Electrical Units

Ratio of SI unit Magnitude of Quantity Symbol Equation SI unit symbol CGS unit SI to cgs unit Z Current I, i I=E/R; I = E/Z; I = Q/t Ampere A Abampere 10-1 Quantity Q, q Q = it; Q = CE Coulomb C Abcoulomb 10-1 Electromotive force E, e E = IR; E = W/Q Volt V Abvolt 108 Resistance R, r R=E/I; R = ␳l /A Ohm Ω Abohm 109 Resistivity ␳␳= RA / I Ohm-metre Ω•m Abohm-cm 1011 Conductance G, g G =␥ A / l G = A/␳l Siemens S Abmho 10-9 Conductivity ␥␥ = 1/␳; ␥ = I/ RA Siemens/meter S/m Abmho/cm 10-11 Capacitance C C = Q/E Farad F Abfarad* 10-9 Permittivity ⑀ Farads/meter F/m Stat farad*/cm 8.85 X 10-12

Relative permittivity ⑀r ⑀r = ⑀ր⑀␱ Numerical Numerical I Self-inductance L L = - N(d␾րdt) Henry H Abhenry 109 1/2 9 Mutual inductance M M = K(L1L2) Henry H Abhenry 10 Energy J J = eit Joule J Erg 107 kwh kwh = kw/3600; 3.6 MJ Kilowatthour kWh 36 X 1012 Active power W W = J/t; W = EI cos ⍜ Watt W Abwatt 107 Reactive power jQ Q = El sin ⍜ Var var Abvar 107 Apparent power VA VA = El Volt-ampere VA Power factor pf pf = W/VA; pf = W/(W + jQ) 1 ␲ Ω 9 Reactance, inductive XL XL = 2 fL Ohm Abohm 10 ␲ Ω 9 Reactance, capacitive XC XC = 1/(2 fC) Ohm Abohm 10 Ω 9 Impedance Z Z = E/I Z = R + j(XL - XC) Ohm Abohm 10 Conductance G G = R/Z2 Siemens S Abmho 10-9 Susceptance B B = X/Z2 Siemens S Abmho 10-9 Admittance Y Y =I/E; Y = G + jB Siemens S Abmho 10-9 Frequency f f = 1/T Hertz Hz Cps Hz 1 Period T T = 1/f Second s Second 1 Time constant T L/R; RC Second s Second 1 Angular velocity ωω= 2πf Radians/second rad/s Radians/second 1

*1 Abfarad (EMU Units) = 9 X 10-20 stat farads (ESU units).

Reproduced with Permission from McGraw-Hill, Marks’ Standard Handbook for Mechanical Engineers, 8th ed. Authors: Baumeister, et al. Copyright 1987

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