11 latent heat & specific latent heat
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LATENT HEAT & SPECIFIC LATENT HEATTOPIC 11: THERMAL PROPERTIES OF MATTER For GCE O-Levels Physics 5054TRANSCRIPT
LATENT HEAT&
SPECIFIC LATENT HEATTOPIC 11: THERMAL PROPERTIES OF MATTER
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WHAT HAPPENS TO MATTER WHEN THERMAL ENERGY IS
APPLIED TO IT?
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OF THE MATTER
THERMAL ENERGY
CAN
Change the temperature
Change the state
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FOCUS ONCHANGING THE
STATE OF THE SUBSTANCE
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WHAT DO WE KNOW?When heat is applied to a substance.
The substance may undergo changes of state.
Heat Energy Applied
Change in State of substance
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WHAT DO WE KNOW?When heat is removed from a substance.
The substance may undergo changes of state.
Heat Energy Removed
Change in State of substance
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LATENT HEATThe thermal energy absorbed or released
during a change of state.
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Energy absorbed Energy released
The unit of latent heat is Joule.
WHY IS ENERGY NEEDED
DURING CHANGES OF STATE?
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LATENT HEATIt is used to break the substance’s
intermolecular bonds during changes of state.
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Intermolecular bonds are forces of attraction that holds the particles together.
SOLID LIQUID GAS
BreakingBonds
BreakingBonds
LATENT HEATIt is used to create the substance’s
intermolecular bonds during changes of state.
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Intermolecular bonds are forces of attraction that holds the particles together.
SOLIDLIQUIDGAS
CreatingBonds
CreatingBonds
LATENT HEATAll the thermal energy is used to break / create the
substance’s intermolecular bonds during changes of state.
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SOLIDLIQUIDGAS
CreatingBonds
CreatingBonds
BreakingBonds
BreakingBonds
None of the thermal energy is used to change the kinetic energy of the particles during changes of state.
There will be no change in the temperature of the substance.
LATENT HEATThe thermal energy absorbed or released
during a change of state.
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Latent Heat of
Fusion
Latent Heat of
Vaporisation
The two types of latent heat:
LATENT HEAT OF FUSIONThe thermal energy required to change a substance
from solid to liquid state, or vice versa, without a change in temperature.
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SOLID LIQUID
L f
L f
The SI unit of latent heat is Joule.
LATENT HEAT OF VAPORISATIONThe thermal energy required to change a substance
from liquid to gaseous state, or vice-versa, without any change in temperature.
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LIQUID GAS
Lv
Lv
The SI unit of latent heat is Joule.
“The latent heat of fusion of a block of ice is 5000 J”
What does this statement means?
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The heavier the mass of the substance, more thermal energy needed to change state,
the bigger the latent heat.
m
L
mL
LATENT HEAT
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For any states of matter,
EXAMINE THE LATENT HEAT PER
UNIT MASS OF THE SUBSTANCE
OR THE SPECIFIC LATENT
HEAT
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“The specific latent heat of vaporisation of steam is
2.26 MJ/kg”
What does this statement means?
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SPECIFIC LATENT HEATThe thermal energy required to change the
state of 1 kg substance, without any change in temperature.
SpecificLatent Heat
Latent HeatLl = m
Mass
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The SI unit of specific latent heat is J/kg.
SPECIFIC LATENT HEAT OF FUSION
The thermal energy required to change 1 kg of the solid to liquid, or vice versa, without a change in temperature.
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SOLID LIQUID
l f
l f1 kg 1 kg
The SI unit of specific latent heat is J/kg.
SPECIFIC LATENT HEAT OF FUSION
The thermal energy required to change 1 kg of the solid to liquid, or vice versa, without a change in temperature.
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The SI unit of specific latent heat is J/kg.
SpecificLatent Heat of Fusion
Latent Heat of Fusion
Ll = mMass
ff
SPECIFIC LATENT HEAT OF VAPORISATION
The thermal energy required to change 1 kg of the liquid to gas, or vice-versa, without any change in temperature.
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LIQUID GAS
l v
l v1 kg 1 kg
The SI unit of specific latent heat is J/kg.
SPECIFIC LATENT HEAT OF VAPORISATION
The thermal energy required to change 1 kg of the liquid to gas, or vice-versa, without any change in temperature.
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The SI unit of specific latent heat is J/kg.
SpecificLatent Heat of
Vaporisation
Latent Heat of
VaporisationLl = m
Mass
vv
Which process is easier to achieve?Melting the ice or Boiling water.
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Ice
= 336 000 J/kgl f
Steam
= 2 260 000 J/kgl v
Thermal Energy = Power X Time Taken
WHAT ARE THE SOURCES OF THERMAL ENERGY?
Bunsen Burner Electric Burner
= P x tQ Heater
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An ice-cream has a mass of 150g. If the specific latent of fusion of ice
cream is 340000 Jkg-1, find the thermal energy required to
melt the ice cream.
