1. Transfer of Thermal Energy
Name: ________________
Class: _________________
Index: ________________

2. Learning Objectives
show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature.
describe, in molecular terms, how energy transfer occurs in solids.
describe, in terms of density change, convection in fluids.
explain that energy transfer of a body by radiation does not require a material medium and the rate of energy transfer is affected by:
(i) colour and texture of the surface.
(ii) surface temperature.
(iii) surface area.
apply the concept of thermal energy transfer to everyday applications.

3. In general, heat travels from higher temperature region to lower temperature region.
There are three processes by which heat may transmitted:
Conduction,
Convection, and
Radiation

4. Conduction of Heat

5. Conduction
Conduction is the process by which heat is transmitted through a medium from one
particle to another.

6. Conduction Heat travels through some material better than others.
Pins are hold by wax
Material Time taken for pin to drop(s)
wood did not drop
copper
steel
aluminium

7. A good conductor has a high conductivity.
Metals are good conductors of heat. Silver and copper have highest conductivity.
Non-metals, such as plastic and air, are poor
Conductor (insulators).
Liquids and gases are usually poor conductors
The poorest conductor is vacuum.

8. Test Yourselves the water at the top starts boiling, the ice cube
Heat a test tube of water near the top with a
‘weighted’ ice cube near the bottom. Even when
the water at the top starts boiling, the ice cube
does not melt. Why ?
Ans.: Water is a poor
conductor of heat.

9. Insulators Insulators are used to lag pipes,
lofts, hot water tanks and many
other objects.
Insulators are very poor
conductors.
Wrapped round hot objects,
they act as barrier, reducing
the heat loss.
Fibre glass insulation is a
popular choice for insulating
the loft of homes.

10. Insulation
Lagging in a cavity wall
provides extra insulation

11. Convection of Heat

12. Convection Convection is the process by which heat is
transmitted from one place to another by
the movement of heated particles of a gas
or liquid due to difference in density.

13. Convection in Liquids To demonstrate convection in water, drop a
few tiny crystals of potassium permanganate
into a flask filled with water.
Gently heat the flask, purple streaks of water
will rise upwards and then fan outwards.
The water becomes uniformly purplish after
some time.
The circulation of a liquid in this matter is
called a convection current.
Thinking :
Why hot water rises and cold water sinks ?

14. Convection in Liquids

15. Convection in Gases
The figure below shows a simple demonstration of convection of gas. The hot gases from the burning candle go straight up the chimney above the candle. Cold air is drawn down the other chimney to replace the air leaving the room.

16. Concept on ventilation of a room with fire place

17. Sea Breeze
Land Breeze
(continue on next slide)

18. Sea Breeze
During the day the sun heats the land must faster than the sea.
(Why ?)
The air above the land is heated, expands and rises.
Cold air from the sea moves inland to take its place.
Hence, sea breeze is obtained.
Discussion : How land breeze is produced ?

19. Test Yourselves 1. Usually, it is better to install air conditioner
in the higher portion of the wall. Please
explain this statement with reasons.
2. How are winds forms ?
3. Why is the heating coil of an electric kettle
placed near the bottom of the vessel ?

20. Radiation of Heat

21. Radiation
Radiation is a method of heat transfer that does not require any medium.
It can take place in a vacuum.
In radiation, heat transmits energy in the form of
waves.

22. Heat Radiation All objects emit heat radiation.
Heating an object up make it radiate more energy.
A dull dark surfaces is a better emitter or radiator than a shiny one.

23. What type of surface is the best absorber of heat
Fig. below shows one way to test different surfaces.
Results from this type of test show that,
a dull black surface is the best absorber of radiation,
a shiny silvery surface is the worst absorber of radiation.

24. What type of surface is the best absorber of heat

25. A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material.
A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.

26. The radiation energy per unit time from a blackbody is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law as
q = σ T4 A         (Not Tested)
where
q = heat transfer per unit time (W)
σ = (W/m2K4) - The Stefan-Boltzmann Constant
T = absolute temperature Kelvin (K)
A = area of the emitting body (m2)
Therefore, the rate of energy transfer increases with temperature and surface area.

27. Worked Example

28. Vacuum Flask The flask is a double-walled glass bottle.
The space between the two
walls is a vacuum. This can
stop energy transfer out by
conduction and convection.
The shiny bright silvering
coated glass surfaces can
reduced the heat lost by
radiation. (Why ?)

29. Test Yourselves 1. Brightly polished kettle do not lose much energy
by radiation. Why ?
2. The cooling fins on the back of a refrigerator,
in a car radiator should be dull black. Why ?

30. References: