Common applications of conduction


Uses of good conductors of heat


1) Cooking utensils
2) Soldering iron rods
3) Heat exchangers





Uses of insulators

1) Handles of appliances and utensils
2) Table mats
3) Sawdust
4) Wooden Clothes
5) Fibreglass, felt and expanded poolystyrene foam


Common applications of convection

1) Electic kettles
2) Air conditioners


Common applications of radiation

1) Teapots
2) Greenhouses
3) Vacuum flasks

Radiation



Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.


It is the only process that does not require medium to transfer energy. It can travel in vacuum unlke conduction and convection. Heat rays from the sun reaches Earth due to radiation.


Factors affecting rate of infrared radiation


1) Colour and texture of the surface

- Dull black surfaces absorb and emit inrared radiation easily and are better at it than shiny white surfaces.
- Shiny white surfaces are better reflectors.


2) Surface temperature

-The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.


3) Surface area

Convection

Convection is the transfer of thermal energy by means of current in liquid or gas.
It involves the movement of the fluids from a hotter region to a colder region.
It only takes place in liquid and gas.



How does it work?


Based on the theory of warm air rises and cool air sinks, convection happens. It only takes place in liquids and gases since it involves the bulk movement of the fluids which solids can never do since their particles are packed tightly together.

Conduction

Conduction is the process of thermal energy transfer without any flow of the material medium.







Note: Different materials conduct heat at different rates. Examples of good conductors of heat are steel, iron, etc. Poor conductors of heat is called insulator and one of them is wood.


How does it work?




Solids are made up of particles called atoms and molecules. When thermal energy is supplied to one end of the rod (using candle flame this time), the particles at that end will vibrate vigorously. They will then collide with neighbouring particles, making them vibrate as well. This is how the kinetic energy is transferred.


There is no transfer of particles but just the thermal energy. In metals, another process called free electron diffusion takes place. (Only metals have free moving electrons). This results in the faster transferring of energy as compared to other solids.


Condution in liquids and gases is much inefficient since the particles are spaced much further apart.


You should be able to:

1)Give an account of thermal energy conduction in metals and non-metals.
2)Explain why good conductors of thermal energy are also good conductors of electricity.


Question:

Is the heat transferred from a barbeque fire to a person standing in front of it a good example of heat transfer by conduction. Explain.


Answer:

Because the person and the pit is separated by air, which is a poor conductor of heat. Thus, it is not a good conductor of heat.

The 3 different processes for transfer of thermal energy:

1)Conduction
2)Convection
3)Radiation



Qns:

During winter, it is common for people to say "keep the cold out of the house". Is this statement correct? Comment.


Ans:

Transfer of thermal energy cannot be stopped but an equilibrium temperature can be achieved between the house and the surrounding outside. Thus, it is impossible to "keep the cold out of the house". However, it is possible to reduce the transfer of thermal energy.

Normally, the house will be the warmer region while the surrounding will be the colder one. So, according to the theory of thermal transfer, heat will actually flow from the house to the surrounding, and then until they both reached an equilibrium temperature.

What the people said about "keeping the cold out of the house" may actually mean to close the doors to prevent the loss of heat quickly.

Introduction to thermal energy


Heat: the transfer of thermal energy from a hotter to a colder region
Thermal energy always flows from a region of higher temperature to a region of lower temperature.







Thermal equilibrium exists when there is no net gain or lost of thermal energy between objects in contact.

Objects in thermal equilibrium have the same temperature.