# Hertz (unit)

Physical unit
Unit name hertz
Unit symbol ${\ displaystyle \ mathrm {Hz}}$
Physical quantity (s) frequency
Formula symbol ${\ displaystyle f, \ nu}$
dimension ${\ displaystyle {\ mathsf {T ^ {- 1}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, Hz = 1 \, {\ frac {1} {s}}}}$
Named after Heinrich Hertz
Derived from second

The Hertz (with the unit symbol Hz) is the derived SI unit for frequency . It indicates the number of repetitive processes per second in a periodic signal . The unit was named in 1930 after the German physicist Heinrich Hertz .

## history

The unit was proposed in 1930 by the "technical committee for electrical and magnetic quantities and units" of the International Electrotechnical Commission and introduced in 1935 as part of the " Giorgi unit system" or MKS unit system, named after Heinrich Hertz. At the 11th  CGPM (Conference Générale des Poids et Mesures) in 1960, the MKS system of units was merged into the SI system of units . Since then, the unit Hertz has replaced the unit cycles per second = cps or cps or c / s (cycles per second), which used to be common in English-language literature .

## use

Flashes of light with the frequency , and . x  Hz means that the light flashes x times per second. In addition, the period is specified, which is defined as the reciprocal of the frequency and indicates the time between two light flashes (in seconds).${\ displaystyle f = 0 {,} 5 \, {\ text {Hz}}}$${\ displaystyle f = 1 \, {\ text {Hz}}}$${\ displaystyle f = 2 \, {\ text {Hz}}}$

Despite the definition, the use of the unit is not limited to periodic oscillations . Regularly recurring events, such as the frequency with which a computer backs up files or issues control commands, can also be specified in Hertz (clock frequency). It is also used for scaling a coordinate axis in the frequency space , for example for an absorption spectrum or a wave packet in quantum mechanics .

The unit Hertz, on the other hand, should not be used for the specification of statistically averaged frequencies of random processes or for the specification of an angular velocity or angular frequency . Even if these quantities have the same dimension and they can therefore all be specified in the unit 1 / s, the use of different units serves to emphasize the differences between the quantities. For radioactive decay ( activity ) the unit Becquerel is used, which is also defined as 1 / s.

## Common decimal multiples

The unit Hz is often used with the following prefixes :

unit factor annotation
Kilohertz kHz 10 3 Hz a thousand cycles per second. Very high tones are in the range of a few kHz to 20 kHz.
Megahertz MHz 10 6 Hz a million cycles per second. VHF broadcasters send electromagnetic waves in the range of 100 MHz.
Gigahertz GHz 10 9 Hz a billion cycles per second. Modern processors have a clock frequency in the GHz range.
Terahertz THz 10 12 Hz one trillion cycles per second. Visible electromagnetic waves ( light ) have frequencies in the range of 400–790 THz. See also terahertz radiation .
Petahertz PHz 10 15 Hz one quadrillion cycles per second. X-rays are in this range.
Exahertz EHz 10 18  Hz a trillion cycles per second. Gamma radiation is in this range.

## Examples

A selection of different phenomena with different frequencies can be found in the Order of magnitude (frequency) list .

### rotational speed

In vibration measurement technology, the speed of a machine is specified in Hertz, which is then referred to as rotational frequency . Apart from that, the unit Hertz according to DIN 1301 should not be used for the speed. Instead, the number of revolutions per minute is often given here:

1 Hz = 60 / min

### Electromagnetic wave

Electromagnetic waves propagate in free space at the speed of light . For example, a wave with a frequency of one megahertz ( radio wave ) has a wavelength of about 300 meters.

In the case of electromagnetic waves with frequencies in the gigahertz range, the wavelength is smaller, for example: wavelength in a microwave oven around 12 cm, wavelength in satellite television reception around 2.5 cm.

Green light with a wavelength of 555 nm has a frequency of 540 THz, which in turn corresponds to an energy of 2.2 eV ( electron volts ).

### Sound wave

Concert pitch a 1 , audio sample

In a flute or pipe, air vibrates periodically. The speed of propagation of the sound wave is around 343 meters per second ( speed of sound at an air temperature of 20 ° C). The audible sound frequencies are in the range from about 16 Hz to 20 kHz and correspond to wavelengths from a few meters to a few centimeters . The concert pitch a 1 is fixed at 440 Hz.

### Standing wave

Consider a rope that is attached at one end and moved up and down the other. This rope swings - with a little skill - as a standing wave . The length of this wave depends on two factors, the speed of wave propagation on the rope and the frequency with which the rope is moved at the unattached end.

### Frequency spectrum

A wave or oscillation of any shape can be represented as a superposition of sine functions of different frequencies in a frequency spectrum in which the amplitude is plotted as a function of the frequency. The frequency axis is usually scaled in Hertz.

Wiktionary: Hertz  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

1. IEC History
2. ^ Historical context of the SI
3. The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e édition, 2006)". In: PTB-Mitteilungen . tape  117 , no. 2 , 2007 ( Online [PDF; 1.4 MB ]).