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In the International System of Units (SI), the unit of time is the second (symbol: s). It has been defined since 1967 as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", and is an SI base unit. [12]
Time unit used for sedimentation rates (usually of proteins). picosecond: 10 −12 s: One trillionth of a second. nanosecond: 10 −9 s: One billionth of a second. Time for molecules to fluoresce. shake: 10 −8 s: 10 nanoseconds, also a casual term for a short period of time. microsecond: 10 −6 s: One millionth of a second. Symbol is μs ...
In physics, time is a fundamental concept to define other quantities, such as velocity. To avoid a circular definition, [16] time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second. [6] [17] [18] Although this aids in practical measurements, it does not address the essence ...
1 fs: The cycle time for ultraviolet light with a wavelength of 300 nanometres; The time it takes light to travel a distance of 0.3 micrometres (μm). 7.58fs: The period of vibration of a hydrogen molecule. 140 fs: The time needed for electrons to have localized onto individual bromine atoms 6 Ångstrom apart after laser dissociation of Br 2. [11]
Jiffy can be an informal term for any unspecified short period, as in "I will be back in a jiffy". From this, it has acquired a number of more precise applications as the name of multiple units of measurement, each used to express or measure very brief durations of time.
The time required for a quantity to fall to half its value as measured at the beginning of the time period. In physics, half-life typically refers to a property of radioactive decay, but may refer to any quantity which follows an exponential decay. Hamilton's principle Hamiltonian mechanics harmonic mean heat
Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true ...
In physics and engineering, the time constant, usually denoted by the Greek letter τ (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system. [1] [note 1] The time constant is the main characteristic unit of a first-order LTI system. It gives speed of the response.