Basic Electronics on the Go - Electrical Units of Measure

From http://www.electronics-tutorials.ws/dccircuits/dcp_3.html

 The following table gives a list of some of the standard electrical units of measure used in electrical formulas and component values.

Standard Electrical Units

 

Electrical Parameter

Measuring Unit

Symbol

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.

As well as the “Standard” electrical units of measure shown above, other units are also used in electrical engineering to denote other values and quantities such as:

• •  Wh – The Watt-Hour, The amount of electrical energy consumed by a circuit over a period of time. Eg, a light bulb consumes one hundred watts of electrical power for one hour. It is commonly used in the form of: Wh (watt-hours), kWh (Kilowatt-hour) which is 1,000 watt-hours or MWh (Megawatt-hour) which is 1,000,000 watt-hours.
• •  dB – The Decibel, The decibel is a one tenth unit of the Bel (symbol B) and is used to represent gain either in voltage, current or power. It is a logarithmic unit expressed in dB and is commonly used to represent the ratio of input to output in amplifier, audio circuits or loudspeaker systems.
  For example, the dB ratio of an input voltage (Vin) to an output voltage (Vout) is expressed as 20log₁₀ (Vout/Vin). The value in dB can be either positive (20dB) representing gain or negative (-20dB) representing loss with unity, ie input = output expressed as 0dB.
• •  ω – Angular Frequency, Another unit which is mainly used in a.c. circuits to represent the Phasor Relationship between two or more waveforms is called Angular Frequency, symbol ω. This is a rotational unit of angular frequency 2πƒ with units in radians per second, rads/s. The complete revolution of one cycle is 360 degrees or 2π, therefore, half a revolution is given as 180 degrees or π rad.
• •  τ – Time Constant, The Time Constant of an impedance circuit or linear first-order system is the time it takes for the output to reach 63.7% of its maximum or minimum output value when subjected to a Step Response input. It is a measure of reaction time.