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**.
Question
Convert 25 °C to kelvin.
Answer
**298 K** (T/K = θ/°C + 273). A change of 25 °C is also a change of 25 K.
Question
What does the kelvin temperature measure?
Answer
The **average random kinetic energy** of the particles: **Ē_k = 3/2 k_B T**.
Question
What decides the direction of the net thermal energy transfer?
Answer
The **temperature difference**: from higher to lower temperature, until thermal equilibrium (not from more internal energy to less).
Question
How do you estimate the spacing of atoms from the volume per atom V?
Answer
Spacing ≈ **∛V** (each atom sits in a small cube).
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Topic 2.1 hub
Thermal energy transfers
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