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
|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|
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.