Convection Currents in Science, What They Are and How They Work

Water boiling in a pot on a stove.

Three-shots/Pixabay

Convection currents are flowing fluid that is moving because there is a temperature or density difference within the material.

Because particles within a solid are fixed in place, convection currents are seen only in gases and liquids. A temperature difference leads to an energy transfer from an area of higher energy to one of lower energy.

Convection is a heat transfer process. When currents are produced, matter is moved from one location to another. So this is also a mass transfer process.

Convection that occurs naturally is called natural convection or free convection. If a fluid is circulated using a fan or a pump, it's called forced convection. The cell formed by convection currents is called a convection cell or Bénard cell.

Why They Form

A temperature difference causes particles to move, creating a current. In gases and plasma, a temperature difference also leads to regions of higher and lower density, where atoms and molecules move to fill in areas of low pressure.

In short, hot fluids rise while cold fluids sink. Unless an energy source is present (e.g., sunlight, heat), convection currents continue only until a uniform temperature is reached.

Scientists analyze the forces acting on a fluid to categorize and understand convection. These forces may include:

  • Gravity
  • Surface tension
  • Concentration differences
  • Electromagnetic fields
  • Vibrations
  • Bond formation between molecules

Convection currents can be modeled and described using convection-diffusion equations, which are scalar transport equations.

Examples of Convection Currents and Energy Scale

  • You can observe convection currents in water boiling in a pot. Simply add a few peas or bits of paper to trace the current flow. The heat source at the bottom of the pan heats the water, giving it more energy and causing the molecules to move faster. The temperature change also affects the density of the water. As water rises toward the surface, some of it has enough energy to escape as vapor. Evaporation cools the surface enough to make some molecules sink back toward the bottom of the pan again.
  • A simple example of convection currents is warm air rising toward the ceiling or attic of a house. Warm air is less dense than cool air, so it rises.
  • Wind is an example of a convection current. Sunlight or reflected light radiates heat, setting up a temperature difference that causes the air to move. Shady or moist areas are cooler, or able to absorb heat, adding to the effect. Convection currents are part of what drives global circulation of the Earth's atmosphere.
  • Combustion generates convection currents. The exception is that combustion in a zero-gravity environment lacks buoyancy, so hot gases don't naturally rise, allowing fresh oxygen to feed the flame. The minimal convection in zero-g causes many flames to smother themselves in their own combustion products.
  • Atmospheric and oceanic circulation are the large-scale movement of air and water (the hydrosphere), respectively. The two processes work in conjunction with each other. Convection currents in the air and sea lead to weather.
  • Magma in the Earth's mantle moves in convection currents. The hot core heats the material above it, causing it to rise toward the crust, where it cools. The heat comes from the intense pressure on the rock, combined with the energy released from natural radioactive decay of elements. The magma can't continue to rise, so it moves horizontally and sinks back down.
  • The stack effect or chimney effect describes convection currents moving gases through chimneys or flues. The buoyancy of air inside and outside of a building is always different due to temperature and humidity differences. Increasing the height of a building or stack increases the magnitude of the effect. This is the principle on which cooling towers are based.
  • Convection currents are evident in the sun. The granules seen in the sun's photosphere are the tops of convection cells. In the case of the sun and other stars, the fluid is plasma rather than a liquid or gas.