Thermal physics

Learning objectives

• Distinguish temperature from thermal energy.
• Calculate energy for a temperature change.
• Explain conduction, convection and thermal radiation.

The big idea

A particle model helps explain expansion, gas pressure and changes of state. Temperature indicates how hot a body is; heating transfers energy because of a temperature difference.

Materials require different energy inputs for the same temperature rise. During a change of state, energy changes the arrangement or separation of particles while temperature can remain constant. Convection needs bulk motion of a fluid; radiation does not require a medium.

Topics to connect

• Particle model
• Thermal properties
• Specific heat capacity
• Melting and boiling
• Thermal expansion
• Conduction, convection and radiation

1. Particles explain states

A solid’s particles vibrate about positions; a liquid’s remain close but can rearrange; a gas has widely separated particles with random motion. Heating usually raises their average kinetic energy. Gas pressure comes from particle impacts on walls, not from particles expanding in size. Compressing a fixed gas at constant temperature increases collision rate with the walls and raises pressure. Molecular drawings are models, not scale photographs.

2. Thermal properties and expansion

Many substances expand when heated because mean particle separation increases. This can matter in railway gaps, liquid thermometers and bimetallic strips. Temperature compares thermal states; internal energy includes energy associated with microscopic motion and interactions. A large cool bath can have more internal energy than a small hot cup. A thermometer must approach thermal equilibrium with what it measures, and its chosen property must change reproducibly.

3. Heating and changes of state

Specific heat capacity is the energy per kilogram for a one-degree temperature increase, so Q = mcΔT applies without a change of state. For 0.2 kg of water with c = 4200 J/(kg K), a 5 K rise needs about 4200 J if losses are neglected. At melting or boiling, energy can change particle arrangement without raising temperature. Evaporation occurs at a liquid surface below boiling; escaping higher-energy particles can cool the remaining liquid.

4. Conduction, convection and radiation

Conduction transfers energy through interactions within matter, with mobile electrons contributing strongly in metals. Convection moves warmer and cooler fluid regions as bulk flow; the fluid itself travels. Infrared radiation transfers energy electromagnetically and needs no medium. A vacuum flask combines a vacuum, reflective surfaces and a narrow insulating stopper because no one feature prevents every transfer route. Dark, matt surfaces are generally effective thermal emitters and absorbers compared with shiny surfaces under comparable conditions.

A method you can reuse

1. Decide whether temperature changes, state changes or both.
2. Identify mass and the relevant material property.
3. Use a temperature interval, not an absolute temperature, in mcΔT.
4. Name each possible transfer route and any losses.
5. Explain the particle mechanism as well as the observed effect.

Equations and relationships

Investigate and explain

Compare cooling curves for equal volumes of warm water in insulated and uninsulated containers, with teacher supervision.

Use particle and heating models to distinguish temperature, energy and state. They simplify material properties and heat losses; measured specific heat capacities are still needed for numerical work.

Open the interactive investigation

Check your understanding