2. PREREQUISITE KNOWLEDGE
▪ For mastery of the content presented in this
presentation, students should have been taught:
– Forms of energy
– The difference between the caloric and kinetic theories of heat eighteenth
– Phases of Matter
3. PRESENTATION OBJECTIVES
▪ At the end of this presentation, you will be able to:
I. Define the term heat
II. Explain the three modes of heat transfer
III. Define the term latent heat
IV. Apply the relationship EH = ml
V. Determine the specific latent heat of vaporization l and fusion, l of
water
4. WHAT IS HEAT?
▪ Heat(or thermal energy) may be defined as energy in
transit from a high temperature object to a lower
temperature object.
5. HEAT TRANSFER
section B: Specific objective 2.1
▪ It is the temperature
difference between the
two neighboring objects
that causes this heat
transfer. Heat transfer is
the exchange of thermal
energy between bodies
due to the difference in
temperatures between
bodies.
6. HEAT TRANSFER
section B: Specific objective 2.1
▪ Heat transfer or heat exchange
is the transition of thermal
energy from hotter object or
area to cooler object or area
▪ The heat transfer continues
until the two objects have
reached thermal equilibrium
(i.e. are at the same
temperature).
7. METHODS OF HEAT TRANSFER
section B: Specific objective 4.1
▪ Heat can be transferred from one
place to another by three
methods: conduction in solids,
convection of fluids (liquids or
gases), and radiation through
anything that will allow radiation
to pass.
▪ Conduction and convection
involve particles, but radiation
involves electromagnetic waves.
8. HEAT TRANSFER
section B: Specific objective 2.1
▪ Conduction is the
movement of heat from
one solid to another one
that has different
temperature when they
are touching each other.
9. HEAT TRANSFER
section B: Specific objective 2.1
▪ Convection is heat transfer
by mass motion of a fluid
such as air or water when
the heated fluid is caused
to move away from the
source of heat, carrying
energy with it
10. HEAT TRANSFER
section B: Specific objective 2.1
▪ Radiation is the transfer of
heat through space in the
form of electromagnetic
waves.
11. LATENT HEAT
▪ Definition 1:The quantity of heat absorbed or released by a
substance undergoing a change of state (solid to liquid,
liquid to gas, etc. ) at constant temperature and pressure.
▪ Definition 2: The heat absorbed or radiated during a
change of phase at constant temperature and pressure.
12. LATENT HEAT
▪ The latent heat associated with melting a solid or freezing
a liquid is called the latent heat of fusion (Lf); that
associated with vaporizing a liquid or a solid or condensing
a vapor is called the latent heat of vaporization (Lv).
13. LATENT HEAT
Latent heat energy is absorbed or given out while a
substance undergoes state change.The average kinetic
energy of the molecules does not change so that the
temperature remains constant.
I. During melting, heat absorbed by the solid is used to
break the inter-molecular bonds between the molecules
of solid substance.
II. During vaporization, heat absorbed by the liquid is used
to break the inter-molecular bonds completely between
the molecules of liquid substance.
14. SPECIFIC LATENT HEAT OF FUSION
▪ Specific latent heat of fusion ( Lf ) of a substance is defined
as the amount of heat required to change 1kg of a
substance from solid to liquid state, or vice versa, without
any change in the temperature.
15. SPECIFIC LATENT HEAT OF VAPORIZATION
▪ Specific latent heat of vaporization, lv, of a substance is
defined as the amount of heat required to change 1Kg of a
substance from liquid state to gaseous state, or vice versa,
without a temperature change.
16. CALCULATING LATENT HEAT OF FUSION
section B: Specific objective 3.5
SI unit of specific latent heat of fusion is joule per kilogram
(Jkg-1)
EH =m Lf , where
EH = amount of thermal energy absorbed or released
m = mass of substance
Lf = specific latent heat of fusion.
17. EXAMPLE 1
Calculate the thermal energy required to convert 2kg of ice at 00c to
water at 00c
(Specific latent heat of fusion of ice = 3.4 x 105 J/Kg)
Solution
EH =m Lf
EH = 2kg x 3.4 x 105 J/Kg
EH = 680,000J or 680KJ (answered)
18. CALCULATING LATENT HEAT OF VAPORIZATION
section B: Specific objective 3.5
SI unit of specific latent heat of vaporization, lv, of a
substance is joule per kilogram (Jkg-1)
EH =m L v where
EH = amount of thermal energy absorbed or released
m = mass of substance
L v = specific latent heat of vaporization.
19. Example 1
Calculate the thermal energy required to convert 2kg of
water at 1000c to steam at 1000c.
(Specific latent heat of vaporization of water = 2.3 x 106 J/Kg)
Solution
EH =m L v
EH = 2kg x 2.3 x 106 J/Kg
EH = 4,600,000J or 4.6MJ
20. QUESTION 1
▪ All are examples of Convection EXCEPT :
a. Hot air balloons rise due to the propensity of warmer air to be less
dense than the air around it.
b. Macaroni rising and falling in a pot of boiling water
c. Lying out in the sun to get a tan
d. the warm air in a radiator being replaced by cold air.
21. QUESTION 2
All are examples of Radiation EXCEPT :
a. Heat from the sun warming your face
b. Heat from a lightbulb
c. Heat from a fire
d. Boiling water by placing a red-hot piece of iron into it
22. QUESTION 3
All are examples of Conduction EXCEPT :
a. A pot sitting on a hot burner
b. Touching a metal spoon that is sitting in a pot of boiling water
c. Picking up a hot cup of coffee
d. A person placing their cold hands over a warm fire
23. QUESTION 4
Water boils at 100oc. Calculate the heat energy which must
be supplied in order to completely convert 2kg of water to
steam.
(Specific latent heat of vaporization of water= 2.3 x 106 J/Kg)
CSEC Physics past paper May 2006 paper 2
