Measurement means comparing a physical quantity with a fixed standard called a unit. Physics uses the International System of Units (SI), adopted in 1960, to keep measurements uniform worldwide. Questions on SI units appear in almost every general science paper.
Fundamental Quantities and Units
There are seven base (fundamental) quantities in the SI system.
- Length - metre (m).
- Mass - kilogram (kg).
- Time - second (s).
- Electric current - ampere (A).
- Temperature - kelvin (K).
- Amount of substance - mole (mol).
- Luminous intensity - candela (cd).
Derived Units
Derived quantities are formed from base quantities.
- Force - newton (N).
- Work / energy - joule (J).
- Power - watt (W).
- Pressure - pascal (Pa).
- Frequency - hertz (Hz).
Advertisement
Prefixes and Large or Small Units
- Kilo (k) = 1,000; Mega (M) = 10 lakh.
- Milli (m) = one thousandth; Micro = one millionth.
- Light year measures astronomical distance, not time.
- Angstrom measures very small lengths (atoms).
Measuring Instruments
- Vernier callipers and screw gauge measure small lengths.
- Barometer measures atmospheric pressure.
- Ammeter measures current; voltmeter measures voltage.
Quick Revision Points
- SI system was adopted in 1960.
- There are seven base SI units.
- SI unit of force is newton; of energy is joule.
- SI unit of temperature is kelvin.
- Light year is a unit of distance.
- Angstrom measures atomic-scale length.
- Pressure unit is pascal; power unit is watt.