase in speed corresponds to a larger average distance travelled between collisions, which correspond to a larger volume being occupied by the same number of molecules.

Because volume is inversely proportional to density for the same mass, this increase in volume with an increase in temperature corresponds to a decrease in density. Less dense fluids float on top of more dense fluids, so the hotter, lighter gas floats on the cooler, denser gas. Similar principles can be applied to liquids, which undergo less expansion and contraction with heating and cooling respectively.

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## Related Questions

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Laminar and turbulent flows are the two major forms of fluid flow, with laminar flow being an orderly flow with all the fluid molecules moving parallel in the direction of flow, whereas in turbulent flow, different streams of fluid form eddies and unpredictable currents in a different direction than the overall direction of flow. Which type of flow arises in a fluid depends on its speed relative to its viscosity and the size and shape of the pipe or other channel carrying the fluid. Viscosity is the resistance to flow, and more inherently viscous fluids are more resistant to turbulence.

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Convection works by transferring heat from a hot substance to a cooler one through the motion of one of the substances. For example, as wind passes over a hot substance, heat from the substance transfers to the air particles, cooling the hot substance and slightly warming the air.

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Non-Newtonian fluids are those that behave differently than Newtonian fluids do with regard to viscosity. Typically, stress causes non-Newtonian fluids to alter their flow patterns or viscosity. The stress can cause the viscosity to increase or decrease, depending on the particular non-Newtonian fluid.