Convection

• Convection is the process where heat is transferred through a fluid (gas and liquid) by the movement of heated particles of the liquid

• Recall movement of molecules in kinetic theory and the energy of the molecules.

Convection in Liquids

• Hot liquids rise as they expand under heat (less dense)

• Cool liquids fall (relatively denser)

• Movement of liquids due to this effect is called the ‘convection current’

Before AfterVideo Demo

Convection in Liquids

Convection in Gases

• Hot gases rise.• Cool gases fall.• Using this effect, ventilation of a room can be

improved by purposeful design, or hot air retained in the winter for better effect.

Convection in Nature

Convection in Nature

• Land heats up faster in the day than the sea.• Air above land is hotter and rises.• Cooler air over water will come in to replace the risen hot air.• Night?

Earth Moving Convection Currents

Think about…

• Why place the air-con on the top of the wall?

• How do planetary winds form? (not the land/sea breezes)

• Why is the ventilation fan in the toilet near the top of the wall or on the ceiling?

Cold air flows down to the room. The warm air at the bottom will rise and get cooled. The convection current will form.

Hot air from equator flows to the colder regions of the planetary poles. Cold air from the poles is forced down to the equator, forming the planetary winds.

Radiation

• Radiation is a form of heat transfer that requires no medium to travel.

Heat is transmitted in the form of energy waves.

Energy travels until it gets absorbed somehow.

Radiation Emission

• All objects above 0K emit heat radiation• This energy form is also known as infra-red.• Heating objects will make it radiate more

heat• Dull and dark surfaces will radiate better

than shiny bright surface.• Bigger surfaces radiate more as well.

Infrared Picture of Earth

infrared visible ultraviolet

extreme ultraviolet X-ray Gamma rays

Radiation Absorption

• What kind of surfaces absorb radiation best?

• Fill middle jar with hot water and measure temperature of adjacent jars over time.

HOT water

Radiation Emission

• Fill 2 similar flasks with boiling hot water.

• Take the temperatures of both flasks.

Radiation Absorption /Emission Results

• Dull and black surfaces absorb and emit more heat.

• Shiny and bright surfaces absorb and emit less heat.

Think about…

• Kettles are shiny to reduce heat lose through radiation. It is shiny outside, but does it matter to be shiny on the inside? (its dark inside no?)

• Car radiators are black. What other technology are also black for the same reason?

• What colour of winter clothing should you wear in winter?

Conduction, Convection and Radiation – Applying knowledge

• The Vacuum Flask (Thermos Flask)

• Keeps water hot (or cold) by harnessing knowledge of heat transfer reduction

Parts of the Vacuum Flask

• Double Glass Walls

• Silvered Surfaces

• Vacuum

• Insulation Supports

• Insulated Cover

- poor conductor

- prevents radiation (poor absorber and emitter)

- No conduction or convection

- Reduce conduction and support the vacuum chamber

- Prevent conduction of heat and convection of hot air

Think about…

• What other piece(s) of technology makes use of knowledge of the 3 ways of heat transfer?

• Your room is hot. Aside from turning on the air-con, is it more efficient to have a fan sucking the hot air out the window or blowing the cooler air into the room? (i.e. which will cool your room faster?)

Fridge, Heat Sinks, Car radiator, air con duct insulation