CT: Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.
Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.
The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.
S: http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/heatingandcooling/heatingrev4.shtml (last access: 18 February 2015)
N: 1. 1620s, from Latin convectionem (nominative convectio) “the act of carrying,” noun of action from past participle stem of convehere “to carry together,” from com- “together” (see com-) + vehere “to carry” (see vehicle). Related: Convective. Convection current recorded from 1868.
2. Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.
S: 1. OED – http://www.etymonline.com/index.php?term=convection (last access: 18 February 2015). 2. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html (last access: 18 February 2015).