The standard SI units used for the measurement of voltage, current and resistance are the Volt [ V ], Ampere [ A ] and Ohm [ 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 electrical units of measure used 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 V_{L} = -L(di/dt) |
Power | Watts | W | Unit of Power P = V × I or I^{2} × R |
Impedance | Ohm | Z | Unit of AC Resistance Z^{2} = R^{2} + X^{2} |
Frequency | Hertz | Hz | Unit of Frequency ƒ = 1 ÷ T |
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 | 10^{12} |
Giga | G | 1,000,000,000 | 10^{9} |
Mega | M | 1,000,000 | 10^{6} |
kilo | k | 1,000 | 10^{3} |
none | none | 1 | 10^{0} |
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:
To convert from one prefix to another it is necessary to either multiply or divide by the difference between the two values. For example, convert 1MHz into kHz.
Well we know from above that 1MHz is equal to one million (1,000,000) hertz and that 1kHz is equal to one thousand (1,000) hertz, so one 1MHz is one thousand times bigger than 1kHz. Then to convert Mega-hertz into Kilo-hertz we need to multiply mega-hertz by one thousand, as 1MHz is equal to 1000 kHz.
Likewise, if we needed to convert kilo-hertz into mega-hertz we would need to divide by one thousand. A much simpler and quicker method would be to move the decimal point either left or right depending upon whether you need to multiply or divide.
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 20log_{10} (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.
• θ – Phase Angle, The Phase Angle is the difference in degrees between the voltage waveform and the current waveform having the same periodic time. It is a time difference or time shift and depending upon the circuit element can have a “leading” or “lagging” value. The phase angle of a waveform is measured in degrees or radians.
• 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.
In the next tutorial about DC circuit theory we will look at Kirchoff’s Circuit Law which along with Ohms Law allows us to calculate the different voltages and currents circulating around a complex circuit.
I want more tutorials on micrometer screw gauge and venier calipers
240V or 240 V ( Space between Integers and unit) which one is correct?
Both.
Jules or Jules ?? What are they
besic of electrical
Colour code plz
Please use the site correctly and look at the Resistors section, or do a site search for colour codes.
very easy
Nice
hi guys, does anyone knws how to identify colour codes of resistors
http://www.electronics-tutorials.ws/resistor/res_2.html
or
http://www.electronics-tutorials.ws/resistor/resistor-colour-code-wheel.html
can any one tell me the exact years in which these Units of measurement of electricity was proposed ? for example, the year the unit of measurement Volts was suggested and by whom ?simiarly al other units of measurement .I believe that because we could esablish these units of measurement and actually measure them could we progress with electrical science or bring the phenomena of electricty into the realm of science which till then hovered in the ream of meta physics.
Units of measurement leads to measurements leads to Quantification which helps in creating mathematical models which can then be studies in much more detail than physical experiments without the problems of accuracies in measurements. Then all it needs is to check out the results of mathematical findings with experimentally measured data and confirm theory for affecting further progress .
Hence it is so important to establish units of measurement after quantizing continuous energies like light, heat , temperature, pressure and so on and design suitable instruments and check out the maths models created theoretically .
If we can apply the same procedure of dealing with and undertsandng matter to the non-material entity , the “mind” then we can digitilize, quantify and build maths models to study deeply the mind too and bring it into the remof Science which now it is not but mind is still in the ream of metaphysics.
To achieve this aim, life-scientists and biologists should venture out and not just the physicists . The language of the mind is avaiabe in software and information technology which needs to be exploited..
Explanation are short and abriviated