EXAMPLE EXERCISE 1
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EXAMPLE EXERCISE 1
=Qice mice X
= 0.150 kg 340 000 J/kgX
=
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Mass Specific Latent Heat of Fusion of Ice
= X
Thermal Energy to melt Ice = Latent Heat of Fusion of Ice
icelf
Qice
A heater which supplies thermal energy at a constant rate of 1000 W is completely
immersed in a 3.00 kg block of ice at 00C.
The block of ice takes 1020s to melt completely.
Calculate a value for the specific latent heat of fusion of ice.
EXAMPLE EXERCISE 2
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EXAMPLE EXERCISE 2
=
Thermal Energy supplied to ice by heater
=Thermal Energy use to
melt the ice
mice XP X t =heater
Q =heater
= 3kgX X1000W 1020s
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icelf
iceLf
icelficelf
A jet of steam at 1000C is directed for a short time at a large block of ice at 00C.
Some of the steam condenses to form water and some ice is melted. The condensed steam
forms 0.40 kg of water at 00C.
EXAMPLE EXERCISE 3
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EXAMPLE EXERCISE 3
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Calculate the thermal energy given out by this mass of steam in changing to water at 1000C.
Calculate the thermal energy given out by this water as it cools to the temperature of ice.
specific latent heat of vaporisation of water = 2200 kJkg-1
specific heat capacity of water = 4.2 kJkg-1K-1
EXAMPLE EXERCISE 3
=Qsteam m steam X
= 0.40 kg 2200 kJ/kgX
=
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Mass Specific Latent Heat of Vaporisation
= X
Thermal Energy to condense Steam
= Latent Heat of Vaporisation of Steam
steaml v
Qsteam
EXAMPLE EXERCISE 3
=Q water
=
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Mass Specific Heat Capacity
=
XThermal Energy Lost by water
=
mwater cwater water
X Change in Temperature
0.4kg 4.2kJ/kg0C 0C(100- 0)XX
Q water
EXAMPLE EXERCISE 4
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What is the amount of energy required to change 10g of ice at 0ºC
to water at 20ºC?
specific latent heat of fusion of ice = 336 J/g
specific heat capacity of water = 4.2 J/gºC
EXAMPLE EXERCISE 4
Total Thermal Energy Needed
= Thermal Energy to melt Ice
=Q Total
= mwater cwater water
=
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Thermal Energy to increase temperature
of water+
Q WatericeLf +
m ice icel f + (( ) )
10g 4.2J/g0C 0C(20 - 0)XX ][+10g 336J/g X( )
=Q Total
EXAMPLE EXERCISE 5
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A glass contains 250g of hot tea at 90ºC.What is the minimum amount of ice at 0ºC
needed to cool the drink to 0ºC?
specific latent heat of fusion of ice = 336 J/g
specific heat capacity of tea = 4.2 J/gºC
EXAMPLE EXERCISE 5
Initial Final
Hot Tea
= 4.2 J/g0C c tea
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Ice00C 900C
Iced Tea00C
250g
mice = ?
= 336 J/gl icef
EXAMPLE EXERCISE 5
=
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=mtea c tea tea m ice icel f
250g 4.2J/g0C 0C(90 - 0)XX ][ 336J/g X( )
Q Tea iceLf=
Thermal Energy lost by hot tea
Thermal Energy gained by melting ice=
m ice
m ice
SUMMARIZATION
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QL =
The amount of thermal energy required to change the state
of a substance without a change in temperature.
“Latent Heat, L”
Ll =m
The amount of thermal energy required to
change the state of 1 kg of a substance without a change in temperature
“Specific Latent Heat, l”
J/kg Joule (J)
SUMMARIZATION
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The thermal energy required to change a
substance from solid to liquid state, or vice versa, without a change in temperature.
“Latent Heat of Fusion”
The thermal energy required to change a
substance from liquid to gaseous state, or vice-
versa, without any change in temperature.
“Latent Heat of Vaporisation”
SI unit = Joule
“L ”f “L ”v
SI unit = Joule
The heavier the mass of the substance, more thermal energy needed to change state,
the bigger the latent heat.
m
L
mL
LATENT HEAT
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For any states of matter,
Ll =mf
f Ll =mv
v
SUMMARIZATION
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The thermal energy required to change 1 kg of the solid
to liquid, or vice versa, without a change in
temperature.
“Specific Latent Heat of Fusion”
The thermal energy required to change 1 kg of the liquid to gas, or vice-
versa, without any change in temperature.
“Specific Latent Heat of Vaporisation”
SI unit = J/kg SI unit = J/kg
SUMMARIZATION
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Thermal Energy supplied to substance by heater
=Thermal Energy to change the state of the substance
=
=
SUMMARIZATION
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Thermal Energy lost by Substance A
=Thermal Energy gained by
Substance B to change state
=
=
SUMMARIZATION
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Total Thermal Energy Needed
=Thermal Energy
Needed to Change State
Thermal Energy Needed to Change
Temperature+
=Q Total Q substancesubstanceL +
=Q Total
=Q Total
=Q Total