PHYSICS PROJECT (:

Convection
Definition:
It is the main mode of heat transfer in fluids- liquids and gases (does not occur in solids!), which involves movement of hotter fluids from hot region to cool region, with the medium moving.

Why only occur in liquids/gases, not in solids?
In liquids and gases, thermal energy is transferred through the movement of particles, while in solids, thermal energy can only be transferred through vibrations of particles and particles are not able to move. Convection requires the movement of medium, particles in solids are not able to move, hence it only occurs in liquids and gases.

Formation of Convection Current
Convection Current is set up due to the difference in density.

Convection in liquids

Diagram shows direction of convection current in liquids.

1) When heated, water at the bottom expands.
2) Expanded water becomes less dense than cooler water.
3) Hotter, expanded water becomes lighter and starts to rise.
4) Cooler water is denser than the hotter water.
5) Cooler water starts to sink.
6) An upward current moves in the direction as shown, current forces cooler liquid to replace the hotter liquid.

The same concept applies to convection in gases.

Warm air rises, cool air sinks.

Main concept from both diagrams:
Convection current is set up by the difference in density,
with less dense warm fluids carrying thermal energy upwards.
hot liquid/gas rises, cool liquid/gas sinks.

Applications of convection
1) Land & Sea Breeze

Land is heated faster in the day, warm air rises, cooler air sinks and moves in.
Convection Current is set up.


Land is cooled faster in the night, sea becomes warmer,
warm air from sea rises, cooler air on land sinks.

2) Cooking appliance (eg oven)

Heating element is found at the bottom, so that convection current can be set up to increase temperature of appliance, warms up food. Heating element causes air at the bottom to be warmer. Warm air rises, cool air sinks. Convection currents help to heat up appliance.

What happens if heating element is placed on top?
Convection current is not set up. Heating element causes air at the top to be warmer. Hot air unable to rise, cool air remains, temperature in appliance remains the same, appliance is unable to heat up.

3) Refridgerator

Freezing unit is placed on top, so as to set up convection current. Air on top is cooled by the cooling unit, becomes denser, hence sinks. Warm air at the bottom is less dense, hence rise. Cooler air on top pushes the warm air at bottom to the top. The entire space in the refidgerator is cooled.