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Physical
Convection Currents
Convection currents are flows in a fluid, such as air or water, that happen when warmer, less-dense fluid rises and cooler, denser fluid sinks.
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The idea
A convection current is a flow inside a fluid, such as air or water, caused by differences in density. The most familiar kind is natural convection. When part of a fluid is heated, it usually expands and becomes less dense, so it rises. Cooler, denser fluid sinks to take its place, gets heated in turn and rises, so the motion keeps going in a loop.
Convection is also one of the main ways heat moves. Conduction passes heat through direct contact, and radiation sends it as light, but convection carries heat along with the moving fluid itself. A pot of soup on a stove shows this, because the warm soup at the bottom rises, cools near the top and sinks again.
Convection currents are common in the atmosphere and the oceans, and they also happen in Earth's mantle, in stars and even in some solids such as ice. People rely on them in ventilation and cooling systems. Sometimes the moving fluid organizes itself into repeating loops called convection cells. Whether natural convection starts depends on the balance between buoyancy, which drives the flow, and viscosity and diffusion, which work against it.
Check yourself
Question 1 of 2
What are repeating loops of rising and sinking fluid called?
Composition
what it's made of
- Density differences — Differences in temperature or composition make parts of a fluid lighter or heavier than the fluid around them.
- Buoyancy — The upward push that makes less-dense fluid rise while denser fluid sinks.
- Heat source — Warming from below, such as a stove burner or sunlit ground, starts many convection currents.
- Sinking flow — Cooler, denser fluid moving down to replace the fluid that rose.
- Convection cells — Repeating loops of rising and sinking fluid that can form a regular pattern.
Connection
how it relates
Under Thermal Energy
Change
how and why it evolves
- Past
- People used rising warm air long before scientists explained it, for example in chimneys that carry smoke up and away. In the 1830s, the term convection appeared in a scientific sense in a treatise by William Prout.
- Now
- Today the word convection has different but related meanings in different fields of science and engineering. Air cooling without fans works this way, from tiny computer chips to large factory equipment.
- Next
- Better computer models will probably improve weather forecasts and help explain how mantle convection pushes tectonic plates.