Units & Measurement (GA Physics) Notes for SSC CGL, CHSL & Railways

Units & Measurement (GA Physics) Notes for SSC CGL, CHSL & Railways

Table of Contents

Physical Quantity

A physical quantity is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.

  • A physical quantity can be expressed as the combination of a magnitude (expressed by a number – usually a real number) and a unit.

Unit of measurement

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity.

International System of Units (SI Units)

The International System of Units (SI, abbreviated from the French Système international (d’unités)) is the modern form of the metric system, and is the most widely used system of measurement.

  • It comprises a coherent system of units of measurement built on 7 base units and a set of 20 prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
  • The system also specifies names for 22 derived units.

SI Base Units

Symbol Name Quantity
A ampere electric current
K kelvin temperature
s second time
m metre length
kg kilogram mass
cd candela luminous intensity
mol mole amount of substance

Important SI Derived Units

Symbol Name Quantity
rad radian angle
Hz hertz frequency
N newton force, weight
m metre length
Pa pascal pressure, stress
J joule energy, work, heat
W watt power, radiant flux
C coulomb electric charge
V volt voltage (electric potential), emf
F farad capacitance
Ω ohm resistance, impedance, reactance
S siemens electrical conductance
Wb weber magnetic flux
T tesla magnetic flux density
H henry inductance
°C degree Celsius temperature
lm lumen luminous flux
lx lux illuminance
Bq becquerel radioactivity

Other Important Units

intensity of Sound decibel (dB)
acceleration m s-2
entropy J K-1
jerk m s-3
magnetic field strength A m-1
relative density no units
Torque N m
momentum kg m s-1

Important Practical Units


  • Astronomical Unit (au): Distance Between the centre of Earth and the centre of the Sun (1 au = 1.5 x 1011 m)
  • Light Year (ly): Distance travelled by light in vacuum in one year.
  • 1 Angstrom = 10-10 m

mass & weight

  • The largest practical unit of mass is Chandrasekhar limit (CSL).
  • Atomic or nuclear masses are made in terms of a unit called amu (atomic mass unit).
          1 CSL = 1.4 times the mass of sun. 
   1 metric ton = 1000 kg
        Quintal = 100 kg
           Slug = 14.57 kg

Important Measuring Devices

  • tensiometer: To measure soil-moisture tension (matric water potential)
  • hygrometer: To measure humidity in the atmosphere
  • hydrometer: To measure the density of liquids
  • fathometer: To measure depth of oceans
  • anemometer: To measure the speed of the wind, or of any current of gas
  • pyrometer: To measure high temperatures, especially in furnaces and kilns.
  • barometer: To measure atmospheric pressure
  • lactometer: To measure the density of milk
  • sphygmomanometer: To measure blood pressure
  • Karatmeter : To measure the purity of gold
  • luxmeter: To measure intensity of luminescence/ light
  • seismograph (seismometer): To measure motion of the ground (based on Richter magnitude scale)
  • polygraph (lie detector): To measure and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while a person is asked and answers a series of questions.

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