Heat and Thermodynamics

The One Thing

Thermodynamics describes how temperature differences drive heat transfer and how heat and work change a system’s internal energy. Its laws also determine the direction of natural processes and place limits on the efficiency of thermal devices.

Reactions, Processes and Experiments

Key Terms

• Thermal equilibrium: A condition in which two systems have the same temperature and no net heat flows between them.
• Temperature: A measure related to the average kinetic energy of particles in a system.
• Heat: Energy transferred from a body at higher temperature to a body at lower temperature because of a temperature difference.
• Zeroth law of thermodynamics: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
• 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.
• Specific heat capacity: The heat required to raise the temperature of unit mass of a substance by one kelvin.
• Heat capacity: The heat required to raise the temperature of an entire body by one kelvin.
• Calorimetry: The measurement of heat exchanged during physical or chemical processes using the principle of conservation of energy.
• Latent heat: Heat absorbed or released during a change of state without a change in temperature.
• Conduction: Transfer of heat through a material without bulk movement of the material, mainly by particle collisions and free electrons.
• Convection: Transfer of heat by the bulk movement of a fluid caused by density differences.
• Radiation: Transfer of heat through electromagnetic waves, which does not require a material medium.
• Internal energy: The total microscopic kinetic and potential energy of the particles of a system.
• Thermodynamic system: A chosen quantity of matter or region of space studied in thermodynamics.
• Thermodynamic process: A change from one equilibrium state to another, such as an isothermal, adiabatic, isobaric, or isochoric process.
• Work done by a gas: Energy transferred when a gas expands or contracts against an external pressure; for constant pressure, W = P change in volume.
• First law of thermodynamics: The change in internal energy equals heat supplied to the system minus work done by the system: change in U = Q - W.
• Second law of thermodynamics: Heat naturally flows from hot to cold, and no heat engine can convert all absorbed heat into useful work.
• Entropy: A thermodynamic quantity that measures energy dispersal or the number of possible microscopic arrangements; for an isolated system, entropy does not decrease.
• Heat engine: A device that absorbs heat from a high-temperature source, performs work, and rejects some heat to a low-temperature sink.
• Refrigerator: A device that uses external work to transfer heat from a low-temperature region to a high-temperature region.
• Coefficient of performance: For a refrigerator, COP equals heat removed from the cold region divided by work input.
• Thermal conductivity: The property represented by k in the rate of heat conduction through a uniform slab.
• Isothermal process: A thermodynamic process at constant temperature.
• Adiabatic process: A thermodynamic process in which no heat is exchanged.
• Isochoric process: A thermodynamic process at constant volume.
• Isobaric process: A thermodynamic process at constant pressure.
• Ideal gas: A gas described by PV = nRT, whose internal energy depends only on temperature, not directly on pressure or volume.
• Black body: The body referred to in Stefan's law as emitting total radiant power proportional to the fourth power of absolute temperature.
• Carnot engine: A reversible heat engine whose efficiency is determined by the temperatures of its hot source and cold sink.
• Absolute temperature: Temperature measured in kelvin, required in gas laws, efficiency relations, and radiation laws.

Easily Confused

• Temperature and heat: Temperature describes the thermal state or average particle kinetic energy, 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 the entire body.
• Sensible heating and latent heating: Sensible heating changes temperature through Q = mc change in T; latent heating changes state at constant temperature through Q = mL.
• Conduction, convection and radiation: Conduction transfers heat through a material without bulk movement, convection uses bulk fluid movement, and radiation uses electromagnetic waves without requiring a medium.
• Heat engine and refrigerator: A heat engine uses heat transfer to produce work, whereas a refrigerator uses external work to transfer heat from a cold region to a hot region.
• Isothermal and adiabatic processes: An isothermal process has constant temperature and, for an ideal gas, Q = W; an adiabatic process has Q = 0.
• Isochoric and isobaric processes: An isochoric process has constant volume and zero work; an isobaric process has constant pressure and W = P(V2 - V1).
• First and second laws of thermodynamics: The first law concerns energy conservation, while the second law concerns the direction of heat flow, entropy, and the impossibility of complete heat-to-work conversion.
• Heat capacity and latent heat: Heat capacity concerns raising the temperature of a body by one kelvin; latent heat concerns a change of state without a temperature change.
• Heat engine efficiency and refrigerator coefficient of performance: Efficiency compares work output with heat absorbed from the source; COP compares heat removed from the cold region with work input.

What Gets Asked

• Definitions and distinctions: Questions may ask for definitions of temperature, heat, thermal equilibrium, internal energy, entropy, or the three mechanisms of heat transfer. Marks are lost by treating heat as a stored property rather than energy transferred because of a temperature difference.
• Numerical heat calculations: Problems may require Q = mc change in T, C = Q divided by change in T = mc, or Q = mL. Marks are lost by confusing specific heat capacity with heat capacity or by applying the sensible-heating equation during a change of state.
• Calorimetry: Questions may use the principle that heat lost by hot bodies equals heat gained by cold bodies in an insulated calorimeter.