Second Term Lesson Note for Week Five
Class : SSS 2
Subject : Physics
Topic : Latent Heat
Duration : 40 Minutes
Period : Single Period
Reference Book :
New School Physics for Senior Secondary School, SSS 1 – 3.
Lagos State Unified schemes of work for Senior Secondary School, SSS 1 – 3.
Online Resources
Instructional Material : Chart showing latent heat and types of latent heat.
Learning Objectives : By the end of the lesson learners will be able to :
i. Define Latent Heat
ii. List the types of Latent Heat
iii. Explain each type of Latent Heat.
Content :
LATENT HEAT
Sometimes when heat energy is given to a substance, it does not increase its temperature, rather, it changes the state of the substance; such heat is referred to as latent heat.
There are two types of latent heat:
- Latent heat of fusion
- Latent heat of vaporization
Latent heat of Fusion
Latent heat of fusion: This is the quantity of energy needed to change solid to liquid without any change in temperature.
Latent heat of vaporization
Latent heat of vaporization: This is the amount of energy needed to change liquid to gases (vapor) or steam at constant temperature.
SPECIFIC LATENT HEAT
This is the quantity of heat required to change a unit mass of a substance from one state to another without a change in its temperature.
(a) SPECIFIC LATENT HEAT OF FUSION (Lf)
This is the quantity of heat required to change a unit mass of a solid to liquid without a change in its temperature. The specific latent heat of fusion of ice is 33600 J/kg.
…1
(b) SPECIFIC LATENT HEAT OF VAPORISATION (Lv)
This is the quantity of heat required to convert a unit mass of liquid to vapor (gaseous state) without any change in temperature. For water the value is 2.26 x 106 J/kg
…2
EXAMPLE
1. How much heat energy is needed to change 3g of ice at 0°C to steam at 100°C? (Lf = 336 KJ/kg, Cw = 4200 J/kgk, Lv = 2.26 x 106 J/kg
First stage:
Heat required for the ice to melt at 0°C
Second Stage:
Heat required to raise the temperature of the melted ice from 0°C to 100°C
Third Stage:
Heat required to convert the liquid to steam
The total energy,
Evaluation :
1. Differentiate between specific heat capacity and specific latent heat
2. Calculate the total energy required to evaporate completely 1kg of ice that is initially at – 10°C. Given that:
[Specific capacity of Ice = 2.2 x 103 J/kgk, Specific heat that capacity of water = 4.2 x 103 J/kgk, Specific latent heat of fusion of Ice = 3.36 x 105 J/kg, Specific latent heat of vaporization = 2.26 x 106 J/kg]
CONCLUSION : the teacher summarizes the lesson again for a better understanding to the learners.
ASSIGNMENT
SECTION A
1. Determine the heat required to change 10g of ice at 0°C to water at 10°C if specific latent heat of ice is 335J/g and specific heat capacity of water is 4.2J/gk (a) 3.77 x 103J (b) 4.00 x 103J (c) 4.50 x 103J (d) 1.33x 103J (e) 4.9x 103J
2. Which of the following is the correct SI unit of specific latent heat? (a) Jkgk (b) Jkg-1k-1 (c) Jkgk-1 (d) Jkg-1 (e) Jkg
3. Latent heat of fusion changes substances from its (a) solid state to liquid state (b) from liquid to solid (c) from liquid to gas
4. The heat required to change 1kg of ice at 0°C to water at the same temperature is called ____________ (a) specific latent heat of vaporization (b) specific heat capacity (c) specific latent heat of fusion (d) heat capacity (e) specific heat capacity
SECTION B
1. An electric heater immersed in water of mass m, raised the temperature of the water from 40°C to 100°C in 5.0 minutes. After another 11.25minutes, one-quarter of the water has been converted to steam. Calculate the specific latent heat of vaporization of water. [Specific heat capacity of water = 4200 Jkg-1K-1]
2. Calculate the energy required to vaporize 50g of water initially at 80°C. [Specific heat capacity of water = 4.2Jg-1K-1; specific latent heat of vaporization of water = 2260 Jg-1]