IP Physics Notes (Upper Secondary, Year 3-4): 9) Thermal Physics

Quick recap -- Temperature tracks particle kinetic energy, while heating changes the internal energy of matter through conduction, convection, or radiation. Quantify changes with specific heat and latent heat relations.

Temperature & Thermometry

• Temperature: measure of hotness, proportional to average kinetic energy of particles; SI base unit kelvin (K).
• Convert: \( T_\pu{K} = T_\pu{^\circ C} + 273.15 \).
• Thermometers rely on thermometric properties (e.g., expansion of liquids, emf in thermocouples, electrical resistance).
• Calibration uses fixed points: ice point (\(\pu{0 ^\circ C}\)), steam point (\(\pu{100 ^\circ C}\)).

Kinetic Model of Matter

• Solids: tightly packed, vibrate about fixed positions; strong intermolecular forces.
• Liquids: particles slide past each other; weaker forces, definite volume but no fixed shape.
• Gases: particles far apart, random motion, negligible forces.
• Brownian motion evidences particle movement.
• Gas behaviour:
  • At constant volume, increasing temperature raises pressure (particles strike walls harder and more often).
  • At constant pressure, raising temperature expands volume (particles move faster, need bigger container to maintain pressure).
  • Boyle's law (Chapter 5) links pressure and volume at fixed temperature.

Transfer of Thermal Energy

• Heat flows from higher to lower temperature regions.
• Conduction: particle collisions/free electron diffusion in solids. Metals conduct well due to mobile electrons; insulators do not.
• Convection: bulk fluid movement; heated regions expand, become less dense, and rise while cooler fluid sinks to replace them.
• Radiation: emission of electromagnetic waves (mainly infrared); no medium required. Rate increases with higher surface temperature, larger surface area, and matte black surfaces.
• Applications: vacuum flasks, radiators, coastal breezes, heat sinks.

Internal Energy & Specific Heat Capacity

• Internal energy = sum of microscopic kinetic + potential energies.
• Heating a body raises internal energy; some energy changes kinetic (temperature rises), some potential (phase change).
• Specific heat capacity \( c \): energy required per kilogram per kelvin change, units ( \pu{J.kg-1.K-1} ).
• Thermal energy change: \[ Q = m c \Delta \theta \]
• Experimentally determine ( c ) using electrical heating: measure voltage \( V \), current \( I \), time \( t \); equate \( V I t = m c \Delta \theta \).

Worked Example: Heating Aluminium

A \( \pu{0.80 kg} \) aluminium block (\( c = \pu{900 J.kg-1.K-1} \)) warms from \( \pu{22 ^\circ C} \) to \( \pu{75 ^\circ C} \).

\[ Q = m c \Delta \theta = 0.80 \times 900 \times (75 - 22) = \pu{3.8 \times 10^4 J}. \]

Change of State & Latent Heat

• During melting/boiling, temperature stays constant while energy changes molecular potential energy.
• Specific latent heat \( \ell \): energy per kilogram needed for phase change at constant temperature.
  • Fusion: solid to liquid, \( \ell_\text{f} \).
  • Vaporisation: liquid to gas, \( \ell_\text{v} \).
• Energy for phase change: \[ Q = m \ell \]
• Electrical method: \( V I t = m \ell \) (measure mass melted or evaporated).

Boiling vs Evaporation

Cooling & Heating Curves

• Plotting temperature vs time at constant heating/cooling power shows plateaus during phase changes.
• Longer plateau at boiling because \( \ell_\text{v} > \ell_\text{f} \) for most substances.
• Gradient in each sloped section inversely proportional to specific heat capacity in that state.

Key Takeaways

• Temperature is a measure of average kinetic energy; Kelvin scale shifts Celsius by 273.15.
• Conduction, convection, and radiation each dominate under different circumstances.
• Use \( Q = m c \Delta \theta \) for sensible heating and \( Q = m \ell \) for latent heating.
• Cooling curves visualise internal energy shifts between kinetic and potential contributions.