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A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world , is the second , defined as about 9 billion oscillations of the caesium atom.
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived.
- Second – Unit of Time. Symbol: s. The second is defined by taking the fixed numerical value of the cesium frequency ∆ν, 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.
- Meter – Unit of Length. Symbol: m. The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s, where the second is defined in terms of ∆ν.
- Kilogram – Unit of Mass. Symbol: kg. The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 ×10 when expressed in the unit J s, which is equal to kg m2 s, where the meter and the second are defined in terms of c and ∆ν.
- Ampere – Unit of Electric Current. Symbol: A. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 × 10 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.
Jul 15, 2024 · Second (s) 1 s. The second is the base unit of time in the International System of Units (SI), defined by the oscillation of cesium atoms. It is universally used in science and everyday life to measure durations and intervals.
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- Length: Meter (m) The meter is the metric unit of length. It's defined as the length of the path light travels in a vacuum during 1/299,792,458 of a second.
- Mass: Kilogram (kg) The kilogram is the metric unit of mass. It's the mass of the international prototype of the kilogram: a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).
- Time: Second (s) The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 oscillations of radiation corresponding to the transition between the two hyperfine levels of cesium-133.
- Electric current: Ampere (A) The basic unit of electric current is the ampere. The ampere is defined as the constant current that, if maintained in two infinitely long straight parallel conductors with a negligible circular cross-section and placed 1 m apart in a vacuum, would produce a force between the conductors equal to 2 x 10-7 newtons per meter of length.
Dec 13, 2023 · To identify the basic units of measurement of the seven fundamental properties. Describe the names and abbreviations of the SI base units and the SI decimal prefixes. Define the liter and the metric ton in these units. Explain the meaning and use of unit dimensions; state the dimensions of volume.
The definitions of the base units specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit. All units of the SI can be written either through a defining constant itself, or through products or quotients of the defining constants.
