Thermal physics

The big picture

Temperature is only one part of thermal reasoning. A sample can absorb energy during a phase change without warming, and a gas can exchange heat while its temperature stays fixed. Track the state, process and system boundary separately.

Begin with observable expansion and calorimetry, then connect temperature to a gas model. In a pressure–volume process, identify which curve you are following before integrating work. A cycle becomes clearer when every heat and work transfer has an explicit sign.

What to read, and why

Start at the first stage if the subject is new. Otherwise, use the descriptions to find the missing step in your understanding. All chapter explanations are open; the labels identify chapters whose solutions and answer checks are also free.

1. STAGE 1

   Describe the thermal state

   Connect observable expansion and temperature to a gas model before following an energy transfer.

   Thermal expansion: geometry, measurement and restraint →Follow what can move, what the ruler measures and when heating produces stress.8 worked examples · 7 checks · Complete free chapterGases: state variables, molecular speeds and collisions →Connect pressure and temperature to particle motion, then distinguish collision distance from collision time.10 worked examples · 10 checks · Open explanation · member practice

2. STAGE 2

   Follow energy through a process

   Distinguish heat, work and state functions; then test the final phase before calculating a mixing temperature.

   Heat, work and thermodynamic paths →Use energy conservation with an explicit system and sign convention, then audit every leg of a gas cycle.12 worked examples · 11 checks · Open explanation · member practiceHeat balances, conduction and cooling →Track energy through mixing and phase changes, then distinguish steady heat flow from a changing temperature.15 worked examples · 14 checks · Open explanation · member practice

3. STAGE 3

   Track transfer and cycles

   Compare radiative exchange with engine and refrigerator accounting.

   Thermal radiation and the wavelength of a spectral peak →Separate emission from net exchange, count the relevant areas and connect temperature to a broad spectrum.5 worked examples · 9 checks · Open explanation · member practiceHeat engines, refrigerators and entropy →Use energy and entropy as separate checks on a complete thermal cycle.8 worked examples · 9 checks · Open explanation · member practice

Choose a method before an equation

Use the physical conditions to narrow the choices. A formula becomes useful after you know why its model applies.

Try a complete free problem

These problems include the hints, worked reasoning and answer feedback. Attempt one before opening the solution.