Heat

Definition and Units of Heat

  • Heat is the transfer of thermal energy between systems due to a temperature difference.
  • It refers to the kinetic energy of vibrating and colliding atoms in a substance.
  • Heat is not the same as internal energy.
  • Thermal energy is stored in the microscopic degrees of freedom of the modes of vibration.
  • The unit of measurement for heat is the joule (J) in the International System of Units (SI).
  • Other traditional units used in engineering include the British thermal unit (BTU) and the calorie.
  • The rate of heating is measured in watts (W).
  • The symbol 'Q' is used to represent heat in equations.
  • Heat flux is defined as the rate of heat transfer per unit cross-sectional area.

Heat and Thermodynamics Laws

  • Heat and entropy are closely related in thermodynamics.
  • The second law of thermodynamics states that heat transfer increases the entropy of a system.
  • In a transfer of energy as heat, there are changes of entropy in both the surroundings and the system.
  • The total change of entropy in the system and surroundings is equal to the uncompensated change of entropy.
  • For a closed system, the change in internal energy is equal to the amount of heat supplied.
  • Enthalpy is a thermodynamic quantity that includes both the internal energy and the pressure-volume work done by a system.
  • The change in enthalpy of a system is equal to the heat transferred at constant pressure.
  • The first law of thermodynamics relates the change in internal energy, heat, and work done by a system.
  • The second law of thermodynamics states that heat cannot spontaneously flow from a colder object to a hotter object.
  • Reversible processes are idealized processes that can be analyzed mathematically.
  • Irreversible processes involve entropy production and are more common in real-world situations.
  • The second law of thermodynamics sets limits on the efficiency of heat engines and refrigerators.

Work and Thermodynamic State Variables

  • Work is the amount of energy transferred to or from a system.
  • The sign convention for work is that work done by the system on its surroundings is negative, while work done on the system by its surroundings is positive.
  • The thermodynamic work done by the system is defined by its state variables, such as volume.
  • It does not necessarily involve mechanisms in the surroundings, such as shaft work.
  • Isochoric work is an example of work that involves variables in the surroundings.
  • The internal energy of a system is a state function.
  • In cyclical processes, state functions return to their initial values upon completion of a cycle.
  • The differential increment for internal energy in an infinitesimal process is an exact differential.
  • In contrast, infinitesimal increments of heat and work are inexact differentials.
  • The integral of any inexact differential in a process where the system returns to the same state does not necessarily equal zero.

Heat Transfer and Laws of Thermodynamics

  • Heat transfer can occur through conduction, convection, and radiation.
  • The second law of thermodynamics states that heat transfer increases the entropy of a system.
  • The second law of thermodynamics states that heat cannot spontaneously flow from a colder object to a hotter object.
  • Reversible processes are idealized processes that can be analyzed mathematically.
  • Irreversible processes involve entropy production and are more common in real-world situations.
  • The second law of thermodynamics sets limits on the efficiency of heat engines and refrigerators.

Perspectives and Terminology of Heat

  • James Clerk Maxwell outlined four stipulations for the definition of heat.
  • Heat can be transferred from one body to another according to the second law of thermodynamics.
  • Heat is a measurable quantity and can be treated mathematically.
  • Heat cannot be treated as a material substance as it can be transformed into non-material substances, such as mechanical work.
  • Heat is one form of energy.
  • The use of 'heat' as an abbreviation for the quantity of energy transferred led to confusion.
  • In classical thermodynamics, heat refers to thermal energy.
  • Enthalpy is recommended for referring to heat content at constant volume.
  • Different perspectives on heat include the jiggling motion of atoms and molecules and the logical structure of thermodynamics.
  • Calorimetry is used to measure the quantity of heat.

Heat Mentions

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Heat Data Sources

Reference URL
Glossary https://www.alternix.com/blogs/glossary-of-terms/heat
Wikipedia http://en.wikipedia.org/wiki/Heat
Wikidata https://www.wikidata.org/wiki/Q44432
Knowledge Graph https://www.google.com/search?kgmid=/m/03k2v
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