General Science(Physics): Quantities, Standards and Units

1. Physics is a quantitative science, based on measurement of physical quantities. Certain physical quantities have been chosen as fundamental or base quantities (such as length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity).
2. Each base quantity is defined in terms of a certain basic, arbitrarily chosen but properly standardised reference standard called unit (such as metre, kilogram, second, ampere, kelvin, mole and candela). The units for the fundamental or base quantities are called fundamental or base units.
3. Other physical quantities, derived from the base quantities, can be expressed as a combination of the base units and are called derived units. A complete set of units, both fundamental and derived, is called a system of units.
4. The International System of Units (SI) based on seven base units is at present internationally accepted unit system and is widely used throughout the world. The SI units are used in all physical measurements, for both the base quantities and the derived quantities obtained from them. Certain derived units are expressed by means of SI units with special names (such as joule, newton, watt, etc).
5. The SI units have well defined and internationally accepted unit symbols (such as m for metre, kg for kilogram, s for second, A for ampere, N for newton etc.). Physical measurements are usually expressed for small and large quantities in scientific notation, with powers of 10. Scientific notation and the prefixes are used to simplify measurement notation and numerical computation, giving indication to the precision of the numbers.

6. Unit of Length: The SI Unit of length is metre(m). Various other metric units used for measuring length are related to the metre by either multiples or submultiples of 10. Thus,

   • 1 kilometre = 1000 ( or 10³) m
   • 1 centimetre= 1/100 ( or 0^-2) m
   • 1 milimetre=1/1000 ( or 10^-3) m

   Very small distance are measured in micrometre or microns ( µm), angstroms(Å), nanometre (nm) and femtometre(fm).

   • 1m= 10⁶ µm
   • 1m= 10⁹ nm
   • 1m=10¹⁰ Å
   • 1m= 10¹⁵fm

   For really large distances, the light year is the unit of choice. A light year is the distance light would travel in a vacuum after one year. It is equal to some nine quadrillion meters (six trillion miles). 1 light year= 9.46X10¹⁵m.

7. Unit of Mass: The SI Unit of mass is kilogram(kg). Various other metric units used for measuring mass are related to the kilogram by either multiples or submultiples of 10. Thus,

   • 1 tonne(t) = 1000 ( or 10³) kg
   • 1 gram(g)= 1/1000 ( or 0^-32) kg
   • 1 miligram(mg)= 10^-6Kg
8. Unit of Time: The SI unit of time is the second (s).

SI Base Quantities and Units:

Base quantity	SI Units
	Name	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermo dynamic Temperature	kelvin	K
Amount of substance	mole	mol
Luminous	candela	cd

Important Units of Measurement:

Used to Measure	Name of the Unit
Electric Current	Ampere
Wave length of light	Angstrom
Electric charge	Faraday
Magnetic induction	Gauss
Magnetic Flux	Maxwell
Electric Charge	Coulomb
Electric Resistance	Ohm
Electric Tension	Volt
Power	Watt
Intensity of Sound	Bel
Temperature	Celcius, Kelvin, Farenheit
Atmospheric Pressure	Bar
Quantity of heat	Calorie
Force	Dyne
Work or Energy	Joule
Work	Newton
Pressure	Pascal
Luminious Flux	Lumen

Other Measures:

• A nautical mile is now 1852 m (6080 feet), but was originally defined as one minute of arc of a great circle, or 1/60 of 1/360 of the earth's circumference. Every sixty nautical miles is then one degree of latitude anywhere on earth or one degree of longitude on the equator. This was considered a reasonable unit for use in navigation, which is why this mile is called the nautical mile. The ordinary mile is more precisely known as the statute mile; that is, the mile as defined by statute or law. Use of the nautical mile persists today in shipping, aviation, and aerospace.
• Distances in near outer space are sometimes compared to the radius of the earth: 6.4 × 106 m. Some examples: the planet Mars has ½ the radius of the earth, the size of a geosynchronous orbit is 6.5 earth radii, and the earth-moon separation is about 60 earth radii.
• The mean distance from the earth to the sun is called an astronomical unit: approximately 1.5×1011m. The distance from the Sun to Mars is 1.5 AU; from the Sun to Jupiter, 5.2 AU; and from the Sun to Pluto, 40 AU. The star nearest the Sun, Proxima Centauri, is about 270,000 AU away.

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