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Showing 61–90 of 1709 reference entries

Physics · MeasurementElectric current

Current measures charge crossing a chosen surface per unit time.

DefinitionPhysicsElectric field and force

An electric field assigns force per unit test charge.

DefinitionPhysicsMagnetic force on a moving charge

Magnetic force depends on charge motion relative to the field.

ModelPhysicsOhm's law and its domain

A linear current-voltage relation requires suitable conditions.

Empirical findingPhysicsFirst law of thermodynamics

Keep the heat and work sign convention explicit in an energy balance.

ModelPhysicsSecond law: isolated-system entropy

Apply entropy increase to the whole isolated system.

ModelPhysics · ChemistryIdeal-gas equation of state

A simple gas model relates pressure, amount, volume and temperature.

ModelPhysicsPhoton energy and frequency

One photon's energy is proportional to its frequency.

ModelPhysics · Mathematics & logicInvariant spacetime interval

Lorentz transformations preserve a combined space-time separation.

ModelPhysicsNewton's third law

Interaction forces act on different bodies in equal and opposite pairs.

ModelPhysicsAngular momentum and torque

Torque changes angular momentum about a specified origin.

ModelPhysicsHydrostatic pressure

Static fluid pressure rises with depth under gravity.

ModelPhysicsArchimedes' buoyancy principle

The hydrostatic upward force equals displaced-fluid weight.

ModelPhysicsMaxwell equations in free space

Four field equations connect electric and magnetic fields with charge and current.

ModelPhysicsSchrödinger time evolution

A quantum wavefunction evolves according to its Hamiltonian.

ModelPhysicsBorn position probability

A normalized wavefunction assigns probabilities to position intervals.

ModelPhysics · MeasurementElementary charge

The elementary charge has an exact SI numerical value.

DefinitionPhysics · MeasurementBoltzmann constant

The Boltzmann constant has an exact SI numerical value.

DefinitionMeasurement · Chemistry · PhysicsAvogadro constant

The mole is linked to a specified number of entities.

DefinitionMeasurement · PhysicsSI defining luminous efficacy

A specified frequency fixes the photometric SI reference.

DefinitionChemistryAtoms and elemental identity

The proton count of an atomic nucleus identifies its chemical element.

DefinitionChemistryIsotopes and mass number

Isotopes share a proton count but have different neutron counts.

DefinitionChemistryPeriodic organization of the elements

Atomic-number order reveals recurring patterns in electron structure and chemical properties.

ModelChemistryChemical bonds

Bonding describes interactions that bind atoms into molecules or extended structures.

DefinitionChemistry · MeasurementMole and amount of substance

The mole counts specified entities using the exact Avogadro constant.

DefinitionChemistryReaction stoichiometry

Balanced coefficients relate the amounts consumed and produced by a reaction.

MethodChemistryBalancing atoms and electric charge

A chemical equation must preserve each element's atom count and the total charge.

MethodChemistryDynamic chemical equilibrium

At equilibrium, opposing reactions continue without net composition change.

ModelChemistryBrønsted acids and bases

In the Brønsted description, an acid donates a proton and a base accepts it.

DefinitionChemistryGibbs energy and thermodynamic direction

Gibbs-energy change distinguishes thermodynamic favourability from reaction speed.

Model
DefinitionSource checked

SI defining luminous efficacy

MeasurementPhysics

A specified frequency fixes the photometric SI reference.

Key knowledge

The luminous efficacy K_cd of monochromatic radiation at frequency 540 × 10¹² Hz is exactly 683 lm W⁻¹ in the SI definition.

Reference values

K_cd683lm W⁻¹

Exact value · SI definition

Values and equations

K_cd = 683 lm W⁻¹ at f = 540 × 10¹² Hz

K_cd
Defining luminous efficacy at the specified frequency lm W⁻¹
f
Specified radiation frequency Hz

Conditions and scope

  • The exact value refers to the specified monochromatic frequency and photometric convention.
  • It is not the wall-plug efficiency of a lamp or a universal conversion at every wavelength.
Sources · 5

References

Sources retain their recorded scope. A source link alone does not establish verification of every statement.

  1. Bureau International des Poids et Mesures (BIPM)3 sources
    1. SI defining constantsBureau International des Poids et Mesures (BIPM)Reference guidebipm.orgRead source
    2. The International System of Units, ninth edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guidebipm.orgRead source
    3. The International System of Units, 9th edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guidebipm.orgRead source
  2. Meet the ConstantsNational Institute of Standards and Technology (NIST)Reference guidenist.govRead source
  3. 国際単位系（SI）第9版 · Japanese translation, V3.01National Metrology Institute of Japan, AISTReference guide · PDFunit.aist.go.jpRead source

Evidence and review

SI Brochure 2.2 and candela definition 2.3.1, specifying frequency and luminous-flux/radiant-power relation.

2026-10-10 · Exact defining value, frequency and unit only; practical photometric realization remains outside scope.

Foundational review

2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.

Sources from other places

Agreement within this scope

Kcd is exactly 683 lm/W for monochromatic radiation at 540×10^12 Hz.

The defining frequency matters; this is not a lamp’s electrical efficiency or an unqualified spectral maximum.

Related reference entries

Explore relationships within this area of study and its connections to other areas.

Showing 1–1 of 1 relationships

Scientific relationships · 1 / 1

The relationship type describes the connection. Its linked statements, sources and conditions define the scope.

  1. Related to
    Relationship sources · 3

    Linked statements

    • Photopic luminous flux weights spectral radiant flux by the standard V(λ) response. Luminous intensity is luminous flux per solid angle; illuminance is incident luminous flux per receiving area; luminance resolves flux per projected area and direction. Their units are lumen, candela, lux and cd/m² respectively. These quantities are not physical radiant power or a direct measurement of an individual observer’s subjective brightness.

    The exact SI defining K_cd at 540 THz is distinct from the spectrum-weighted quantities and normalized K_m.

Entry detailsPermanent link to this entry ↗
Record identity
si-luminous-efficacy
Revision
core.1
Structured reference record
SI