Lesson Notes By Weeks and Term v4 - SHS 3
HEAT
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HEAT
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Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 17
Grade code: 3.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.1
Indicator code: 3.2.1.LI.2
Theme: ENERGY
Subtheme: HEAT
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This lesson explores thermal expansion, which is the tendency of matter to change its shape, area, and volume in response to a change in temperature. In Ghana, we experience this phenomenon every day, even if we don't notice it. The hot afternoon sun causes the metal roofing sheets on our homes to expand, sometimes making creaking sounds. The Adomi Bridge has special gaps called expansion joints to prevent it from buckling under the heat. Understanding thermal expansion is crucial for engineers, builders, and technicians to design structures and devices that are safe and durable in our warm climate. Today, we will learn how to quantify and calculate this expansion for solids.
A. The Particulate Nature of Thermal Expansion
When a solid is heated, the thermal energy supplied increases the kinetic energy of its constituent atoms or molecules. These particles vibrate more vigorously about their fixed positions. As they vibrate with greater amplitude, they push their neighbours further away, increasing the average distance between them. This small increase in separation, when accumulated over the millions of particles in the solid, results in a macroscopic (observable) increase in the size of the object. This is thermal expansion. B. Linear Expansion
This refers to the expansion of a solid in one dimension – its length. It is most noticeable in long, thin objects like wires, rods, and railway tracks. Key Idea: The change in length (ΔL) of a solid is directly proportional to its original length (L₀) and the change in temperature (ΔT). ΔL ∝ L₀ ΔL ∝ ΔT Formula: Combining these, we get: ΔL = αL₀ΔT Where: ΔL = Change in length (L - L₀) in metres (m). L₀ = Original length in metres (m). L = Final length in metres (m). ΔT = Change in temperature (T - T₀) in Kelvin (K) or degrees Celsius (°C). Since it's a *change*, the value is the same for both scales. α (alpha) = Coefficient of Linear Expansivity. This is a property of the material that tells us how much it expands per unit length per unit degree rise in temperature. Coefficient of Linear Expansivity (α): α = ΔL / (L₀ΔT) The S.I. unit for α is per Kelvin (K⁻¹) or per degree Celsius (°C⁻¹). A material with a large α (like Aluminium) expands more than a material with a small α (like Steel) for the same temperature change.