Conduction, convection and radiation

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**Conduction**, **convection** and **radiation**. Heat always flows from hotter to colder.

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Faster-vibrating hot particles jostle their cooler neighbours, passing **energy** along while the particles stay put. In metals, free **electrons** also carry it (so metals conduct best).

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The **hot fluid itself** moves: warmed fluid expands, becomes less dense and **rises**, carrying its energy with it (only in liquids and gases).

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As **infrared electromagnetic waves**, needing **no material** — so it is the only method that works through a **vacuum** (e.g. the Sun → Earth).

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**Radiation** only — conduction and convection both need particles/material.

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$\dfrac{\Delta Q}{\Delta t} = kA\dfrac{\Delta T}{\Delta x}$ — rate = conductivity × area × (temperature difference ÷ thickness). **Given** in the data booklet.

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The **watt** (W), i.e. joules per second (J s⁻¹) — it is a rate of energy transfer.

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A bigger thickness **Δx** (on the bottom) **slows** conduction: rate ∝ 1 ÷ Δx, so doubling the thickness halves the rate.

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A larger conductivity **k**, larger area **A**, or a larger temperature difference **ΔT**.

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The object cools toward room temperature, so the **temperature difference** driving the heat loss shrinks — a smaller difference means a slower rate, i.e. a flatter graph.

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They contain **free electrons** that move quickly through the metal and carry thermal energy, on top of the usual particle-to-particle vibration.

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From a **hotter** region to a **colder** one, until they reach the same temperature (thermal equilibrium).