UNITS AND MEASUREMENTS

 

MEASUREMENT

Measurement and Measurement Units in Physics

Measurement is a process of detecting an unknown physical quantity by using standard quantity. For example: Take a book and use a ruler (scale) to find its length. Suppose the length was 20 cm. You underwent a process called Measurement where:

·         The unknown physical quantity was the length of the book.

·         The ruler was the standard quantity.

·         20 was the magnitude.

·         cm was the unit of the book-length.

 

Units of Measurement

Units provide specific meaning to the magnitude of a substance. Units of measurement provide a standard to identify the measurement of a physical quantify.

Systems of Unit of Measurement

There are different standards and units of the system used in the word. Few common system of measurements are:

CGS unit system

In the CGS unit system, the length is measured in centimeter, mass is measured in gram, and time is measured in second.

FPS unit system

In the FPS system, the length is measured in the foot, mass is measured in the pound, and time is measured in seconds.

MKS System

In MKS system, the length is measured in meter, mass is measured in kg, and time is measured in second.

SI unit

Different units are used in different countries for the measurement of physical quantities. In the US, pound metrics is used commonly for indicating mass but in India, the kilogram is used. To remove these differences, SI (International System of units) the system was standardized in 1960. In the SI unit,

Name	Abbreviation	Measure
meter	m	Length
kilogram	kg	mass
second	s	time
ampere	A	electric current
Kelvin	K	thermodynamic temperature
mole	mol	amount of substance
candela	cd	luminous intensity

Divisions of units

Fundamental Units (Basic Units)

Fundamental units are those units that can express themselves without the assistance of any other units. For example Kilogram (kg) is a fundamental unit because it is independently expressed and cannot be broken down into multiple units.

Derived Units

Derived units are those units that cannot be expressed in the absence of fundamental units. For example, Newton (N) is a derived unit because it cannot be expressed in the absence of fundamental unit (meter) and can be broken down to multiple units (Newton equals to kg.m /s²).

 

Some common examples of derived units can be mentioned in the following table.

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

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		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.

Table 4.  Examples of SI derived units whose names and symbols include SI derived units with special names and symbols
	SI derived unit
Derived quantity	Name	Symbol
dynamic viscosity	pascal second	Pa·s
moment of force	newton meter	N·m
surface tension	newton per meter	N/m
angular velocity	radian per second	rad/s
angular acceleration	radian per second squared	rad/s2
heat flux density, irradiance	watt per square meter	W/m2
heat capacity, entropy	joule per kelvin	J/K
specific heat capacity, specific entropy	joule per kilogram kelvin	J/(kg·K)
specific energy	joule per kilogram	J/kg
thermal conductivity	watt per meter kelvin	W/(m·K)
energy density	joule per cubic meter	J/m3
electric field strength	volt per meter	V/m
electric charge density	coulomb per cubic meter	C/m3
electric flux density	coulomb per square meter	C/m2
permittivity	farad per meter	F/m
permeability	henry per meter	H/m
molar energy	joule per mole	J/mol
molar entropy, molar heat capacity	joule per mole kelvin	J/(mol·K)
exposure (x and  rays)	coulomb per kilogram	C/kg
absorbed dose rate	gray per second	Gy/s
radiant intensity	watt per steradian	W/sr
radiance	watt per square meter steradian	W/(m2·sr)
catalytic (activity) concentration	katal per cubic meter	kat/m3