Lesson Notes By Weeks and Term v4 - SHS 2
HEAT
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HEAT
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Subject: Physics
Class: SHS 2
Term: 1st Term
Week: 14
Grade code: 2.2.1.LI.4
Strand code: 2
Sub-strand code: 1
Content standard code: 2.2.1.CS.1
Indicator code: 2.2.1.LI.4
Theme: ENERGY
Subtheme: HEAT
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This lesson explores the fascinating concept of "hidden heat," known as latent heat. We experience its effects every day here in Ghana, but we might not know the physics behind it. Why does sweating cool us down on a hot Accra afternoon? How does a block of ice keep the fish fresh at the Tema fishing harbour for so long without getting much colder? Why does boiling water for our banku stay at the same temperature no matter how high we turn the fire? Understanding latent heat answers these questions. It's the energy required to change the state of a substance (e.g., from solid ice to liquid water, or from liquid water to gaseous steam) *without* changing its temperature.
Part 1: Phase Changes - A Quick Review Before we talk about "latent heat," let's remember the states (or phases) of matter: solid, liquid, and gas. A phase change is the transition from one state to another. Melting: Solid to Liquid (e.g., ice turning into water) Freezing (or Solidification): Liquid to Solid (e.g., water in the freezer turning into ice) Vaporisation (Boiling/Evaporation): Liquid to Gas (e.g., water boiling to become steam) Condensation: Gas to Liquid (e.g., steam hitting a cold lid and turning back into water droplets)
The Critical Idea: During a phase change, the heat energy being added or removed does NOT change the temperature of the substance. Instead, this energy is used to change the arrangement and bonding between the molecules. Part 2: What is Latent Heat? Imagine you are heating a block of ice. You put a thermometer in it. As you add heat, the temperature rises from, say, -10°C to -5°C, then to 0°C. But once it reaches 0°C, something strange happens. Even though you are still adding heat, the thermometer reading stays stuck at 0°C until ALL the ice has melted into water.
Where did that heat energy go? It didn't make the molecules move faster (which would increase the temperature). Instead, it was used to break the strong bonds holding the water molecules in their fixed solid (ice) structure. This "hidden" energy is called Latent Heat (Q). Definition: Latent Heat (Q) is the thermal energy absorbed or released by a substance during a phase change at a constant temperature. The unit is Joules (J).
There are two main types of latent heat: Latent Heat of Fusion (L_f): The heat energy required to change a substance from a solid to a liquid at its melting point. It is also the energy released when a substance freezes. Latent Heat of Vaporisation (L_v): The heat energy required to change a substance from a liquid to a gas at its boiling point. It is also the energy released when a substance condenses.