This topic concerns mostly to O-Level students. Conduction, convection and infra red radiation

1. TRANSFER OF
THERMAL ENERGY

2. Transfer of thermal energy
RADIATION
INFRARED RADIATION IS THE TRANSFER OF ENERGY FROM
ONE PLACE TO ANOTHER BY AN ELECTROMAGNETIC WAVE
AND DOES NOT REQUIRE A MEDIUM TO TRAVEL
THROUGH.

3. HOW TO DISTINGUISH BETWEEN GOOD
AND BAD ABSORBERS OF INFRARED
RADIATION
• Apparatus needed: two temperature sensors, data logger, 100
watt bulb, aluminium foil.

4. PROCEDURES:
1.Connect the two sensors to the data logger where a
graph of temperature against time will be plotted.
2.One of the sensor is blackened and the other wrapped
with shiny aluminium foil.
3.Place the 2 sensors at equal distance away from the
one hundred watt bulb.
4.Switch on the sensor and light bulb. Observe the
temperature rise for both sensors.
5.After some time ,switch off the light bulb.

5. CONCLUSION
The blackened sensor shows a higher temperature
recorded than the shiny aluminium foil because black
surfaces are better absorbers of infra-red radiation
than shiny white surfaces.

6. How to distinguish between good and bad
emitters of infrared radiation.
Apparatus needed: two temperature sensors, data logger,
two identical tins(one painted black and one with silvery
surface), boiling water maintained at 100 degree Celsius.

7. 1. Place the temperature sensors into the 2 tins
filled with boiling water.
2. One tin is blackened on the surface and the
other with silvery surface.
3. Start the data logger.
4. Stop the data logger after some time.
5. The graph is plotted with temperature against
time.

8. CONCLUSION
The graph shows that the temperature of the blackened
tin drop faster compared to the water in the silvery tin.
The blackened surface is a better emitter of radiation.

9. There are many good and bad emitters of infrared
radiation.
Nature of surface affects the rate of emission of infrared
radiation.
For example:
Good emitters Bad emitters
Dull surfaces Shiny surface
Black surfaces White surface
Dark coloured surfaces Light coloured surfaces

10. Good absorbers Bad absorbers
Dull surfaces Shiny surface
Black surfaces White surface
Dark coloured surfaces Light coloured surfaces
 The nature of the surface affects its ability to absorb
radiant heat.
 Infrared radiation is absorbed by all objects and surfaces
which causes a temperature rise.
For example:

11. In general, a good emitter of radiant heat is also a good
absorber of radiant heat.
Conversely, a poor emitter of radiant heat is also a poor
absorber of radiant heat.

12. TRANSFER OF THERMAL ENERGY
•HEAT TRANSFER IS THE PROCESS OF THERMAL EXCHANGE
BETWEEN DIFFERENT SYSTEMS. GENERALLY THE HEAT
TRANSFER BETWEEN TWO SYSTEMS WILL BE FROM THE
HOTTER SYSTEM TO THE COOLER SYSTEM.
•HEAT TRANSFER IS PARTICULARLY IMPORTANT IN
BUILDINGS FOR DETERMINING THE DESIGN OF THE
BUILDING FABRIC, AND FOR DESIGNING THE PASSIVE AND
ACTIVE SYSTEMS NECESSARY TO DELIVER THE REQUIRED
THERMAL CONDITIONS FOR THE MINIMUM
CONSUMPTION OF RESOURCES.

13. HEAT TRANSFER TO OR FROM A BUILDING
Conduction, convection, and radiation heat transfer
take place almost everywhere we look. In a building
envelope, conduction primarily takes place through
opaque envelope assemblies, convection is usually the
result of wind or pressure-driven air movement, and
radiant heat transfer is primarily from the sun through
the arrangement of windows in a building. Buildings
are typically designed to provide comfort using
convective or radiant modes of heat transfer.

14. TRANSFER OF HEAT TO OR FROM A
ROOM
Whenever a group of people enter a room,
convection currents are set up. The heat
generated by bodily processes such as
breathing causes the air near the body to be
heated up by conduction. The warm air, being
less dense, rises while the colder, denser air
above the person sinks. This sets up a
convection current which causes heat to
transfer to or from a room.

15. TRANSFER OF HEAT TO OR FROM A
ROOM
HEAT

16. AIR CONDITIONER
The rotary fan inside an air an air-conditioner
forces cool dry air out into the room. The cool
air, being denser, sinks while the warm air
below, being less dense, rises, and is drawn
into the air-conditioner where it is cooled. In
this way, the air is recirculated and the
temperature of the air falls to the value
preset on the thermostat.

18. HEATER
The heater, placed at the bottom, heats the
surrounding air. The warm air, being less dense,
rises while the cool air above, being denser, falls
down, and is heated by the heater. In this way,
the air is recirculated and the temperature of
the air rises to the value preset on the
thermostat.

20. STATE AND EXPLAIN THE USE OF THE
IMPORTANT PRACTICAL METHODS OF
THERMAL INSULATION FOR BUILDINGS.
Thermal insulation is important to reduce energy
consumption in buildings by preventing unwanted
heat gain/loss. There are number of ways to
reduce the heat loss/gain :
Roof
Windows
Walls
Door
Ground

21.  Windows
 Use double
glazing in
windows which
prevent heat to
escape and enter
easily
 Use curtains on
windows to reduce
radiations. Trapped Air
is a good insulator.
Therefore if there is a
trapped air space the
heat will not go out the
window.
 Reduce the
incidence of solar
heat by using
external shading
(louvered shutters,
chajjas,…)

22. • FILL WALL CAVITY WITH INSULATOR
OR INSULATE WALL INTERNALLY OR
EXTERNALLY WHICH WILL NOT LET THE
HEAT ESCAPE OR ENTER THE BUILDING.
• THE WALLS MAY BE CONSTRUCTED
OUT OF SUITABLE HEAT INSULATING
MATERIAL.
 Walls
 Ground
 Sheep wool insulation and
polystyrene insulating board is used
for floor insulation.
 In winter use carpet with underlay
to warm up the building and in
summer remove it.

23.  Reflective insulation
reflects radiant heat,
making it a good
insulation choice for
doors.
 Doors
 Use draught excluder
on doors to eliminate
cold draft and slow
heat loss.
 Roof
 Use Loft and roof insulation to reduce
heat loss.
 Polystyrene insulation and Spray
foam is also used.
 Flat roof may be kept cool by flooded
water either by storing or spraying.

24. THANK YOU