The hertz (symbol: Hz) is a unit of frequency Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. It is defined as the number of cycles Below is a listing of recurring cycles. See also list of wave topics, time, and pattern per second The second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock. It is the basic unit The International System of Units defines seven dimensionally independent SI base units. All other physical units, that are derived from these base units, are known as SI derived units. Additionally, other units, such as the litre, are not SI units but are non-SI units accepted for use with SI of frequency in the International System of Units The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science (SI), and is used worldwide in both general-purpose and scientific contexts. Hertz can be used to measure any periodic event; the most common uses for hertz are to describe radio and audio frequencies, more or less sinusoidal contexts in which case a frequency of 1 Hz is equal to one cycle per second With the organisation of the International System of Units in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second—i.e. the cycle in cycle per second was dropped. Perhaps because of the convenient brevity it brings to both speech and writing, this particular mandate has been so widely adopted as to render the old.

The unit hertz is defined by the International System of Units The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science (SI) such that the hyperfine splitting in the ground state of the caesium Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (83 °F), which makes it one of only five metals that are liquid at or near room temperature. Caesium is most notably used in atomic clocks 133 atom is exactly 9 192 631 770 hertz, ν (hfs Cs) = 9 192 631 770 Hz.^[1] Equivalently, 1 Hz = ¹⁄_{9,192,631,770} ν (hfs Cs). This definition is derived from the SI The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science definition of the second The second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock. Hertz are inverse, s^-1. In practice, the hertz simply replaced the older cycle per second With the organisation of the International System of Units in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second—i.e. the cycle in cycle per second was dropped. Perhaps because of the convenient brevity it brings to both speech and writing, this particular mandate has been so widely adopted as to render the old.

In English, hertz is used as both singular and plural. As any SI unit, Hz can be prefixed An SI prefix is a name or associated symbol that precedes a basic unit of measure (or its symbol) to form a decimal multiple or submultiple. The abbreviation SI is from the French language name Système International d’Unités (also known as International System of Units). SI prefixes are used to reduce the number of zeros shown in numerical; 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 The second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock" (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 Heart Rate is a measure of the number of heart beats per minute (bpm). The average resting human heart rate is about 70 bpm. Heart rate varies significantly between individuals based on fitness, age and genetics. Endurance athletes often have very low resting heart rates. Heart rate can be measured by monitoring one's pulse. Pulse measurement can at 1.2 Hz. Neither the cycle per second nor the hertz, however, are regularly used in nonsinusoidal contexts. The "frequency" (activity) of aperiodic or stochastic events, especially radioactive decay Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. For example: a carbon-14 atom emits radiation and, is expressed in becquerels The becquerel is the SI derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. It is therefore equivalent to s-1. The becquerel is named for Henri Becquerel, who shared a Nobel Prize with Pierre and Marie Curie for their work in discovering radioactivity.

To avoid confusion, periodically varying angles are typically not expressed in hertz, but rather in an appropriate angular unit such as radians The radian is represented by the symbol "rad" or, more rarely, by the superscript c . For example, an angle of 1.2 radians would be written as "1.2 rad" or "1.2c" (the second symbol can be mistaken for a degree: "1.2°"). However, the radian is mathematically considered a "pure number" that needs per second. A disc rotating at 60 revolutions per minute (RPM) can thus be said to be rotating at ≈6.283 rad/s or 1 Hz, where the latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in hertz and an angular frequency ω measured in radians/s are:

ω = 2πf and .

