Conduction, Convection, Radiation

The picture shows all three processes used in one instance.

Conduction, Convection, Radiation

You will be able to see all three heat transfer processes occurring when you stand next to a bonfire.



You will probably need to stand at least 20 feet away from a big bonfire like the one shown in the picture. What keeps you away is heat radiating from the fire through infrared radiation. The flames and smoke are carried upward by convection: Air around the fire heats up and rises. The ground 3 feet beneath the fire will get hot, heated by conduction. The top layer of soil is directly heated (by radiation), and then the heat is conducted through layers of dirt deep into the ground.
To build a good thermos, what you want to do is reduce these three heat transfer phenomena as much as possible.

Examples of heat transfer by radiation

• The thermos or vacuum flask

A practical vacuum flask is a bottle made of glass, metal or plastic with hollow walls; the narrow region between the inner and outer wall is evacuated of air. It can also be considered to be two thin-walled bottles nested one inside the other and sealed together at their necks.

Using vacuum as an insulator avoids heat transfer by conduction or convection. Radiative heat loss can be minimized by applying a reflective coating(silver) to surfaces. The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so does not exchange much heat with the contents, thus does not affect their temperature much.

• The internal silvered surface reflects heat back into the contents.



• The Flask itself consists of a 'double wall' glass vessel.

Heat can only pass through a vacuum by radiation, and radiated heat from outside is being reflected back to the outside by the silvered surface on the outside walls of the glass vessel. In this way, heat transfer into and out of the container (whether hot or cold drinks are contained), will not easily occur. Heat by conduction and convection is also minimised by this construction.

Good and Bad absorbers and radiators of heat

Dull black surfaces are good absorbers and
good radiators of radiation.

Shiny surfaces are poor absorbers and poor
radiators of radiation; they reflect energy
away.

Colour of clothes
Light-coloured clothes absorb less energy by radiation
than dark-coloured clothes under the sun.

Energy transfer by radiation

Solar energy is transferred through a vast empty space, known as vacuum.
An object that is warm/hot radiates energy.
The hotter the object,
The more energy it radiates.
Such radiation is called infra-red radiation., it is invisible.