Internal energy and the particle model
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Question
Define internal energy.
Answer
The **total random kinetic energy** of all the particles **plus** the **total intermolecular potential energy** of all the particles.
Question
What are the two parts of internal energy?
Answer
**Random KE** (the particles' motion) and **intermolecular PE** (energy in the forces between particles).
Question
What makes up the internal energy of a REAL gas?
Answer
Both the **random KE** of the particles **and** the **intermolecular PE** (a real gas has weak forces, so the PE part is not zero).
Question
What does temperature measure?
Answer
The **average random kinetic energy** of the particles (not the potential energy).
Question
When does the intermolecular PE part change most?
Answer
During a **change of state** (melting, boiling) — the spacing of the particles changes there.
Question
Why are most solids denser than their liquids?
Answer
The particles are packed **closer together** in the solid, so there is **more mass per volume**.
Question
Formula for density?
Answer
$\rho = \dfrac{m}{V}$ — mass ÷ volume. **Given** in the data booklet.
Question
Units of density?
Answer
**kg m⁻³** (kilograms per cubic metre).
Question
At what temperature is water densest?
Answer
About **4 °C** — water's density anomaly.
Question
Why does ice float on water?
Answer
Ice (and water below 4 °C) is **less dense** than water at 4 °C, so it rises and floats.
Question
How does the density anomaly help aquatic life?
Answer
Ponds freeze **top-down**; the ice insulates the ≈4 °C water below, so fish survive the winter.
Question
Difference between a real gas and an ideal gas (internal energy)?
Answer
A **real gas** has KE **and** intermolecular PE; an **ideal gas** is modelled with no forces, so its internal energy is the **KE only**.
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Topic 2.1 hub
Thermal energy transfers
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