S.I Units

CHAPTER - 1

Basic Circuit Analysis - DC

S.I UNITS

The International system of units known as SI units derives all the units from the following base units.

TABLE 1

The metre is the length equal to 16, 50, 763.73 wave lengths of the orange line in the spectrum of an internationally specified krypton discharge lamp.

The kilogram is the mass of platinum iridium cylinder preserved at the International Bureau of Weights and Measures at SEVRES, near Paris.

The second is defined as the interval of time taken by 9, 19, 26, 31, 770 cycles of radiation corresponding to the transition of the Cesium 133 atom. The Kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water.

The Candela is the luminous intensity in the perpendicular direction, of a surface 1/6,00,000 square metre of a black body at the temperature of freezing under standard atmospheric pressure.

The ampere is the current which is maintained when two straight infinitely long parallel conductors of negligible cross-section, placed one meter apart in vacuum, would produce a force of 2 × 10^-7 newtons per metre length between these conductors.

The mole is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kg of the carbon isotope. When the mole is used, it is essential to specify the elementary entities under consideration. These may be atoms, molecules, ions, electrons or other particles or group of particles.

Force, Work, Energy, Power

The SI unit of force is the Newton (N), namely the force which when applied to a mass of 1 kg, giving it an acceleration of 1 m/s². The SI unit of work (or) energy is the joule, which is the work done when a force of 1 N acts through a distance of one metre in the direction of the force. The SI unit of power is Joule/s or Watt. Since the watt is very small, the kilowatt is often used. Similarly for energy, it is often convenient to use the kilowatt-hour instead of joules.