Thermal Physics

The One Thing

Thermal physics explains temperature, pressure, changes of state, expansion, and energy transfer in terms of the motion, energy, arrangement, and interactions of particles. Thermal energy transfers from hotter regions to colder regions until thermal equilibrium is reached.

Reactions, Processes and Experiments

In the formula $E=mc\text{ delta theta}$, $E$ is thermal energy transferred in joules, $m$ is mass in kilograms, $c$ is specific heat capacity in joules per kilogram per degree Celsius, and $\text{delta theta}$ is temperature change. In $E=ml$, $E$ is energy transferred in joules, $m$ is mass in kilograms, and $l$ is specific latent heat in joules per kilogram.

Key Terms

• Kinetic particle model: A model stating that matter is made of tiny particles in constant random motion, with the particles held together by forces.
• Solid: A state in which particles are closely packed in fixed positions and vibrate about those positions; solids have a fixed shape and volume.
• Liquid: A state in which particles are close together but can move past one another; liquids have a fixed volume but take the shape of their container.
• Gas: A state in which particles are far apart and move rapidly in random directions; gases have no fixed shape or volume.
• Temperature: A measure related to the average kinetic energy of the particles in a substance.
• Thermal energy: The internal energy associated with the random kinetic and potential energies of particles in a substance.
• Internal energy: The total microscopic kinetic energy and potential energy of all the particles in a substance.
• Heat: Energy transferred from a hotter object or region to a colder object or region because of a temperature difference.
• Thermal equilibrium: A condition in which objects in contact reach the same temperature, so there is no net thermal energy transfer between them.
• Conduction: Transfer of thermal energy through a substance from hotter regions to colder regions without bulk movement of the substance.
• Convection: Transfer of thermal energy in fluids by the movement of warmer, less dense fluid and cooler, denser fluid.
• Radiation: Transfer of thermal energy by electromagnetic waves, mainly infrared radiation, and it does not require particles or a medium.
• Thermal conductor: A material that transfers thermal energy easily, such as most metals.
• Thermal insulator: A material that transfers thermal energy poorly, such as plastic, wood, foam, or trapped air.
• Specific heat capacity: The energy required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius or 1 kelvin.
• Specific latent heat: The energy required to change the state of 1 kilogram of a substance without changing its temperature.
• Melting: The change from solid to liquid when particles gain enough energy to leave their fixed positions.
• Boiling: A change from liquid to gas occurring throughout the liquid at its boiling point.
• Evaporation: A change from liquid to gas occurring at the surface and at temperatures below the boiling point.
• Condensation: The change from gas to liquid when particles lose energy.
• Freezing: The change from liquid to solid when particles lose enough energy to become arranged in fixed positions.
• Thermal expansion: The increase in size of a substance when its particles gain energy and move farther apart on average.
• Absolute zero: The lowest possible temperature, 0 K or approximately -273 degrees Celsius, at which particles have minimum thermal motion.

Easily Confused

• Thermal energy and heat: Thermal energy is energy associated with particles within a substance; heat is energy transferred because of a temperature difference.
• Temperature and thermal energy: Temperature relates to average particle kinetic energy, whereas thermal energy includes the particles’ total microscopic kinetic and potential energies.
• Melting and boiling: Melting changes a solid to a liquid; boiling changes a liquid to a gas throughout the liquid at its boiling point.
• Boiling and evaporation: Boiling occurs throughout a liquid at its boiling point; evaporation occurs only at the surface and can occur below the boiling point.
• Conduction, convection and radiation: Conduction transfers energy through a substance without bulk movement; convection transfers energy by fluid movement; radiation transfers energy by electromagnetic waves and can occur through a vacuum.
• Specific heat capacity and specific latent heat: Specific heat capacity concerns a temperature increase of 1 kilogram by 1 degree Celsius or 1 kelvin; specific latent heat concerns changing the state of 1 kilogram without changing its temperature.
• Thermal energy and internal energy: Internal energy is the total microscopic kinetic and potential energy of particles; thermal energy refers to the internal energy associated with their random kinetic and potential energies.
• Light, shiny surfaces and dark, matt surfaces: Light, shiny surfaces are generally poor absorbers and emitters of radiation; dark, matt surfaces are generally good absorbers and emitters.
• Convection and conduction in a vacuum: A vacuum reduces conduction and convection because it contains very few particles, but radiation can still pass through it.
• Evaporation and condensation: Evaporation changes liquid to gas as higher-energy surface particles escape; condensation changes gas to liquid as particles lose energy.

What Gets Asked

• Explain the kinetic particle model and use it to relate higher temperature to greater average particle kinetic energy. Marks are lost by describing temperature as the total energy of all particles rather than an average kinetic energy.
• Describe melting, boiling, evaporation, condensation, or freezing using particle arrangement and energy. Marks are lost by stating that temperature rises during melting or boiling.
• Use $E=mc\text{ delta theta}$ to calculate thermal energy, identifying $E$, $m$, $c$, and $\text{delta theta}$ with the stated units. Marks are lost by confusing specific heat capacity with specific latent heat.
• Use $E=ml$ for a change of state. Marks are lost by including a temperature change when the material states that the temperature remains constant.
• Compare conduction, convection, and radiation, including the role of mobile electrons in metals, density changes in fluids, and the ability of radiation to travel through a vacuum. Marks are lost by attributing bulk fluid movement to conduction or claiming that radiation requires a medium.
• Explain gas pressure, thermal expansion, or insulation in practical contexts such as bridges, railway tracks, pipes, electrical cables, and insulation systems. Marks are lost by stating that insulation completely stops energy transfer rather than slowing it.