The three ways to transfer heat energy are thermal conduction, convection currents, and radiation. Convection currents can transfer heat in a fluid, which is a liquid or a gas, but can not transfer heat in a solid. The fluid circulation carries thermal energy from the heat source to the other portions of the fluid.
What Causes Convection Currents
Imagine a cool room with a radiator at one end and no fans or any other forced air systems to blow the warm air to the other side of the room. How does the radiator heat the entire room? The key is convection currents. The hot radiator sets up convection currents that transfer thermal energy to the rest of the room and eventually heat the entire room. How do convection currents work?
The hot radiator warms the air that is closest to the radiator. The warm air expands, becomes less dense and rises to the top of the room. When the air reaches the top of the room it is pushed sideways towards the far wall by the more recently warmed air rising from the radiator below. In this way warm air moves to the other side of the room. Once on the other side of the room the air drops down both because it has cooled a little and because the air behind it continues to push on it. The air then continues to circulate back to the radiator and repeat the process.
By continuing to circulate, the convection current transfers heat energy to the other side of the room and heats the entire room. This process can work in any fluid, whether a liquid or a gas. Because matter must circulate for convection currents to transfer thermal energy convection currents can not work in a solid. However they can efficiently transfer heat in a fluid.
When a fan or other mechanism circulates the fluid more rapidly, it is called forced convection. If the fluid circulation is not forced or helped in any way, it is called natural convection.
Examples of Convection Currents
The only things required for convection currents are a heat source and a fluid that can circulate to transfer the heat energy. The air heating a room is a small scale convection current. There are also many larger scale convection currents.
Wind patterns are large scale convection currents in the atmosphere. If there is a warmer spot on Earth, convection currents are set up resulting in wind as the air circulates. Ocean currents are the same effect but in the water rather than the atmosphere. Heat in one portion of the ocean sets up convection currents which result in ocean currents as the water circulates to transfer heat energy.
Beneath the Earth, the mantle seems solid but is really in a slowly flowing plastic putty-like state. Heat sources in Earth's core set up slowly moving convection currents in the mantle. According to the theory of plate tectonics, crustal plates float on the mantle and slowly drift from the convection currents in the mantle.
The Sun's surface has a mottled appearance called granulation. These granules result from convection currents transferring energy from the solar interior to the surface. The individual convection current cells form the granules observed on the Sun's surface.
For a liquid or a gas, which does not conduct heat well, convection currents are usually the most efficient way to transfer thermal energy.
Further Reading
Wilson, J.D., Buffa, A.J., and Lou, B., College Physics 6th ed., Pearson, 2007.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
