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Physics Theory : Units & Measurements

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SI (Systeme Internationale d'Unites) system is the standard system of units used by scientists worldwide.
There are seven base SI units (for length, mass, time, electric current, temperature, luminous intensity, and amount of substance) from which the other units may be derived.


SI unit for length is meter (whose symbol is m).
One meter is defined as the length equal to 1,650,763.73 wavelengths, in vacuum, of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the Krypton-86 atom.


SI unit for mass is kilogram (whose symbol is kg).
The standard kilogram is kept at the Bureau of Weights and Measures at Sevres, France under carefully-controlled conditions.


SI unit for time is second (whose symbol is s).
One second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of the ground state of the Cesium-133 atom.


SI unit for electric current is Ampere (whose symbol is A).
The unit is named after French physicist Andre Marie Ampere, who explored the link between electric current and voltage.
An electric current is a flow of charged particles, typically electrons.


SI unit for temperature is Kelvin (whose symbol is K).
To convert a temperature from Celsius (oC) to Kelvin (K), simply add 273.16.


SI unit for luminous intensity is candela (whose symbol is cd).


SI unit for amount of substance is mole (whose symbol is mol).


SI unit for force is Newton (whose symbol is N).
It is named afer the famous British scientist, Sir Isaac Newton.
One Newton is the force required to accelerate a mass of one kilogram at 1 metre/second2.
Note that force is mass multiplied by acceleration (F = m a) as per Newton's second law of motion.


SI unit for pressure is Pascal (whose symbol is Pa).
The scientist, Blaise Pascal, gave his name to this SI unit.
1 Pa = 1 N/m2.
Note that pressure is defined as force per unit area.
Another unit for pressure is bar, where 1 bar = 105 N/m2.


SI unit of energy is Joule (whose symbol is J).
The unit is named after British physicist James Joule (1819-89), who discovered the first law of thermodynamics (conservation of energy).
Latent heat is measured in Joules (J).
Heat capacity is measured in J/K and specific heat capacity is measured in J/(kg K).
Specific heat capacity is heat capacity per unit mass.
In electricity, one Joule is used every second when one Ampere flows through a resistance of one Ohm.


SI unit of power is Watt (whose symbol is W).
Power is the rate at which work is done or the rate at which electricity is used.
1 W = 1 J/s.


Unit for electric charge is Coulomb (whose symbol is C).
The unit is named after French physicist Charles Coulomb, who studied electrostatic forces.
A Coulomb is the electric charge moved in one second by an electric current of one Ampere.


Unit for electrical potential is Volt (whose symbol is V).
The unit is named after Italian chemist Alessandro Volta, who invented the voltaic pile (the world's first battery).
A battery or generator produces a voltage which causes current to flow in a circuit.


Unit for electrical resistance is Ohm (whose symbol is Ω).
Electrical resistance is defined as the ratio of voltage to electric current as per Ohm's law.
Resistance is the degree to which a conductor of electricity opposes the flow of current.


Unit of capacitance is Farad (whose symbol is F).
Capacitance is defined as the ratio of electric charge to electrical potential as per Faraday's law.


Unit for frequency is Hertz (whose symbol is Hz).
The German physicist, Heinrich Hertz, demonstrated the existence of radio waves.
The unit of frequency (cycles per second) is named after him.


Unit for illumination is lux (whose symbol is lx).
One lux equals one lumen per square meter.


Unit for radiation activity is Becquerel (whose symbol is Bq).
The French physicist, Antoine-Henri Becquerel, is known for his discovery of radioactivity.
The unit to measure radiation activity is named after him.


Physics Quiz on Units & Measurements.
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