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.

Electromagnetic Spectrum

The sun emits radiation in a broad spectrum called the electromagnetic spectrum, a small portion of which is visible. The visible spectrum can be seen when cloud droplets separate light in a rainbow. The visible spectrum is red, orange, yellow, green, blue, indigo and violet. Ultraviolet (UV) Radiation is outside of the visible range near violet, hence the name.

A glass prism can be used to split white light into separate colours that are sensitive to the human eye, ranging from red (longer wavelength) to violet (shorter wavelength). The human eye cannot detect electromagnetic radiation outside this range.

Radiation: Wavelengths

This picture shows us some common wavelengths.

-units of micrometers are often used to characterize the wavelength of radiation

-1 micrometer = 10-6 meters

10.4: RADIATION

Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object.

For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required. When electromagnetic waves comes in contact with an object, the waves transfer the heat to that object.

Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation. When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls

Convection: Example 2

Convection can also lead to circulation in a liquid, as in the heating of a pot of water over a flame. Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed, though they will not be as regular as suggested in the drawing.

Convection: Video 1

Convection: Example 1

The wind we feel outside is often the result of convection currents. You can understand this by the winds you feel near an ocean. Warm air is lighter than cold air and so it rises. During the daytime, cool air over water moves to replace the air rising up as the land warms the air over it. During the nighttime, the directions change - the surface of the water is sometimes warmer and the land is cooler.

10.3: CONVECTION

Convection is the transfer of thermal energy due to the motion of the substance that contains the thermal energy. Although the conduction process can occur in liquids and gases as well as solids, convection may have a much larger effect in liquids and gases where the molecules are free to migrate.A hot cup of coffee transfers thermal energy as heat to the surrounding air by convection. The heated air over the coffee expands and rises. The rising heated air is replaced by cooler, denser air, which in turn, expands and rises. This circulation of air is an example of convection.

Convection occurs because as the temperature is increased, most materials expand. This means that they become less dense. Thus, the warmer air near the hot coffee is floated up to the ceiling just like a piece of cork, which is less dense than water, rises to
the surface of the water.

In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.

So, if you have a hot bowl of soup on the table, it heats a layer of air surrounding the bowl. That layer then rises because it is hotter than the surrounding air. Cold air fills in the space left by the rising hot air. This new cold air then heats up and rises, and the cycle repeats. It is possible to speed up convection -- that is why you blow on hot soup to cool it down. If it weren't for convection your soup would stay hot a lot longer, because it turns out that air is a pretty poor heat conductor.

Conduction: Video

Conduction: Example 3

Consider two substances at different temperatures separated by a barrier which is subsequently removed, as in the figure shown.
When the barrier is removed, the fast ("hot'') atoms collide with the slow ("cold'') ones. In such collisions the faster atoms lose some speed and the slower ones gain speed; thus, the fast ones transfer some of their kinetic energy to the slow ones. This transfer of kinetic energy from the hot to the cold side is called a flow of heat through conduction.

Conduction: Example 2

Think of a frying pan set over an open camp stove. The fire's heat causes molecules in the pan to vibrate faster, making it hotter. These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. This process continues until the entire pan has heated up due to the vibrating and colliding molecules. If you've ever touched the metal handle of a hot pan without a potholder, you have first-hand experience with heat conduction. Therefore, to prevent our hands from being scalded, pots and pans have insulated handles.

Conduction: Example 1

If you put a metal bar into a candle (or bunsen) flame, it gets hot quickly. Soon you can't hold it. If you put a glass bar into a candle, it won't get too hot to hold. But if you touched the end that was in the flame, you would find that it is really hot!

The process in which heat passes through a solid substance is called conduction. Metals are good conductors of heat. Non-metals are generally bad conductors of heat. Liquids and gases are bad conductors of heat as well. A bad conductor of heat is called an insulator.

10.2: CONDUCTION

Conduction is the transport of thermal energy through an object by a series of collisions between atoms, molecules and electrons. Different materials transfer heat by conduction at different rates - this is measured by the material's thermal conductivity.

Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water.

If one end of a metal spoon is held in a pan or a stove, the other end soon becomes hot. This is because, the energy from the stove causes the molecules in the end of the spoon, that is in the pan, to move faster, increasing the rate and size of their vibrations. These molecules transfer some of their extra kinetic energy to neighbouring molecules and these, in turn, affect their neighbours. Therefore, the temperature of the entire spoon increases. If the pan is kept at a constant temperature, overtime, the pan and the spoon will have the same temperature. Most of us would not pick this spoon up without a potholder. The potholder is a poor conductor of heat and a good thermal insulator.

How rapidly an object transports thermal energy by conduction depends, in part, on what material the object is made of.

Chapter 10: Transfer of Thermal Energy

Thermal energy ALWAYS flows from a region of higher temperature to a region of lower temperature. In another words, thermal energy is transferred only when there is a difference in temperature The transfer of thermal energy as heat can take place in three processes: conduction, convection and radiation.