Editing Hertz (section)

== Definition ==
The hertz is defined as one per second for periodic events. The [[International Committee for Weights and Measures]] defined the second as "the duration of {{val|9192631770}} periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the [[caesium]]-133 atom"<ref>{{Cite web |url=https://www.bipm.org/en/publications/si-brochure/ |title=SI Brochure: The International System of Units (SI) § 2.3.1 Base units |year=2019 |publisher=[[International Bureau of Weights and Measures|BIPM]] |page=130 |language=en-gb, fr |edition=9th |format=PDF |access-date=2 February 2021}}</ref><ref>{{Cite web |url=https://www.bipm.org/en/publications/si-brochure/ |title=SI Brochure: The International System of Units (SI) § Appendix 1. Decisions of the CGPM and the CIPM |year=2019 |publisher=[[International Bureau of Weights and Measures|BIPM]] |page=169 |language=en-gb, fr |edition=9th |format=PDF |access-date=2 February 2021}}</ref> and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly {{val|9192631770|u=hertz}}, {{nowrap|1=''ν''<sub>hfs Cs</sub> = {{val|9192631770|u=Hz}}}}." The dimension of the unit hertz is 1/time (T<sup>−1</sup>). Expressed in base SI units, the unit is the reciprocal second (1/s).

In English, "hertz" is also used as the plural form.<ref>[http://physics.nist.gov/Pubs/SP811/sec09.html NIST Guide to SI Units – 9 Rules and Style Conventions for Spelling Unit Names], National Institute of Standards and Technology</ref> As an SI unit, Hz can be [[Metric prefix|prefixed]]; commonly used multiples are kHz (kilohertz, {{val|e=3|u=Hz}}), MHz (megahertz, {{val|e=6|u=Hz}}), GHz (gigahertz, {{val|e=9|u=Hz}}) and THz (terahertz, {{val|e=12|u=Hz}}). One hertz (i.e. one per second) simply means "one periodic event occurs per second" (where the event being counted may be a complete cycle); {{val|100|u=Hz}} means "one hundred periodic events occur per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at {{val|1|u=Hz}}, or a human heart might be said to [[heart rate|beat]] at {{val|1.2|u=Hz}}.

The occurrence [[aperiodic frequency|rate of aperiodic]] or [[stochastic]] events is expressed in ''[[reciprocal second]]'' or ''inverse second'' (1/s or s<sup>−1</sup>) in general or, in the specific case of [[radioactivity]], in [[becquerel]]s.{{efn|"(d) The hertz is used only for periodic phenomena, and the becquerel (Bq) is used only for stochastic processes in activity referred to a radionuclide."<ref name="BIPMtable3">{{cite web |url=http://www.bipm.org/en/publications/si-brochure/section2-2.html#section2-2-2 |title=BIPM – Table 3 |publisher=[[BIPM]] | access-date=2012-10-24}}</ref>}} Whereas {{val|1|u=Hz}} (one per second) specifically refers to one cycle (or periodic event) per second, {{val|1|u=Bq}} (also one per second) specifically refers to one radionuclide event per second on average.

Even though frequency, [[angular velocity]], [[angular frequency]] and radioactivity all have the dimension T<sup>−1</sup>, of these only frequency is expressed using the unit hertz.<ref>{{Cite web |url=http://www.bipm.org/en/si/derived_units/2-2-2.html |title=SI brochure, Section 2.2.2, paragraph 6 |url-status=dead |archive-url=https://web.archive.org/web/20091001192328/http://www.bipm.org/en/si/derived_units/2-2-2.html |archive-date=1 October 2009}}</ref> Thus a disc rotating at 60 revolutions per minute (rpm) is said to have an angular velocity of 2{{pi}}&nbsp;rad/s and a [[rotational frequency|frequency of rotation]] of {{val|1|u=Hz}}. The correspondence between a frequency ''f'' with the unit hertz and an angular velocity ''ω'' with the unit [[radians]] per second is
:<math>\omega = 2\pi f</math> and <math>f = \frac{\omega}{2\pi} .</math>

{{SI unit lowercase|Heinrich Hertz|hertz|Hz}}
{{Clear}}