KENYA FORM 1

1. HEAT TRANSFER

2. HEAT AND TEMPERATURE
Heat is a form of energy which passes from a body at
higher temperature to a body at a lower temperature.
If two bodies at the same temperature are in contact, there is no net heat
flow from one body to the other .
This condition is known as thermal equilibrium.
The SI unit of heat is the joule (J).
NOTE:Heat cannot be measured directly by an instrument
as temperature is by a thermometer.

3. Modes of Heat Transfer
Heat can travel through a medium and also
through vacuum.
There are three modes of heat transfer,
a) Conduction
b) Convection
c) Radiation

4. CONDUCTION
If you stir hot tea using a metal spoon, you will observe
that the handle of the spoon becomes warm.
solids that are good conductors of heat (metals) use both
atom vibration and free electrons to conduct heat.

5. EXPERIMENT 7.1:Comparing thermal conductivities
of various conductors
Different materials have different
thermal conductivities. Metals are
generally good conductors of heat.
Non-metals are poor conductors of
heat (insulator).
NOTE:Conduction is
therefore the transfer of
heat as a result of vibration
of particles.

6. Factors Affecting Thermal Conductivity
Thermal conductivity in materials depends on
the following factors:
(i) The temperature difference(AӨ) between
the ends of the conductor.
(ii) The length of the conductor (l).
(iii) The cross-section area(A) of the conductor.
(iv) The nature of the material (k).

7. In an equation form;

8. Thermal Conductivity in Liquids
Water at the top of the boiling tube boils while the ice
remains unmelted.
Water is a poor conductor of heat.

9. Why Liquids are Poor Conductors of Heat ?
This is because there are large inter-molecular distances
between liquid molecules.There are also fewer and rare
collisions between the molecules.
Electrolytes, e.g., saltsolution,are better conductors of
heat than pure liquids because of an increased
compactness of the particles.

10. Thermal Conductivity in Gases
Gases are worse conductors of heat because of
large inter-molecular distance.

11. Some applications of Good and Poor
Conductors of Heat
• Good conductors are used to make cooking
utensils ,soldering irons etc.
• Bad conductors are applied in, fire fighting and
making handles for cooking pots.

12. CONVECTION
Convection is the process by which heat is
transferred through fluids (liquids and gases).
(i) Natural convection; Involves change in
density of the fluid with temperature.
(ii) Forced convection: Mixing of hot and cold
parts of the fluid through some external stirring,
like a fan or pump.

13. To demonstrate convection in
liquids

14. When a liquid is heated ,it expands and this lowers its density.
The less dense liquid rises and its place is taken by more dense
colder liquid. This movement of liquid forms convection currents.

15. Convection Current in Gases
Convection currents are set up when air or gas
is heated.

16. Molecular Explanation of Convection in
Fluids
Molecules in fluids are further apart and have
negligible cohesive force.
Convection currents are set up much faster in
gases than in liquids because of the extremely
low cohesive forces existing between the
molecules of the gases.

17. Some Applications of Convection in Fluids
Domestic Hot Water System
A domestic hot water supply systemj works on
the principle of convection current.

19. RADIATION
Heat transfer through vacuum is called thermal
radiation. All bodies absorb and emit radiation.
An electric bulb in a room produces both light and radiant
heat. The radiant heat is absorbed by the materials in the
room, which in turn give out radiant heat of lower
energy, see figure.

20. Nature of Radiant Heat
Because of the nature of production, radiant
heat is an electromagnetic wave that causes
heating effect in objects that absorb it.
Radiation is also described as the flow of heat
from one place to another by means of
electromagnetic waves.
The paper burns out.

21. Emission and Absorption of Radiation
From the experiments
above, black surfaces are
better emitters of heat
than shiny surfaces.

22. Applications of Thermal Radiation
1.Electric kettles ,iron boxes and cooking pans
have polished surfaces to reduce heat loss
through radiation.
2. Petrol tanks are painted silvery bright to
reflect away as much heat as possible.
3. Houses in hot areas have their walls and
roofs painted with bright colours to reflect away
heat, while those in cold regions have walls and
roofs painted with dull colours.

23. 4. In solar concentrators, electromagnetic
waves in form of radiant heat are reflected to a
common point (focus) by a concave reflector.The
temperature at this point can be sufficiently high
to boil water.

25. Vacuum flask
A vacuum flask or thermos flask can keep the
substance inside hot or cold for a long period of
time.It designed such that the heat transfer by
conduction ,convection and radiation between
the contents of the flask and its surroundings is
reduced to a minimum.

26. SUCCESS NOTES
 Heat is a form of energy which flows as a
result of temperature difference between two
regions.
 There are three modes of heat transfer namely
conduction,convection and radiation.
 In conduction and convection a material is
required while in radiation,heat travels
through a vacuum.
 Thermal conductivity depends on:
• Temperature difference-the greater the
temperature difference the higher the rate of
conduction.
• Cross- sectional area-thicker materials
conduct heat faster than thin ones.

27. • Length- rate of conduction decreases with
increase in length of material.
• Nature of material-different materials conduct
heat differently.
 Metals are good conductors of heat than non-
metals.
 Water is not o good conductor of heat.
 The amount of radiation absorbed by a given
surface depends on its nature. Black/dull
surfaces are better absorbers of radiation than
polished surfaces.

28. QUESTIONS
1.Explain ?
(a) Soup cools faster in a steel
bowl than in a glass bowl.

29. Since metals are better conductors of heat
than non-metals,steel bowl, a metal alloy,
conducts heat away faster from the soup than
glass. Hence, the soup in steel bowl cools faster.

30. (b) Air conditioner unit is placed near the top
of the ceiling in a room.

31. Convection currents occur when hot air rises
and cold air sinks. The warmer air which is less
dense rises to the top and after being cooled
down by the air conditioner unit, the cooler air
sinks. The convection currents cause the room to
be cooled efficiently.

32. 2.The figure shows the heating element of a
heater being placed near the bottom of the
liquid.
(a) Explain why the beaker is placed on a base
made of saw dust.

33. Saw dust is a poor conductor of heat, so it can
reduce heat loss from the beaker through
conduction.

34. b.)What is the advantage of placing the
heating element near the bottom of the liquid?

35. It allows convection currents to be set up
quickly. The cooler liquid which sinks can be
heated up quickly by the heating element placed
near the bottom.

36. 4) Mrs. Chen has two woks. A black one made
of iron and a shiny one made of stainless steel.
She prefers using the black one because she
says it allows her to cook faster.
(a) Is she right? Explain your answer.

37. Yes. Black is a better absorber and emitter of
heat so food cooks faster in black wok.

38. b)What forms of heat transfer take place when
she fries vegetables in the wok?

39. Conduction and radiation from the flame to the
wok.Conduction from the wok surface to the
food.

40. 5)The figure shows a beaker of water
containing a small amount of blue dye crystals.
When the container is heated, the blue dye
crystals dissolve in the water and create a
pattern in it.
Explain why wire gauze is used in this experiment.

41. It is used to spread heat of a Bunsen flame.

42. (i) Wire gauze to glass of beaker.
(ii) within water in the beaker

43. i) Conduction
ii) Convection

44. Hw.page 196
Revision Exercise 7