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The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, Δν Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1." [1]
1 Hz The inverse second or reciprocal second ( s −1 ), also called per second , is a unit defined as the multiplicative inverse of the second (a unit of time ). It is applicable for physical quantities of dimension reciprocal time , such as frequency and strain rate .
The dimension of the unit hertz is 1/time (T −1). Expressed in base SI units, the unit is the reciprocal second (1/s). In English, "hertz" is also used as the plural form. [5] As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 10 3 Hz), MHz (megahertz, 10 6 Hz), GHz (gigahertz, 10 9 Hz) and THz (terahertz, 10 12 Hz).
Metric time is the measure of time intervals using the metric system. The modern SI system defines the second as the base unit of time, and forms multiples and submultiples with metric prefixes such as kiloseconds and milliseconds. Other units of time – minute, hour, and day – are accepted for use with SI, but are not part of it
The period T is the time taken to complete one cycle of an oscillation or rotation. The frequency and the period are related by the equation [ 4 ] f = 1 T . {\displaystyle f={\frac {1}{T}}.} The term temporal frequency is used to emphasise that the frequency is characterised by the number of occurrences of a repeating event per unit time.
In astrophysics and quantum physics a jiffy is, as defined by Edward R. Harrison, [13] the time it takes for light to travel one fermi, which is approximately the size of a nucleon. One fermi is 10 −15 m, so a jiffy is about 3 × 10 −24 s. It has also more informally been defined as "one light-foot", which is equal to approximately one ...
In an increasing system, the time constant is the time for the system's step response to reach 1 − 1 / e ≈ 63.2% of its final (asymptotic) value (say from a step increase). In radioactive decay the time constant is related to the decay constant ( λ ), and it represents both the mean lifetime of a decaying system (such as an atom) before it ...
16.67 milliseconds (1/60 second) – a third, cycle time for American 60 Hz AC electricity (mains grid) 16.68 milliseconds (1/59.94 second) – the amount of time one field lasts in 29.97 fps interlaced video (commonly erroneously referred to as 30 fps) 20 milliseconds – cycle time for European 50 Hz AC electricity; 31.25 milliseconds – a ...