The HERTZ (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second . It is named for Heinrich Rudolf Hertz , 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), and 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 speeds at which computers and other electronics are driven. CONTENTS * 1 Definition * 2 History * 3 Applications * 3.1 Vibration * 3.2 Electromagnetic radiation * 3.3 Computers * 4 SI multiples * 5 See also * 6 Notes and references * 7 External links DEFINITION The hertz is equivalent to cycles per second , i.e., "1/second" or s 1 {displaystyle {text{s}}^{-1}} . The International Committee for Weights and Measures defined the second as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" and then adds the obvious conclusion: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, ν(hfs Cs) = 9 192 631 770 Hz." In English, "hertz" is also used as the plural form. 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 . Whereas 1 Hz is 1 cycle per second , 1 Bq is 1 aperiodic radionuclide event per second. Even though angular velocity , angular frequency and the unit hertz all have the dimension 1/s, angular velocity and angular frequency are not expressed in hertz, but rather in an appropriate angular unit such as radians 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 = 2 f {displaystyle omega =2pi f,} and f = 2 {displaystyle f={frac {omega }{2pi }},} . This SI unit is named after Heinrich Hertz . As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (Hz). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (hertz)—except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case . Note that "degree Celsius " conforms to this rule because the "d" is lowercase.— Based on The International System of Units, section 5.2. HISTORY The hertz is named after the German physicist Heinrich Hertz (1857–1894), who made important scientific contributions to the study of electromagnetism . The name was established by the International Electrotechnical Commission (IEC) in 1930. It was adopted by the General Conference on Weights and Measures (CGPM) (Conférence générale des poids et mesures) in 1960, replacing the previous name for the unit, cycles per second (cps), along with its related multiples, primarily kilocycles per second (kc/s) and megacycles per second (Mc/s), and occasionally kilomegacycles per second (kMc/s). The term cycles per second was largely replaced by hertz by the 1970s. One hobby magazine, Electronics Illustrated, declared their intention to stick with the traditional kc., Mc., etc. units. APPLICATIONS A sine wave with varying frequency A heartbeat is an example of a non-sinusoidal periodic phenomenon that may be analyzed in terms of frequency. Two cycles are illustrated. VIBRATION
Sound
ELECTROMAGNETIC RADIATION Electromagnetic radiation is often described by its frequency—the number of oscillations of the perpendicular electric and magnetic fields per second—expressed in hertz.
Radio
COMPUTERS For more details on why the frequency, including for gigahertz (GHz) etc., is a flawed speed indicator for computers, see Megahertz myth . In computers, most central processing units (CPU) are labeled in
terms of their clock rate expressed in megahertz or gigahertz (106 or
109 hertz, respectively). This number refers to the frequency of the
CPU's master clock signal ("clock rate "). This signal is a square
wave , which is an electrical voltage that switches between low and
high values at regular intervals.
Hertz has become the primary unit of
measurement accepted by the general populace to determine the
performance of a CPU, but many experts have criticized this approach,
which they claim is an easily manipulable benchmark as some processors
use multiple clock periods to perform a single operation, while others
can perform multiple operations in a single cycle. For home-based
personal computers, CPU clock speeds have ranged from approximately 1
MHz in the late 1970s (
Atari
Various computer buses , such as the front-side bus connecting the CPU and northbridge , also operate at various frequencies in the megahertz range. The "speed" or the more correct term bandwidth of networks, including wireless , such as Wi-Fi , 3G , 4G , LTE , etc. is not affected by the myth described above. In general, higher frequency allows for higher possible bandwidth or bit rates, and lowers the time it takes to download large files. In general, however, higher frequencies do not pass through walls, making lower frequencies an advantage indoors. Most common equipment reports the highest frequency available, but usually also works at lower ones. Bandwidth is often equated with "speed", while latency ("lag") is the perceived problem (and is a measure distinct from bandwidth); higher bandwidth has an influence on latency. SI MULTIPLES SI multiples for hertz (Hz) SUBMULTIPLES MULTIPLES VALUE SI SYMBOL NAME VALUE SI SYMBOL NAME 10−1 Hz dHz decihertz 101 Hz daHz decahertz 10−2 Hz cHz centihertz 102 Hz hHz hectohertz 10−3 Hz mHz millihertz 103 Hz KHZ KILOHERTZ 10−6 Hz µHz microhertz 106 Hz MHZ MEGAHERTZ 10−9 Hz nHz nanohertz 109 Hz GHZ GIGAHERTZ 10−12 Hz pHz picohertz 1012 Hz THZ TERAHERTZ 10−15 Hz fHz femtohertz 1015 Hz PHz petahertz 10−18 Hz aHz attohertz 1018 Hz EHz exahertz 10−21 Hz zHz zeptohertz 1021 Hz ZHz zettahertz 10−24 Hz yHz yoctohertz 1024 Hz YHz yottahertz Common prefixed units are in bold face. Higher frequencies than the International System of Units provides prefixes for are believed to occur naturally in the frequencies of the quantum-mechanical vibrations of high-energy, or, equivalently, massive particles, although these are not directly observable and must be inferred from their interactions with other phenomena. By convention, these are typically not expressed in hertz, but in terms of the equivalent quantum energy, which is proportional to the frequency by the factor of Planck\'s constant . SEE ALSO *
Alternating current
*
Bandwidth (signal processing)
*
Electronic tuner
NOTES AND REFERENCES * ^ "hertz". (1992). American Heritage Dictionary of the English
Language (3rd ed.), Boston: Houghton Mifflin.
* ^ "SI brochure: Table 3. Coherent derived units in the SI with
special names and symbols".
* ^ " CIPM, 1964 – Atomic and molecular frequency standards"
(PDF). SI brochure, Appendix 1.
* ^ NIST Guide to SI Units – 9 Rules and Style Conventions for
Spelling Unit Names, National Institute of Standards and Technology
* ^ "(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." "
BIPM
EXTERNAL LINKS * BIPM |