Lesson Notes By Weeks and Term v4 - SHS 2
WAVES
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WAVES
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Subject: Physics
Class: SHS 2
Term: 1st Term
Week: 15
Grade code: 2.2.1.LI.4
Strand code: 2
Sub-strand code: 2
Content standard code: 2.2.1.CS.1
Indicator code: 2.2.1.LI.4
Theme: ENERGY
Subtheme: WAVES
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This lesson explores the concept of "latent heat," a fundamental principle in thermal physics. We often think that when we add heat to something, its temperature must rise. However, there are crucial moments during melting or boiling where heat is absorbed without any temperature change. Understanding this "hidden" heat is essential. In Ghana, we see these principles in action every day: when we boil water for *banku* or *fufu*, when shea butter melts in the sun, when wet clothes dry on the line, or even when we feel cool after sweating.
A. Review of Changes of State Before we discuss latent heat, let's refresh our memory on the changes of state we see around us. Melting (Fusion): The process where a solid turns into a liquid at a constant temperature (the melting point). Example: Ice turning into water. Freezing (Solidification): The process where a liquid turns into a solid at a constant temperature (the freezing point). Example: Water turning into ice in a freezer. Boiling: A rapid process where a liquid turns into a gas (vapour) throughout the bulk of the liquid at a specific temperature (the boiling point). Example: Water bubbling violently and turning into steam at 100°C. Evaporation: A slower process where a liquid turns into a gas from its surface at any temperature below the boiling point. Example: A puddle of water drying up on a hot day. B. Sensible Heat vs. Latent Heat
Imagine you take a block of ice from a deep freezer at -10°C and start heating it. -10°C to 0°C: As you add heat, the temperature of the ice rises. The molecules vibrate faster. This heat you add, which causes a change in temperature, is called Sensible Heat. We calculate it using the formula Q = mcΔT. At 0°C: Something interesting happens. Even though you are still adding heat, the thermometer reading stays fixed at 0°C. The ice begins to melt and turn into water. All the heat energy being supplied is used to break the strong bonds holding the water molecules in a fixed solid structure (ice). It is NOT used to increase the kinetic energy of the molecules, so the temperature does not rise. 0°C to 100°C: Once all the ice has melted, the temperature of the water will start to rise again as you continue to add heat. This is again Sensible Heat. At 100°C: The temperature stops rising again! The water starts to boil, turning into steam. All the heat energy is now being used to break the remaining intermolecular forces and allow the molecules to escape as a gas. Above 100°C: If you could trap the steam and continue heating it, its temperature would rise above 100°C.
The heat absorbed or released during a change of state (at a constant temperature) is called Latent Heat (L). The word "latent" means "hidden". C. Latent Heat of Fusion and Vaporisation Latent Heat of Fusion (L_f): The heat energy required to change a substance from a solid to a liquid state *without any change in temperature*. Latent Heat of Vaporisation (L_v): The heat energy required to change a substance from a liquid to a gaseous state *without any change in temperature*. D. Specific Latent Heat (l)
The amount of heat needed to melt a large block of ice is more than what is needed for a small one. Therefore, latent heat depends on the mass of the substance. To create a standard measure, we define Specific Latent Heat.