Sociology, asked by monuamitcool9377, 11 months ago

Why the metric units of length, mass and time are considered as the SI base units and other units like those of area, volume, capacity and mass as derived units?

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Answered by gopalberma
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SI base units

The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent, as given in Table 1.

Table 1. SI base units

SI base unit

Base quantity Name Symbol

length meter m

mass kilogram kg

time second s

electric current ampere A

thermodynamic temperature kelvin K

amount of substance mole mol

luminous intensity candela cd

For detailed information on the SI base units, see Definitions of the SI base units and their Historical context.

SI derived units

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.

Table 2. Examples of SI derived units

SI derived unit

Derived quantity Name Symbol

area square meter m2

volume cubic meter m3

speed, velocity meter per second m/s

acceleration meter per second squared m/s2

wave number reciprocal meter m-1

mass density kilogram per cubic meter kg/m3

specific volume cubic meter per kilogram m3/kg

current density ampere per square meter A/m2

magnetic field strength ampere per meter A/m

amount-of-substance concentration mole per cubic meter mol/m3

luminance candela per square meter cd/m2

mass fraction kilogram per kilogram, which may be represented by the number 1 kg/kg = 1

For ease of understanding and convenience, 22 SI derived units have been given special names and symbols, as shown in Table 3.

Table 3. SI derived units with special names and symbols

SI derived unit

Derived quantity Name Symbol Expression

in terms of

other SI units Expression

in terms of

SI base units

plane angle radian (a) rad - m·m-1 = 1 (b)

solid angle steradian (a) sr (c) - m2·m-2 = 1 (b)

frequency hertz Hz - s-1

force newton N - m·kg·s-2

pressure, stress pascal Pa N/m2 m-1·kg·s-2

energy, work, quantity of heat joule J N·m m2·kg·s-2

power, radiant flux watt W J/s m2·kg·s-3

electric charge, quantity of electricity coulomb C - s·A

electric potential difference,

electromotive force volt V W/A m2·kg·s-3·A-1

capacitance farad F C/V m-2·kg-1·s4·A2

electric resistance ohm Omega V/A m2·kg·s-3·A-2

electric conductance siemens S A/V m-2·kg-1·s3·A2

magnetic flux weber Wb V·s m2·kg·s-2·A-1

magnetic flux density tesla T Wb/m2 kg·s-2·A-1

inductance henry H Wb/A m2·kg·s-2·A-2

Celsius temperature degree Celsius °C - K

luminous flux lumen lm cd·sr (c) m2·m-2·cd = cd

illuminance lux lx lm/m2 m2·m-4·cd = m-2·cd

activity (of a radionuclide) becquerel Bq - s-1

absorbed dose, specific energy (imparted), kerma gray Gy J/kg m2·s-2

dose equivalent (d) sievert Sv J/kg m2·s-2

catalytic activity katal kat s-1·mol

(a) The radian and steradian may be used advantageously in expressions for derived units to distinguish between quantities of a different nature but of the same dimension; some examples are given in Table 4.

(b) In practice, the symbols rad and sr are used where appropriate, but the derived unit "1" is generally omitted.

(c) In photometry, the unit name steradian and the unit symbol sr are usually retained in expressions for derived units.

(d) Other quantities expressed in sieverts are ambient dose equivalent, directional dose equivalent, personal dose equivalent, and organ equivalent dose

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