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Hertz

The '''hertz''' (symbol: '''Hz''') is the International System of Units|SI unit of frequency defined as the number of cycle per second|cycles per second of a periodic phenomenon."hertz". (1992). ''American Heritage Dictionary of the English Language'', 3rded. Boston: Houghton Mifflin. One of its most common uses is the description of sinusoidal waves, particularly those used in radio and audio applications.

Definition

The hertz is equivalent to cycle per second|cycles per second. In defining the second the Comité International des Poids et Mesures|CIPM declared that "''the standard to be employed is the transition between the hyperfine levels F = 4, M = 0 and F = 3, M = 0 of the ground state 2S_1/2 of the caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9 192 631 770 hertz''" thereby effectively defining the hertz and the second simultaneously. In English, hertz is used as both singular and plural. As an SI unit, Hz can be SI prefix|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 heart rate|beat at 1.2 Hz. The "frequency" (activity) of aperiodic or stochastic events, such as radioactive decay, is expressed in becquerels. Even though angular velocity, angular frequency and hertz all have the dimensions of ''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 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 are:
-

\omega = 2\pi f

and

f = \omega/(2\pi) \,

History

The hertz is named after the Germany|German physicist Heinrich Hertz, who made important scientific contributions to electromagnetism. The name was established by the International Electrotechnical Commission (IEC) in 1930.IEC History 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, ''cycle per second|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. The term "gigahertz", most commonly used in computer processor clock rates and radio frequency (RF) applications, can be pronounced either , with a hard sound, or , with a soft . The prefix "giga-" is derived directly from the Greek language|Greek "."

Applications

Vibration

Sound is a traveling wave which is an oscillation of pressure. Humans perceive frequency of sound waves as Pitch (music)|pitch. Each musical note corresponds to a particular frequency which can be measured in hertz. An infant's ear is able to perceive frequencies ranging from 20 Hz to 20,000 Hz; the average human can hear sounds between 20 Hz and 16,000 Hz.Dominant spectral region The range of ultrasound, infrasound and other physical vibrations such as molecular vibrations extends into the megahertz range and well beyond.

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 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. 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. Even higher frequencies exist, such as that of gamma rays, 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 or photon Energy|energies: for a more detailed treatment of this and the above frequency ranges, see electromagnetic spectrum.)

Computing

In computing, most central processing units (CPU) are labeled in terms of their clock rate expressed in megahertz or gigahertz (10⁹ hertz). This number refers to the frequency of the CPU's master clock signal ("Clock rate"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals. This signal is also referred to as a square wave. 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.Good Riddance, Gigahertz For home-based personal computers, the CPU has ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to up to 6 GHz in the present (IBM POWER processors). Various Bus (computing)|computer buses, such as the front-side bus connecting the CPU and northbridge (computing)|northbridge, also operate at different frequencies in the megahertz range (for modern products). Cathode ray tube|CRT television and monitor Refresh rate|refresh rates are measured in hertz.

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 constant|Planck's constant.