1.3 Thermal energy transfer by convection - mass movement of fluid due to a difference in density

• Convection also involves heat transfer via particles but this involves bulk movement of particles in liquids or gases (fluids) and cannot take place in solids.

• Heating a gas or liquid increases its thermal energy store which will distributes itself through the kinetic energy stores of the particles. Although from the point of heating, conduction will be slow, most heat will be transferred to the bulk of the fluid by convection.

• Convection occurs when hotter/warmer less dense fluid (gas/liquid) flows and rises, is replaced by cooler more dense fluid moving (flowing) downwards.

• This cycle of events is called a convection current and its 'mechanism' is explained below.

• When a material is heated the particles have more KE, move faster and tend to push each other further apart, ie the material expands, becoming less dense and rise, floating above the more dense cooler fluid.

• It is this change in density that cause convection to happen as gases and liquids are fluids - they can flow in convection currents.

• This is how hot water is produced in the hot water tank in the home, or the heating of water in a kettle where convection currents flow from the heating element so enabling all the water to be heated up.

• Note that the heating element must be near the bottom of the tank or kettle to produce the convection circulation to heat up all the water! If you put the heating element at the top there is no convection and all you do is heat up the top layer of water!

• Despite the name, radiators on the walls heat rooms up mainly by convection (there is some radiation too). Heat is transferred to the air particles when they collide with the radiator, and warm less dense air rises from the radiator towards the ceiling. The elevated air cools as the heat is distributed to the cooler air, which falls on the other side of the room. At the same time cooler air is drawn in at the base of the radiator to replace it - hence you get a convections current situation that gradually warms up all of the room.

  • See also notes on density

• The hot gases from a gas fire will always rise due to the immediate formation of a rising convection current (recycling air flow) which carries the heat around the room. The chemical energy store of the fuel gas increases the thermal energy of the contents of the room.

  • The energy is transferred in several ways.

  • (i) The hot flame gases will heat the immediate surrounding air by conduction.

    • This will cause lots of hot air to rise towards the ceiling.

    • Heat is dissipated to the contents of the room and surroundings and the risen air begins to cool and become less dense.

    • The more dense air falls and eventually circulate back round near the fire where cool air is drawn in to the fire.

    • The process repeats itself continuously as long as the fire is lit - hence the formation of the convection current flow of air.

  • (ii) The heat is also transferred by infrared radiation emitted from the hot flame - from the higher temperature flame to the lower temperature regions of the room.

  • Hot water or electrical radiators work in exactly the same way.

    • The surrounding air next to the radiator is heated by conduction.

    • The thermal energy from the hot water or electrical element heats the particles of radiator case.

    • The increased kinetic energy of the hot metal/concrete particles is transferred to the air particles in contact with the radiator i.e. those that collide with the radiator.

    • The air particles move faster and spread out to decrease the density.

    • The less dense air rises to start the convection current and are replaced by cooler air coming up and passing by the radiator surface.

    • Again, some energy will be transferred to the room from the radiator surface by infrared radiation - you can feel this for yourself by placing your hand near, but not touching the radiator.

• See also More on methods of reducing heat transfer