Hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI) and is defined as one cycle per second.[1][2] The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one hertz is the reciprocal of one second.[3] It is named after Heinrich Rudolf Hertz (1857–1894), the first person to provide conclusive proof of the existence of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz (103 Hz, kHz), megahertz (106 Hz, MHz), gigahertz (109 Hz, GHz), terahertz (1012 Hz, THz).
Some of the unit's most common uses are in the description of sine waves and musical tones, particularly those used in radio- and audio-related applications. It is also used to describe the clock speeds at which computers and other electronics are driven. The units are sometimes also used as a representation of the energy of a photon, via the Planck relation E=hν, where E is the photon's energy, ν is its frequency, and the proportionality constant h is Planck's constant.
The hertz is defined as one cycle per second. The International Committee for Weights and Measures defined the second as "the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom"[4][5] and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9192631770 hertz, ν(hfs Cs) = 9192631770 Hz." The dimension of the unit hertz is 1/time (1/T). Expressed in base SI units, the unit is 1/second (1/s). Problems can arise because the unit of angular measure (radian) is sometimes omitted in SI.[6][7][8][9]
In English, "hertz" is also used as the plural form.[10] As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz, 106 Hz), GHz (gigahertz, 109 Hz) and THz (terahertz, 1012 Hz). One hertz simply means "one cycle per second" (typically that which is being counted is a complete cycle); 100 Hz means "one hundred cycles per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz.
The occurrence rate of aperiodic or stochastic events is expressed in reciprocal second or inverse second (1/s or s−1) in general or, in the specific case of radioactive decay, in becquerels.[11] Whereas 1 Hz is one cycle per second, 1 Bq is one aperiodic radionuclide event per second.
Even though angular velocity, angular frequency and the unit hertz all have the dimension 1/T, angular velocity and angular frequency are not expressed in hertz,[12] but rather in an appropriate angular unit such as the radian per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π rad/s or 1 Hz, where the former measures the angular velocity and the latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in hertz and an angular velocity ω measured in radians per second is
