Electrical Units of Measurement are employed to articulate standard electrical units, and their prefixes come into play when the units become either too small or too large to express as a base unit.
The standard electrical units of measure employed for expressing voltage, current, and resistance are the Volt [V], Ampere [A], and Ohm [Ω], respectively.
These electrical units of measurement adhere to the International System of Units (SI), also known as the metric system, with other commonly used electrical units derived from SI base units.
In electrical or electronic circuits and systems, there are instances where it becomes necessary to use multiples or sub-multiples (fractions) of these standard electrical measuring units, particularly when the quantities being measured are very large or very small.
Standard Electrical Units of Measure
The following table provides a list of some standard electrical units of measure utilized in electrical formulas and component values.
| 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) |
These standard electrical units play a crucial role in describing and calculating various electrical parameters in circuits, ensuring a standardized and consistent representation in electrical and electronic engineering.
Multiples and Sub-multiples
In electrical and electronic engineering, a wide range of values is encountered for standard electrical units, spanning from very small to very large values. To simplify the representation of these values and avoid excessive use of zeros, multiples and submultiples of the standard units are employed. The table below provides the names and abbreviations for these multiples and submultiples.
| Multiple/Submultiple | Abbreviation | Multiplier (in powers of 10) |
|---|---|---|
| Peta | P | 10¹⁵ |
| Tera | T | 10¹² |
| Giga | G | 10⁹ |
| Mega | M | 10⁶ |
| Kilo | k | 10³ |
| Hecto | h | 10² |
| Deca | da | 10¹ |
| Base Unit (no prefix) | – | 10⁰ |
| Deci | d | 10⁻¹ |
| Centi | c | 10⁻² |
| Milli | m | 10⁻³ |
| Micro | μ | 10⁻⁶ |
| Nano | n | 10⁻⁹ |
| Pico | p | 10⁻¹² |
These prefixes allow engineers to express values more conveniently, making calculations and measurements easier to work within a wide range of scenarios.
To express the units or multiples of units for Resistance, Current, or Voltage, the following examples can be used:
- 1 kV=1 kilo-volt1kV=1kilo-volt – equivalent to 1,0001,000 Volts.
- 1 mA=1 milli-amp1mA=1milli-amp – equivalent to one thousandth (1/1000) of an Ampere.
- 47 kΩ=47 kilo-ohms – equivalent to 47,000 Ohms.
- 100 μF=100 micro-farads– equivalent to 100100 millionths (100/1,000,000) of a Farad.
- 1 kW=1kilo-watt – equivalent to 1,000 Watts.
- 1 MHz=1 mega-hertz– equivalent to one million Hertz.
To convert from one prefix to another, it is necessary to multiply or divide by the difference between the two values. For instance, to convert 1MHz into kHz:
Since 1 MHz is 1,000,000 Hertz and 1 kHz is 1,000 Hertz, 1 MHz is 1,000 times bigger than 1 kHz. Therefore, to convert Mega-hertz into Kilo-hertz, multiply mega-hertz by one thousand, as 1 MHz is equal to 1000 kHz.
Similarly, to convert kilo-hertz into mega-hertz, divide by one thousand. A more straightforward method is to move the decimal point either left or right, depending on whether multiplication or division is needed.
In addition to the “Standard” electrical units of measure mentioned above, other units are used in electrical engineering to denote various values and quantities, such as:
- Wh – The Watt-Hour, represents the amount of electrical energy consumed by a circuit over a period. Commonly used in the form of WhWh (watt-hours), kWh (Kilowatt-hour, 1,000 watt-hours), or MWh (Megawatt-hour, 1,000,000 watt-hours).
- dB – The Decibel, a logarithmic unit used to represent gain or loss in voltage, current, or power. For instance, the dB ratio of input to output voltage (VIN/VOUT) is expressed as 20log10(VOUT/VIN).
- θ – Phase Angle, indicating the difference in degrees between voltage and current waveforms with the same periodic time.
- ω – Angular Frequency, used in AC circuits to represent the phasor relationship between waveforms. It is a rotational unit in radians per second (rads/srads/s).
- τ – Time Constant, representing the time it takes for the output of a circuit to reach 63.7% of its maximum or minimum value when subjected to a Step Response input.
In the next tutorial on DC circuit theory, we will explore Kirchhoff’s Circuit Law, which, along with Ohm’s Law, enables the calculation of various voltages and currents in complex circuits.
| Read more Tutorials on DC Circuits | |
|---|---|
| 1. | DC Circuit Theory |
| 2. | Ohms Law and Power |
| 3. | Electrical Units of Measure |
| 4. | Kirchhoffs Circuit Law |
| 5. | Kirchhoffs Current Law |
| 6. | Kirchhoffs Voltage Law |
| 7. | Mesh Current Analysis |
