Hertz

The Hertz ( with the unit symbol Hz ) is the SI derived unit of frequency. It indicates the number of repeating operations 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 system of units " or MKS System of Units, named after Heinrich Hertz. At the 11th CGPM ( Conférence Générale des Poids et Mesures ) in 1960, the MKS unit system went on in the SI unit system. Since then, the unit Hertz replaced the previously common in English-language literature unit cycles per second = cps or cps and c / s ( cycles per second).

Use

Despite the definition of the use of the unit is not limited to periodic oscillations. Also, regularly recurring events, such as the frequency with which a computer creates backup copies of files or issue control commands can be in units of Hertz specify ( clock frequency ). Furthermore, it is used for the scaling of a coordinate axis in the frequency domain, for example in an absorption spectrum or the wave packet in the quantum mechanics.

The unit Hertz is, however, not be used for specifying statistically averaged frequencies of random processes or for indicating an angular velocity or angular frequency. Although these values ​​have the same dimension, and all of them can therefore be expressed in the unit 1 / s, the use of different units serves to emphasize the differences of the sizes. A frequency can be converted by multiplying by the factor in an angular frequency.

Common decimal multiples

The unit Hz is often used with the following prefixes:

Examples

A selection of various phenomena of different frequencies is in the list of magnitude ( frequency).

Number of revolutions

In the vibration measurement technique the speed of a machine is expressed in hertz, then one speaks of rotational frequency. Otherwise at rotational speeds, the number of revolutions per minute is often specified:

Electromagnetic wave

Electromagnetic waves propagate in free space speed of light. A wave having a frequency of one megahertz ( radio wave ), for example about the wavelength of 300 meters.

Green light having a wavelength of about 500 nm has a frequency of 605 THz, which in turn corresponds to an energy of 2.5 eV (electron volts).

Wherein electromagnetic waves having frequencies in the gigahertz range is greater than the wavelength, for example: wavelength in the microwave oven for about 12 cm, the local satellite television reception wavelength about 2.5 cm.

Shock wave

In a flute or whistle air vibrates periodically. The speed of propagation of the acoustic wave is approximately 343 meters per second (speed of sound at 20 ° C air temperature). The audible sound frequencies are in the range of 20 Hz to 20 kHz corresponding to wavelengths of a few meters up to a few centimeters. The concert pitch a1 is set to 440 Hz.

Standing wave

If a rope that fastened at one end and at the other is high and is moved down. This rope swings - with some skill - as a standing wave. The length of this wave depends on two factors, the velocity of wave propagation in the cable as well as the frequency at which the cable is moved to the non- fixed end.

Frequency spectrum

A shaft or the vibration of any shape can be represented as a superposition of sine functions of different frequencies in a frequency spectrum, wherein the amplitude is plotted as a function of frequency. The scaling of the frequency axis is carried out usually in Hertz.