1. Volume B Chapter 25
Transmission of Heat

2. Myth or Truth? Oh no! You will spoil the flask
if you put cold water in it.
After this, the flask cannot be used
to keep water hot anymore.
Myth or Truth?

3. Why are the handles of saucepans usually made of plastic?

4. Why is food packed in styrofoam boxes?

5. Cooling fins at the back of refrigerators.
Why are they always painted black?

6. What we will learn…
Explain what is meant by conduction, convection & radiation
The volume & density of a substance will change during thermal expansion
Different materials have different rates of heat flow

7. Heat Moves!
Heat is a form of energy that is transmitted from a region of higher to lower temperature.

8. Heat Moves!
IN 3 WAYS…
CONDUCTION
CONVECTION
RADIATION

9. 1 represents conduction. 2 represents convection. 3 represents
They are three processes of heat transfer shown in the picture.
1 represents
conduction.
2 represents
convection.
3 represents
radiation.

10. CONDUCTION
Conduction is transmission of heat through a material from the hot regions to the cold regions without any visible movement of the material.

11. Thus, kinetic energy is transferred more quickly in solids.
CONDUCTION
Solids are better conductors of heat than liquids and gases as solid particles are closer together.
Thus, kinetic energy is transferred more quickly in solids.
See movie

12. How Conduction Works in solids
When one end of a rod is heated, particles there gain energy and vibrate faster.
These particles collide with less energetic neighbors.

13. How Conduction Works in solids
Some of their energy is transferred to neighboring particles which thus gains kinetic energy.
This goes on until the cold end reaches same temperature as hot end
See movie

14. Conduction of heat through solids
Wood
Iron
Aluminium
Copper
Observation:
Observe the lengths of wax that have melted on the different rods in a given interval of time.

15. Conduction of heat by metals
Metals conduct heat much faster then non-metals as they have free electrons.
Process:
When a metal is heated, free electrons gain energy and move faster.
They move in the spaces between atoms before colliding with some atoms and transferring energy to them.

16. Good & Bad Conductors of Heat
Materials conduct heat at different rates
Good conductors of heat allow heat to flow through rapidly
Bad conductors or good insulators of heat allow heat to flow through slowly

17. Poor conductors (insulators)
Good & Bad Conductors of Heat
Good conductors
Poor conductors (insulators)
Silver
Copper
Aluminum
Brass
Iron
Lead
Stainless Steel
Vacuum
Air
Styrofoam
Body fat
Cork
Water
Glass

18. Application of Conduction
1. Uses of Good Conductors
Used where heat has to be quickly transmitted
E.g. Pans, kettles, cooking utensils
Usually made of metals like aluminium, copper, steel

19. Application of Conduction
2. Uses of Poor Conductors (Insulators)
Use where unwanted heat has to be kept away or to prevent heat loss.
E.g. vacuum flask, handles of saucepan
Made of Cloth, plastics, wood etc
insulator

20. Application of Conduction
Trapped air between 2 layers of glass is good insulator, reduces conduction of heat

21. Good Conductors Good Insulators
Try This: Fill in the table appropriately
Brass
Wood
Aluminium
Air
Foam
Copper
Iron
Water
Cotton
Plastic
Mercury
Gold
Good Conductors
Good Insulators .

22. Good Insulators (or Bad Conductors)
ANSWER: Fill in the table appropriately
Brass
Wood
Aluminium
Air
Foam
Copper
Iron
Water
Cotton
Plastic
Mercury
Gold
Good Conductors
Good Insulators (or Bad Conductors)
Brass
Aluminium
Wood
Air
Copper
Iron
Foam
Water
Mercury
Gold
Cotton
Plastic .

23. CONVECTION
Convection is the transmission of heat through a fluid (liquid or gas) from the hot to the cold region by the circulating movement of the fluid.
cool
(higher density)
hot
(lower density)
liquid or gas

24. CONVECTION occurs in fluids (liquids and gases), but not in solids
When one part of fluid is heated, it expands, becomes less dense and rises
The surrounding cooler and denser fluid sinks
This produces a circulating movement called convection current.

25. CONVECTION IN LIQUIDS
See movie

26. CONVECTION IN LIQUIDS
Add potassium permanganate (purple) crystals into a flask of water
Heat flask gently just below the crystals
Purple-streaked water rises upwards and fan outwards.
Water becomes evenly purple in colour after a while.

27. 7.2 Convection Explanation:
When a liquid is heated, it expands, becomes less dense and floats upwards.
Cold and denser liquid moves down to replace the rising liquid masses.
This cold liquid, in turn, gets heated up.
The cycle repeats until the boiling point of the liquid is reached.

28. 7.2 Convection Application 2: Car Radiator
Car engines are kept cool by a water jacket
When water in the water jacket gets heated, it flows into copper tubes which include many cooling fans.
A fan causes air to flow past the tubes and cool water in them.
Cooled water flows down and back into engine through a hose at the bottom.

