Thermal Properties and Temperature
Section: Thermal Physics | Syllabus: Cambridge AS Level Physics 9702
Thermal Expansion Have you ever noticed gaps between railway tracks or heard a metal lid loosen after being heated? That's thermal expansion in action - when substances expand because their particles move faster and spread out as temperature increases.
Thermal Expansion The increase in size of a substance when its temperature increases, caused by particles gaining kinetic energy and moving faster or further apart. The Particle Explanation When heated, particles gain kinetic energy and move faster.
They vibrate more strongly (in solids) or move apart (in liquids and gases). This increased motion means the average distance between particles increases → the material expands. State Expansion on Heating Explanation Solid Least Particles vibrate but remain tightly packed.
Liquid Moderate Particles move further apart when heated. Gas Most Particles move freely and very far apart as energy increases. Real-Life Examples Gaps between railway lines prevent buckling in hot weather.
Power lines sag in summer and tighten in winter. Bridges have expansion joints to allow safe expansion and contraction. What About Cooling? The opposite happens - as temperature decreases, particles lose energy and move closer together, causing the substance to contract .
Thermal Expansion of Solids, Liquids, and Gases Bar chart or comparison showing expansion amounts: Gases expand the most (particles move freely), Liquids expand moderately, Solids expand the least (particles tightly packed).
All expand when heated as particles gain kinetic energy and move further apart. Specific Heat Capacity Different substances heat up at different rates. For example, a metal spoon gets hot quickly in a cup of tea, but the water warms much slower.
This difference is explained by the property called specific heat capacity (c). Specific Heat Capacity The amount of energy required to raise the temperature of 1 kg of a substance by 1°C (or 1 K). Formula: Q = mcΔ T \(Q\) = heat energy (J) \(m\) = mass (kg) \(c\) = specific heat capacity (J/kg°C) \(Δ T\) = temperature change (°C) Key Concept Substances with a high specific heat capacity (like water) heat up and cool down slowly.
Those with low specific heat capacity (like metals) heat up and cool quickly. Substance Approx. Specific Heat Capacity (J/kg°C) Behavior Water 4200 Heats/cools slowly Aluminium 900 Moderate heating rate Copper 385 Heats up rapidly Investigating Specific Heat Capacity To measure specific heat capacity in the lab, you supply known energy and measure the temperature change.
Method (Electrical Heating) Weigh the block (mass = m). Insert a heater and thermometer into the block. Connect the heater to a power supply and ammeter/voltmeter. Record current (I), voltage (V), and time (t).
Calculate energy supplied: \(Q = VIt\). Measure the temperature change \(Δ T\). Find specific heat capacity: \(c = Q/m Δ T\). Temperature Rise vs Time (Specific Heat) Graph with Time on x-axis and Temperature on y-axis.
Multiple lines showing different materials being heated with same power. Copper (low c) rises steeply, Aluminium rises moderately, Water (high c = 4200 J/kg°C) rises slowly. Higher specific heat capacity = slower temperature rise.
Experimental Errors Heat loss to surroundings → use insulation. Uneven temperature → stir the substance. Latent Heat When a substance changes state, temperature stays constant even though energy is still being absorbed or released.
That energy changes the potential energy of particles (breaking or forming bonds), not their kinetic energy. Latent Heat The energy required to change the state of a substance without changing its temperature.
Formula: Q = mL \(Q\) = energy (J) \(m\) = mass (kg) \(L\) = specific latent heat (J/kg) Types of Latent Heat Latent Heat of Fusion (L f ): energy needed to melt or freeze (solid ⇄ liquid). Latent Heat of Vaporisation (L v ): energy needed to boil or condense (liquid ⇄ gas).
Heating Curve for Water Graph with Time/Energy on x-axis and Temperature on y-axis. Five sections: (1) Ice warming (rising line). (2) Melting at 0°C (flat horizontal line - latent heat of fusion). (3) Water warming (rising line).
(4) Boiling at 100°C (flat horizontal line - latent heat of vaporisation). (5) Steam warming (rising line). Remember Temperature stays constant during melting and boiling because all heat energy goes into changing the state - not raising temperature.
Evaporation Evaporation happens when the most energetic particles at the surface of a liquid escape into the air, even below the boiling point. Evaporation The change from liquid to gas that occurs at the surface of a liquid at any temperature, when the most energetic particles escape.
Key Points: Only surface particles escape. Faster (higher energy) particles leave → remaining liquid has lower average energy → temperature decreases . This is why evaporation causes cooling (e.g. sweating).
Factors Affecting Evaporation Rate: Temperature (higher → faster evaporation) Surface area (larger → fas…
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