This SI The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science unit is named after Heinrich Hertz Heinrich Rudolf Hertz was a German physicist who clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol A symbol is something such as an object, picture, written word, sound, or particular mark that represents something else by association, resemblance, or convention. For example, a red octagon may stand for "STOP". On maps, crossed sabres may indicate a battlefield. Numerals are symbols for numbers is uppercase Capital letters or majuscules [IPA pronunciation: /məˈdʒʌskjuls, ˈmædʒəˌskjuls/], in the Roman alphabet A, B, C, D, etc., may also be called capitals, or caps. Upper case, upper-case, or uppercase is also often used in this context as synonym of capital. Manual typesetters kept them in the upper drawers of a desk or in the upper type case, (Hz). When an SI unit is spelled out in English, it should always begin with a lowercase Lower case , minuscule, or small letters are the smaller form of letters, as opposed to upper case or capital letters, as used in European alphabets (Greek, Latin, Cyrillic, and Armenian). For example, the letter "a" is lower case while the letter "A" is upper case letter (hertz), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as serve as a unit increment to indicate a temperature interval (a difference between" 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 Germany /ˈdʒɜrməni/ , officially the Federal Republic of Germany (German: Bundesrepublik Deutschland (help·info), IPA: [ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant]), is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and physicist Heinrich Hertz Heinrich Rudolf Hertz was a German physicist who clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves, who made important scientific contributions to electromagnetism Electromagnetism is the physics of the electromagnetic field, a field which exerts a force on particles with the property of electric charge and which is reciprocally affected by the presence and motion of such particles. The name was established by the International Electrotechnical Commission The International Electrotechnical Commission is a not-for-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology". IEC standards cover a vast range of technologies from power (IEC) in 1930.^[2] It was adopted by the General Conference on Weights and Measures The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention) of 1875. It meets in Sèvres (in the southwestern suburbs of Paris) (CGPM) (Conférence générale des poids et mesures) in 1960, replacing the previous name for the unit, cycles per second With the organisation of the International System of Units in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second—i.e. the cycle in cycle per second was dropped. Perhaps because of the convenient brevity it brings to both speech and writing, this particular mandate has been so widely adopted as to render the old (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.

The term "gigahertz", most commonly used in computer processor speed and radio frequency Radio frequency is a frequency or rate of oscillation within the range of about 3 Hz to 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations (RF) applications, can be pronounced either /ˈgɪgaˌhɝts/, with a hard /g/ sound, or /ˈʒɪgaˌhɝts/ or /ˈdʒɪgaˌhɝts/, with a soft /ʒ/ or /dʒ/. The prefix "giga-" is derived directly from the Greek Greek is an Indo-European language native to the southern Balkan peninsula, the language of the Greeks. It forms an independent branch within Indo-European. It has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical Ancient Greek literature and "γιγας" and hence the preferred pronunciation is /ˈgɪga/.

Applications

Vibration

Sound For humans, hearing is normally limited to frequencies between about 12 Hz and 20,000 Hz , although these limits are not definite. The upper limit generally decreases with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. As a signal perceived by one of the major senses, sound is is a traveling wave which is an oscillation of pressure Pressure is an effect which occurs when a force is applied on a surface. The symbol of pressure is p .[citation needed]. Humans perceive frequency of sound waves as pitch Pitch represents the perceived fundamental frequency of a sound. It is one of the three major auditory attributes of sounds along with loudness and timbre. When the actual fundamental frequency can be precisely determined through physical measurement, it may differ from the perceived pitch because of overtones, also known as partials, harmonic or. Each musical note In music, the term note has two primary meanings: 1) a sign used in musical notation to represent the relative duration and pitch of a sound; and 2) a pitched sound itself. Notes are the "atoms" of much Western music: discretizations of musical phenomena that facilitate performance, comprehension, and analysis corresponds to a particular frequency which can be measured in hertz. An infant's ear is able to perceive frequencies ranging from 16 Hz to 20,000 Hz; the average human can hear sounds between 20 Hz and 16,000 Hz.^[3] The range of ultrasound Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound. The production of ultrasound is used in many different fields,, infrasound Infrasound is sound that is lower in frequency than 20 cycles per second, the normal limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher levels it is possible to and other physical vibrations such as molecular vibrations extends into the megahertz range and well beyond.

