Chapter 2 Units And Measurements
UNITS
Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted standard called unit.
The units for fundamental or base quantities are called the fundamental units or base units.
The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units.
SI BASE QUANTITIES AND UNITS
- Base quantity: Length
Unit name: metre
Symbol: m
Definition: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
- Base quantity: Mass
Unit name: kilogram
Symbol: kg
Definition: The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at the Bureau of Weights and Measures at France.
- Base quantity: Time
Unit name: second
Symbol: s
Definition: The second is 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 cesium-133 atom.
- Base quantity: Electrical current
Unit name: ampere
Symbol: A
Definition: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10-7 newton per metre of length.
- Base quantity: Thermodynamic temperature
Unit name: kelvin
Symbol: K
Definition: The kelvin is the fraction 1/213.16 of the thermodynamic temperature of the triple point of water.
- Base quantity: Amount of substance
Unit name: mole
Symbol: mol
Definition: The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.
- Base quantity: Luminous intensity
Unit name: candela
Symbol: cd
Definition: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The result of every measurement by any measuring instrument contains some uncertainty. This uncertainty is called error.
The accuracy of a measurement is a measure of how close the measured value is to the true value of the quantity.
The precision tells us to what resolution or limit the quantity is measured.