Thermal Properties of Matter

The One Thing

Heat is energy transferred because of a temperature difference, always flowing spontaneously from higher temperature to lower temperature until thermal equilibrium is reached. Matter responds to heat through changes in temperature, dimensions, state, and the mechanisms of conduction, convection, and radiation.

Reactions, Processes and Experiments

Key Terms

• Temperature: A measure related to the average kinetic energy of the particles in a substance.
• Heat: Energy transferred from a body at higher temperature to a body at lower temperature.
• Thermal equilibrium: The condition in which bodies in thermal contact have the same temperature and no net heat flows between them.
• Zeroth law of thermodynamics: If two bodies are separately in thermal equilibrium with a third body, they are in thermal equilibrium with each other.
• Temperature scales: Common scales include Celsius, Fahrenheit, and Kelvin; the Kelvin scale is the SI scale of temperature.
• Thermal expansion: The increase in length, area, or volume of a substance when its temperature rises.
• Coefficient of linear expansion: The fractional change in length per unit rise in temperature: alpha = change in length divided by original length and temperature change.
• Coefficient of area expansion: The fractional change in area per unit rise in temperature; for an isotropic solid, it is approximately 2 alpha.
• Coefficient of volume expansion: The fractional change in volume per unit rise in temperature; for an isotropic solid, it is approximately 3 alpha.
• Specific heat capacity: The heat required to raise the temperature of unit mass of a substance by one degree Celsius or one kelvin.
• Heat capacity: The heat required to raise the temperature of an entire body by one degree Celsius or one kelvin.
• Calorimetry: The measurement of heat transfer using the principle that heat lost by one body equals heat gained by another in an insulated system.
• Latent heat: The heat absorbed or released during a change of state without a change in temperature.
• Specific latent heat: The heat required to change the state of unit mass of a substance at constant temperature.
• Conduction: Transfer of heat through a substance without bulk movement of its particles, common in solids.
• Convection: Transfer of heat by the actual movement of fluid particles.
• Radiation: Transfer of heat through electromagnetic waves, which does not require a material medium.
• Thermal conductivity: A property indicating how effectively a material conducts heat.
• Newton's law of cooling: For small temperature differences, the rate of cooling is approximately proportional to the temperature difference between a body and its surroundings.
• Anomalous expansion of water: Water contracts when heated from 0 degrees Celsius to 4 degrees Celsius and expands when heated above 4 degrees Celsius; its density is maximum at 4 degrees Celsius.
• Kelvin: The SI unit of temperature, represented by K.
• Joule: The SI unit of heat or energy, represented by J.
• Specific latent heat of fusion: The heat required for the solid-liquid change of unit mass at constant temperature.
• Specific latent heat of vaporisation: The heat required for the liquid-gas change of unit mass at constant temperature.
• Isotropic solid: A solid for which relevant physical properties are the same in all directions.

Easily Confused

• Temperature and heat: Temperature measures the thermal state of a substance, whereas heat is energy transferred because of a temperature difference.
• Specific heat capacity and heat capacity: Specific heat capacity applies to unit mass; heat capacity applies to an entire body.
• Sensible heating and latent heating: Sensible heating changes temperature according to Q = mc delta T, whereas latent heating changes state at constant temperature according to Q = mL.
• Specific latent heat of fusion and specific latent heat of vaporisation: Fusion concerns the solid-liquid change; vaporisation concerns the liquid-gas change.
• Conduction, convection, and radiation: Conduction transfers heat without bulk particle movement, convection involves actual fluid movement, and radiation uses electromagnetic waves and requires no material medium.
• Linear, area, and volume expansion: Linear expansion changes length, area expansion changes area, and volume expansion changes volume.
• Conduction and thermal conductivity: Conduction is a heat-transfer process; thermal conductivity is a material property indicating how effectively the material conducts heat.
• Heat capacity and thermal conductivity: Heat capacity concerns the heat required for a temperature rise, whereas thermal conductivity concerns the rate of heat conduction through a material.
• Ordinary expansion of water and anomalous expansion of water: Water normally expands on heating above 4 degrees Celsius, but anomalously contracts when heated from 0 degrees Celsius to 4 degrees Celsius.
• Radiated power and net radiation exchange: Stefan's law gives the power radiated by an ideal black body, whereas net radiation exchange accounts for radiation exchanged with surroundings at temperature T0.

What Gets Asked

• Definitions and distinctions: Questions may ask for definitions of temperature, heat, thermal equilibrium, specific heat capacity, latent heat, conduction, convection, radiation, or thermal conductivity. Marks are lost by treating heat and temperature as interchangeable.
• Numerical calculations: Questions may require Q = mc delta T, C = Q/delta T = mc, Q = mL, or the thermal expansion relations delta L = alpha L delta T, delta A = beta A delta T, and delta V = gamma V delta T. Marks are lost by using specific heat capacity where heat capacity is required, or by ignoring the phase-change condition.
• Calorimetry problems: Students may be asked to apply heat lost = heat gained in an insulated system. Marks are lost by failing to state or use the assumption that no heat is lost to the surroundings.
• Expansion relations: Questions may test beta is approximately 2 alpha and gamma is approximately 3 alpha for an isotropic solid, or applications to railway tracks, bridges, and overhead wires. Marks are lost by confusing length, area, and volume expansion.
• Heat-transfer mechanisms: Questions may require identification or comparison of conduction, convection, and radiation. Marks are lost by assigning convection to solids, or by stating that radiation requires a material medium.
• Radiation and cooling laws: Questions may use H/t = kA(T1 - T2)/L, P = sigma A T^4, P = sigma A(T^4 - T0^4), lambda_max T = constant, or Newton's law of cooling. Marks are lost by using Celsius rather than absolute temperature in radiation equations, or applying Newton's law of cooling when the temperature difference is not small.
• Anomalous expansion of water: Questions may ask why aquatic life survives in cold regions. Marks are lost by stating that water has minimum density at 4 degrees Celsius instead of maximum density, or by omitting that ice floats and water below can remain near 4 degrees Celsius.