Hertz

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The hertz (symbol: Hz) is the SI unit of frequency. 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).

One hertz simply means one per second (typically that which is being counted is a complete cycle); 100 Hz means one hundred 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 frequencies of aperiodic events, such as radioactive decay, are expressed in becquerels.

To avoid confusion, periodically varying angles are typically not expressed in hertz, but rather in an appropriate angular unit such as radians per second. A disc rotating at 1 revolution per minute (RPM) can thus be said to be rotating at 0.105 rad/s or 0.017 Hz, where the latter reflects the number of complete revolutions per second. The conversion between Hz and rad/s is rad/s = 1 Hz × 2 .

[edit] History

The hertz is named after the German physicist Heinrich Rudolf Hertz, who made important scientific contributions to electromagnetism. The name was established by the IEC in 1930.^[1] It was adopted by the 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 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. The prefix “giga-” is derived directly from the Greek “γιγας” and hence the preferred pronunciation is /ˈgɪga/. Some electrical engineers use /ˈdʒɪga/, by analogy with “gigantic”.

[edit] Applications

[edit] Vibration

Sound is a traveling wave which is an oscillation of pressure. Humans perceive frequency of sound waves as pitch. Each musical note corresponds to a particular frequency which can be measured in hertz. Although the human ear is able to perceive frequencies ranging from 16 Hz to 20,000 Hz at the age of a baby, the average human can hear sounds between 20 Hz and 15,000 Hz. ^{[citation needed]} The range of ultrasound and other physical vibrations such as molecular vibrations extends into the megahertz and well beyond.

[edit] 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 (rarely) 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 energies: for a more detailed treatment of this and the above frequency ranges, see electromagnetic spectrum.)

[edit] Computing

In computing, most CPUs 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"). 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.^[2] For home-based personal computers, the CPU has ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to nearly 10 gigahertz^[3] for modern-day Macintosh and PC machines.

Various computer buses, such as memory buses connecting the CPU and system RAM, also transfer data using clock signals operating at different frequencies in the megahertz ranges (for modern products).

[edit] 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 energy.