Convection
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Convection is the transfer of thermal energy through a fluid (liquid or gas) by the bulk movement of the fluid itself, driven by differences in density caused by temperature differences.
Mechanism of Convection
Convection occurs because most fluids expand when heated. As a region of fluid is heated, its particles move faster and spread further apart, causing the fluid in that region to expand and become less dense than the surrounding cooler fluid. The less dense, warmer fluid rises, while the denser, cooler fluid sinks to take its place. This creates a continuous circulation pattern known as a convection current.
The sequence of events can be summarised as follows:
- A region of fluid near a heat source is heated and expands.
- Its density decreases; it becomes less dense than the surrounding fluid.
- The less dense, warmer fluid rises (due to upthrust).
- Cooler, denser fluid flows in to replace it from the sides or below.
- The cooler fluid is in turn heated, and the cycle repeats, forming a convection current.
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Convection cannot occur in solids because the particles in a solid are held in fixed positions and cannot move from place to place to carry energy.
Natural Convection
Natural (or free) convection arises purely from density differences due to temperature gradients. No external device is needed to drive the fluid movement. Examples include the circulation of water when heated in a beaker, and the movement of air above a radiator.
Everyday Examples
- Domestic hot water systems: A boiler heats water at the bottom of a tank. The hot water rises to the top while cool water descends to be heated, setting up a convection current that gradually heats all the water in the tank.
- Room heating: A hot radiator heats the air in contact with it. The warm air rises and spreads across the ceiling, while cooler air near the floor moves toward the radiator to be heated, circulating warm air around the room.
- Sea breezes: During the day, land heats up faster than the sea. The air above the land becomes warmer and less dense, rises, and cool air from over the sea flows inland to replace it, creating a sea breeze.
- Land breezes: At night, land cools faster than the sea. The air above the sea is warmer and rises, drawing cooler air from the land out to sea.
- Gliders and birds of prey: Rising columns of warm air (thermals) created by convection over land are used by gliders and soaring birds to gain altitude without powered flight.