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Physics · Ch 1 — Nature of Physical World and Measurement

SI Unit System

1.4.5

SI Unit System

Since 1960, the internationally agreed system of units has been the International System of Units, or SI (from the French Système International) -- what scientists and engineers everywhere now call simply 'the metric system'. Its standard scheme of symbols, units and abbreviations was recommended by the General Conference on Weights and Measures (GCWM) in 1971, for uniform use in scientific, technical, industrial and commercial work worldwide.

SI has three defining advantages:

  1. Rational -- it uses exactly one unit for one physical quantity (no competing units for the same thing).
  2. Coherent -- every derived unit is obtained straightforwardly from the base and supplementary units, with no extra conversion factors.
  3. Metric -- multiples and sub-multiples are always powers of 10, so scaling a measurement up or down is just moving a decimal point (with the prefixes of Table 1.4, section 1.5.1).

SI recognises seven base quantities and units (Table 1.2): length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol), and luminous intensity (candela, cd) -- each pinned down by a precise physical definition (e.g. the second is defined by 9,192,631,770 periods of a specific Cesium-133 radiation, and historically the metre was tied to the speed of light in vacuum). Table 1.3 shows how a long list of everyday derived quantities -- area, velocity, force, pressure, energy, power, and so on -- are built purely from combinations of these seven base units. …

Table 1.2SI Base Quantities and Units
Base QuantityUnitSymbolDefinition
LengthmetremThe length of the path travelled by light in vacuum in 1/299,792,458 of a second (1983)
MasskilogramkgThe mass of the prototype cylinder of platinum-iridium alloy (height equal to diameter) preserved at the International Bureau of Weights and Measures, Sevres, near Paris (1901)
TimesecondsThe duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium-133 atom (1967)
Electric currentampereAThe constant current which, maintained in two straight parallel conductors of infinite length and negligible cross-section, 1 m apart in vacuum, produces a force per unit length of 2×10−72\times10^{-7} N/m between them (1948)
TemperaturekelvinKThe fraction 1/273.161/273.16 of the thermodynamic temperature of the triple point of water (1967)
Amount of substancemolemolThe amount of substance containing as many elementary entities as there are atoms in 0.012 kg of pure carbon-12 (1971)
Table 1.3Derived Quantities and their Units
Physical quantityExpressionUnit
Plane anglearc / radiusrad
Solid anglesurface area / radius2^2sr
Arealength ×\times breadthm2^2
Volumearea ×\times heightm3^3
Velocitydisplacement / timem s−1^{-1}
Accelerationvelocity / timem s−2^{-2}
Angular velocityangular displacement / timerad s−1^{-1}
Angular accelerationangular velocity / timerad s−2^{-2}
Densitymass / volumekg m−3^{-3}
Linear momentummass ×\times velocitykg m s−1^{-1}
Moment of inertiamass ×\times (distance)2^2kg m2^2
Forcemass ×\times accelerationkg m s−2^{-2} or N
Pressureforce / areaN m−2^{-2} or Pa
Energy (work)force ×\times distanceN m or J
Powerwork / timeJ s−1^{-1} or watt
Impulseforce ×\times timeN s
Surface tensionforce / lengthN m−1^{-1}
Moment of force (torque)force ×\times distanceN m
Electric chargecurrent ×\times timeA s or C
Current densitycurrent / areaA m−2^{-2}
Magnetic inductionforce / (current ×\times length)N A−1^{-1} m−1^{-1} or tesla
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