LEARNING OBJECTIVES
What you will be able to do
- apply the particle model to solids, liquids and gases
- relate gas pressure and volume at constant temperature
- explain thermal expansion and specific heat capacity
- compare conduction, convection and radiation
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Use particles and energy transfers to explain temperature, state changes and heating.
Thermal physics explains visible changes by tracking microscopic particles. Temperature indicates the average kinetic energy of particles, whereas internal energy includes the particles' kinetic and potential energy.
Heating can raise temperature, change state or both. The pathway—conduction, convection or radiation—depends on the material and surroundings.
SECTION 01
Particle model and gas pressure
Solid particles vibrate about fixed positions, liquid particles move past one another, and gas particles move rapidly and randomly with large separations. Brownian motion gives evidence for invisible molecular collisions.
Gas pressure results from particles striking container walls. At constant temperature, compressing a fixed mass of gas makes collisions more frequent, so pressure rises.
Compressing a gas
- A gas occupies 120 cm³ at 100 kPa.
- It is compressed at constant temperature to 80 cm³.
- p₂ = 100 × 120 ÷ 80.
Answer: The final pressure is 150 kPa.
SECTION 02
Thermal expansion and thermometry
Most substances expand when heated because particles vibrate or move farther apart. Gases expand most and solids least. Expansion is useful in liquid thermometers and bimetallic strips but requires gaps in bridges and rails.
A thermometer uses a physical property that changes predictably with temperature. Fixed points and a linear scale allow calibration; sensitivity, range and response time determine suitability.
| Feature | Improvement | Effect |
|---|---|---|
| Sensitivity | narrow capillary or large bulb | larger movement per degree |
| Response time | thin glass and small bulb | quicker thermal equilibrium |
| Range | suitable liquid and scale | measures required extremes |
SECTION 03
Specific heat capacity and state changes
A material with high specific heat capacity needs more energy for the same mass and temperature rise. During melting or boiling, energy separates particles without raising their average kinetic energy, so temperature stays constant for a pure substance.
Evaporation occurs at the surface below boiling point. Faster particles escape, reducing the average kinetic energy of the remaining liquid and causing cooling.
Heating water
- A 0.50 kg sample of water warms by 12 °C.
- Use c = 4200 J/(kg °C).
- Energy = 0.50 × 4200 × 12.
Answer: 25 200 J, or 25.2 kJ, is transferred.
SECTION 04
Conduction, convection and radiation
Conduction transfers thermal energy through collisions and vibrations; free electrons make metals especially effective conductors. Convection occurs in fluids when warmer, less dense regions rise and cooler, denser regions sink.
Infrared radiation is electromagnetic and needs no medium. Dull black surfaces are strong absorbers and emitters; shiny light surfaces are weak absorbers and emitters and good reflectors.
Choosing a vacuum flask feature
- A vacuum contains very few particles.
- Conduction and convection require particles.
- Silvered surfaces reflect infrared radiation.
Answer: The vacuum reduces conduction and convection; silvering reduces radiation.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Temperature and internal energy are related but not identical.
- Gas pressure comes from particle collisions.
- Specific heat capacity controls temperature rise for a given energy input.
- Conduction, convection and radiation require different explanations.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- apply the particle model to solids, liquids and gases
- relate gas pressure and volume at constant temperature
- explain thermal expansion and specific heat capacity
- compare conduction, convection and radiation