Electrical Quantities

The standard SI units used for the measurement of voltage, current and resistance are the Volt [ V ],Ampere [ A ] and Ohms [ Ω ] respectively. Sometimes in electrical or electronic circuits and systems it is necessary to use multiples or sub-multiples (fractions) of these standard units when the quantities being measured are very large or very small. The following table gives a list of some of the standard units used in electrical formulas and component values.

Standard Electrical Units

 Parameter Symbol Measuring Unit Description Voltage Volt V or E Unit of Electrical Potential V = I × R Current Ampere I or i Unit of Electrical Current I = V ÷ R Resistance Ohm R or Ω Unit of DC Resistance R = V ÷ I Conductance Siemen G or ℧ Reciprocal of Resistance G = 1 ÷ R Capacitance Farad C Unit of Capacitance C = Q ÷ V Charge Coulomb Q Unit of Electrical Charge Q = C × V Inductance Henry L or H Unit of Inductance VL = -L(di/dt) Power Watts W Unit of Power P = V × I  or  I2 × R Impedance Ohm Z Unit of AC Resistance Z2 = R2 + X2 Frequency Hertz Hz Unit of Frequency ƒ = 1 ÷ T

Multiples and Sub-multiples

There is a huge range of values encountered in electrical and electronic engineering between a maximum value and a minimum value of a standard electrical unit. For example, resistance can be lower than 0.01Ω’s or higher than 1,000,000Ω’s. By using multiples and submultiple’s of the standard unit we can avoid having to write too many zero’s to define the position of the decimal point. The table below gives their names and abbreviations.

 Prefix Symbol Multiplier Power of Ten Terra T 1,000,000,000,000 1012 Giga G 1,000,000,000 109 Mega M 1,000,000 106 kilo k 1,000 103 none none 1 100 centi c 1/100 10-2 milli m 1/1,000 10-3 micro µ 1/1,000,000 10-6 nano n 1/1,000,000,000 10-9 pico p 1/1,000,000,000,000 10-12

So to display the units or multiples of units for either Resistance, Current or Voltage we would use as an example:

• 1kV = 1 kilo-volt  –  which is equal to 1,000 Volts.
• 1mA = 1 milli-amp  –  which is equal to one thousandths (1/1000) of an Ampere.
• 47kΩ = 47 kilo-ohms  –  which is equal to 47 thousand Ohms.
• 100uF = 100 micro-farads  –  which is equal to 100 millionths (1/1,000,000) of a Farad.
• 1kW = 1 kilo-watt  –  which is equal to 1,000 Watts.
• 1MHz = 1 mega-hertz  –  which is equal to one million Hertz.