Electromagnetic radiation

Electromagnetic radiation Electromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified into several types is often described by its frequency—the number of oscillations Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes is used to be synonymous with "oscillation.& of the perpendicular electric and magnetic fields per second—expressed in hertz.

Radio frequency radiation is usually measured in kilohertz, megahertz, or gigahertz; this is why radio dials are commonly labeled with kHz, MHz, and GHz. Light Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye , or perhaps 380–750 nm. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens (infrared Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light (400-700 nm), but shorter than that of terahertz radiation (100 µm - 1 mm) and microwaves (~30,000 µm). Infrared radiation spans roughly three orders of magnitude (750 nm and 100 µm)) to thousands (ultraviolet Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet) of terahertz. Electromagnetic radiation with frequencies in the low terahertz range, (intermediate between those of the highest normally-usable radio frequencies and long-wave infrared light), is often called terahertz radiation In physics, terahertz radiation refers to electromagnetic waves sent at frequencies in the terahertz range. It is also referred to as submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz. The term is normally used for the region of the electromagnetic spectrum between 300 gigahertz and 3 terahertz (3x1012 Hz),. Even higher frequencies exist, such as that of gamma rays Gamma rays are electromagnetic radiation of high energy. They are produced by sub-atomic particle interactions, such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz and therefore energies above 100, which can be measured in exahertz. (For historical reasons, the frequencies of light and higher frequency electromagnetic radiation are more commonly specified in terms of their wavelengths In physics, wavelength is the distance between repeating units of a waveform. It is commonly designated by the Greek letter lambda . Examples of wave-like phenomena are light, water waves, and sound waves. Waves may have arbitrary shapes, but recur periodically in time or space or both. A wave that does not move in space but oscillates in time is or photon In physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. The effects of this force are easily observable at both the microscopic and macroscopic level, because the energies In physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law. Eight different forms of energy exist to explain all known natural phenomena. These forms include (but are not limited to) kinetic, potential, thermal,: for a more detailed treatment of this and the above frequency ranges, see electromagnetic spectrum The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.)

Computing

In computing, most central processing units A central processing unit or processor is an electronic circuit that can execute computer programs. This topic has been in use in the computer industry at least since the early 1960s (Weik 1961). The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation has remained much the (CPU) are labeled in terms of their clock speed expressed in megahertz or gigahertz (10⁹ hertz). This number refers to the frequency of the CPU's master clock signal In electronics and especially synchronous digital circuits, a clock signal is a signal used to coordinate the actions of two or more circuits ("clock speed"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals. Hertz has become the primary unit of measurement accepted by the general populace to determine the speed of a CPU, but many experts have criticized this approach, which they claim is an easily manipulable benchmark.^[4] For home-based personal computers, the CPU has ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to nearly 4 GHz in the present. This can be increased even further by increasing the frequency of the CPU (overclocking) in the BIOS or other software. (Likewise, speed can also be decreased, or underclocked.)

Various computer buses, such as the front-side bus connecting the CPU and northbridge, also operate at different frequencies in the megahertz range (for modern products).

SI multiples

Frequencies not expressed in hertz

Even higher frequencies are believed to occur naturally, in the frequencies of the quantum-mechanical wave functions of high-energy (or, equivalently, massive) particles, although these are not directly observable, and must be inferred from their interactions with other phenomena. For practical reasons, 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

References

1. ^ BIPM definition
2. ^ IEC History
3. ^ Dominant spectral region
4. ^ Good Riddance, Gigahertz

External links

• BIPM Cesium ion f_Cs definition
• National Research Council of Canada: Generation of the Hz
• National Research Council of Canada: Cesium fountain clock
• National Physical Laboratory: Trapped ion optical frequency standards
• National Research Council of Canada: Optical frequency standard based on a single trapped ion
• National Research Council of Canada: Optical frequency comb
• One Hertz in Radians per Second (Google). Note, as of 06 May 2009 there is an error of 2π.

Categories: SI derived units | Units of frequency

See Also:

• Hertz
• 2600 Hertz

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