The hertz (symbol: Hz) is the International System of Units (SI) base unit of frequency. The International System of Units (SI defines seven dimensionally independent SI base units. Frequency is a measure of the number of occurrences of a repeating event per unit Time. The definition of the hertz is based upon that for the second, namely: 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. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units ^[1]

Its base unit is cycle/s or s^-1 (also called inverse seconds, reciprocal seconds). In English, hertz is used as both singular and plural. As any 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). An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or

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 second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units 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. Measuring heart rate The Pulse rate (which in most people is identical to the heart rate can be measured at any point on the body where an Artery 's pulsation 2 Hz. The frequencies of aperiodic events, such as radioactive decay, are expressed in becquerels. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. The becquerel (symbol Bq) is the SI derived unit of radioactivity.

To avoid confusion, periodically varying angles are typically not expressed in hertz, but rather in an appropriate angular unit such as radians per second. The radian is a unit of plane Angle, equal to 180/ π degrees, or about 57 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 .

History

The hertz is named after the German physicist Heinrich Hertz, who made important scientific contributions to electromagnetism. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Heinrich Rudolf Hertz ( February 22, 1857 – January 1, 1894) was a German physicist who clarified and expanded the electromagnetic theory Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of The name was established by the International Electrotechnical Commission (IEC) in 1930. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes ^[2] 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). The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM The cycle per second was a once-common unit of Frequency. With the organisation of the International System of Units (abbreviated SI from the French in 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 (RF) applications, can be pronounced either /ˈgigaˌhɝts/, with a hard /g/ sound or /ˈʒɪgaˌhɝts/ or /ˈdʒɪgaˌhɝts/, with a soft /ʒ/ sound at the beginning of the word. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz The prefix "giga-" is derived directly from the Greek "γιγας" and hence the preferred pronunciation is /ˈgɪga/. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Some electrical engineers use /ˈdʒɪga/, by analogy with "gigantic". Electrical engineering, sometimes referred to as electrical and electronic engineering, is a field of Engineering that deals with the study and application of

Applications

Vibration

Sound is a traveling wave which is an oscillation of pressure. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Humans perceive frequency of sound waves as pitch. Pitch represents the perceived Fundamental frequency of a sound Each musical note corresponds to a particular frequency which can be measured in hertz. 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; 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, infrasound and other physical vibrations such as molecular vibrations extends into the megahertz range and well beyond. Not to be confused with Supersonic. Ultrasound is cyclic Sound pressure with a Frequency greater than the upper Infrasound is Sound with a Frequency too low to be heard by the human Ear. A molecular vibration occurs when Atoms in a Molecule are in periodic motion while the molecule as a whole has constant translational

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. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states

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 is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens (infrared) to thousands (ultraviolet) of terahertz. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays 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. Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, submillimeter radiation, terahertz waves, terahertz Even higher frequencies exist, such as that of gamma rays, which can be measured in exahertz. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions (For historical reasons, the frequencies of light and higher frequency electromagnetic radiation are more commonly specified in terms of their wavelengths or photon energies: for a more detailed treatment of this and the above frequency ranges, see electromagnetic spectrum. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies )

Computing

In computing, most central processing units (CPU) are labeled in terms of their clock speed expressed in megahertz or gigahertz (10⁹ hertz). The number of megahertz refers to the frequency of the CPU's master clock signal ("clock speed"). In Electronics and especially synchronous Digital circuits a clock signal is a signal used to coordinate the actions of two or more circuits The clock rate is the fundamental rate in cycles per second (measured in Hertz) at which a Computer performs its most basic operations such as adding two 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 used 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 in the BIOS or other software.

Various computer buses, such as memory buses connecting the CPU and system random access memory (RAM), also transfer data using clock signals operating at different frequencies in the megahertz ranges (for modern products). In Computer architecture, a bus is a subsystem that transfers data between computer components inside a Computer or between computers In Personal computers the Front Side Bus ( FSB) is the bus that carries data between the CPU and the northbridge.

Order of magnitude

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. A wave function or wavefunction is a mathematical tool used in Quantum mechanics to describe any physical system For practical reasons, these are typically not expressed in hertz, but in terms of the equivalent energy.

References

See also

• Frequency changer
• Orders of magnitude (frequency)
• Signal bandwidth
• Radian per second

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