29. CONVECTION IN GASES

30. CONVECTION IN GASES
Fit two glass chimneys to the top of a wooden box with plane glass window
Light a candle below one chimney
Heat from this initiates a convection current
Hot air flows out of this chimney
Cold air flows in through other chimney

31. Application of Convection
Sea and Land Breezes

32. Application of Convection
1. Sea Breeze (Daytime)
In the day, land heats up faster than the sea
Air above land expands and rises
Cool air above sea moves in towards the land
This result in sea breeze

33. Application of Convection
Sea and Land Breezes

34. Application of Convection
2. Land Breeze (Night time)
At night, the land cools faster than the sea
Warm air above sea rises
Cool air above land moves out towards the sea
Result in land breeze

35. Application of Convection
Hot Water System

36. Application of Convection
Hot Water System
Boiler at bottom heats up water and causes the hot water to rise
Hot water then moves to the hot water storage tank
Cold water from the cold tank then moves down to replace the hot water
A convection current is set up

37. Application of Convection
Air- Conditioners

38. Application of Convection
Air- Conditioners
Cold air blown out by the air-conditioner sinks because it is denser than warm air.
Warm air rises and is drawn into air-conditioner to be cooled.
A convection current is set up

39. QUIZ TIME!!

40. A. Water B. Oxygen C. Wood D. Aluminium
Q1. Which of the following is a good conductor of heat?
A. Water
B. Oxygen
C. Wood
D. Aluminium

41. Q2. Why does heat conduction take place faster in metals?
A. The molecules are larger than non-metals and this reduces the amount of time taken for energy to transfer.
B. The presence of free electrons causes heat energy to transmit quickly.
The molecules are packed closer than non-metals and this reduces the time taken for energy to transfer.
The molecules gain energy faster in metals.

42. A.Pan: Plastic Handle: Copper Pan: Glass Handle: Glass
Q2. Which of these would you use for the frying pan and its handle?
A.Pan: Plastic Handle: Copper
Pan: Glass Handle: Glass
Pan: Copper Handle: Brass
Pan: Steel Handle: Wood

43. Q3. The purpose of using a double-glazed window is to
A. Save money because glass is expensive.
Provide a double protection against harmful projectiles.
Reduce heat conduction by trapping air inside.
Improve heat convection by trapping air inside.

44. Q4. We need to put on more clothes in winter because
A. The air is cold and we do not want heat to escape from our bodies.
The clothes is able to trap cold air.
The air is cold and we will lose water too quickly.
Winter clothes are nice and trendy.

45. QUIZ TIME!!

46. Q1. Convection takes place because
A. Free electrons make the heat energy travel faster in the fluid.
Hot liquid and cold liquid separates themselves from each other.
Hot fluid rises and colder fluid sinks.
Hot fluid sinks and colder fluid rises.

47. Q2. The purple color stream is seen to travel
Q2. The purple color stream is seen to travel upwards from the center and downwards from the sides because
A. The heat from the fire causes the crystal to react with the water.
it is the only possible way for the crystal to dissolve.
The water at the top starts to cool down and water from the base has to replace it.
The heating at the base stirs up a convection current.

48. A. Alcohol Bricks Nitrogen Paraffin
Q3. Convection of heat cannot take place in
A. Alcohol
Bricks
Nitrogen
Paraffin

49. A. air blown from the top will fall to the ground due to gravity.
Q4. We place air conditioners near the ceiling because
A. air blown from the top will fall to the ground due to gravity.
cold air can be found near the ceiling.
hot air rises to the ceiling and cooling it will create a convection current.
cold air rises to the ceiling and cooling it will create a convection current.

50. Q5. Sea breeze occurs
A. In the daytime.
in the night time.
only at midnight.
only when it rains.

51. Radiation
Radiation is the transmission of heat from one place to another by means of infra-red radiation which can travel through vacuum or air

52. Radiation
Rate of Emitting (giving out) and Absorbing Heat by Radiation is affected by:
(a) surface temperature
(b) colour and texture of the surface
(c) surface area

53. Radiation Dull black surfaces are good emitters and absorbers of heat
Shiny white surfaces are good reflectors (bad emitter and absorber) of heat

54. Radiation
Why does the cork on the dull black surface fall off first?
Ans: Dull black surfaces are good absorbers of heat, thus it melts the wax faster

55. Application of Radiation
Good Emitters
Poor Emitters
Cooling fins at the back of a refrigerator painted dull black
A shiny metal teapot and aluminum foil keep contents warm

56. Application of Radiation
Good Absorbers
Poor Absorbers
Solar heating panels are painted in dull black paint
to absorb heat from sun
Houses in hot countries are painted in white or light-coloured paint, to keep them cooler.
Light colours are chosen for clothes and cars in hot weather

57. Application of Radiation
Vacuum Flask
Purpose:
keeps hot liquids hot and cold liquids cold.
Function:
Reduces transfer of heat by conduction, convection and radiation

58. Application of Radiation

59. Reduces heat loss/gained by:
Application of Radiation
Structure
Reduces heat loss/gained by:
vacuum in between
the double glass walls
conduction or convection
Silvered walls
Outer silvered surface
Inner silvered surface
Radiation
reflects radiation in external surroundings away from flask
reflects radiation from hot fluids back into flask
Foam plastic support
conduction
Plastic cap
convection

61. Purpose (reduces heat loss/gained by)
LET'S RECAP
State the parts of a vacuum flask and explain what their functions are.
Structure
Purpose (reduces heat loss/gained by)

62. 1 represents conduction. 2 represents convection. 3 represents
They are three processes of heat transfer shown in the picture.
1 represents
conduction.
2 represents
convection.
3 represents
radiation.