SI unit of a physical quantity.
This article focuses on basic SI units and derived SI units on electrical engineering and practical derived units in electrical quantity.
The 'International System of Units' can be abbreviated as the SI units. SI unit system was established in 1960. SI unit systems are commonly used to measure different physical quantities and constant terms of measurement throughout the world. SI unit enables us to measure and calculate easily.
Physical quantities*- It is a quantity that can be measured.
Measurement*: Comparing unknown quantity with known quantity to balance or measure it
SI units are based on the MKSA system (meter, kilogram, second, ampere ). SI units are universally accepted one and it is also called the metric system.
SI units can be broadly classified into,
- Basic SI unit.
- Derived or Fundamental SI units.
Most of the practical units in electrical engineering come in the category of Derived SI units.
Base unit.
Length
- The SI unit of length is the 'meter' and the symbol is 'm'.
- Length is used to measure the distance between two quantities.
Time
- The SI unit of time is 'seconds' and the symbol is denoted as 's'.
Temperature
- The SI unit of temperature is Kelvin and it is denoted by the letter 'K'.
Luminous Intensity
- The SI unit of 'Candela' is the luminous intensity and it is denoted as 'Cd'.
Plane angle
- The SI unit of plane angle is 'radians' and the symbol is denoted as 'rad'.
Solid angle
SI unit of solid angle is 'Steradians' and the symbol is denoted as 'Sr'.
Force
The SI unit of force is Newton and its symbol or denoted as N.
Derived SI unit in electrical and its dimension.
Energy or Work or Quantity of Heat. (J)
- Its unit is 'Joule' and the symbol is denoted as 'J'.
- Dimension is 'm^1. L^2. T^-2'.
- Its unit is 'watt' and the symbol is denoted as 'W'.
- Dimension is 'm^2.kg.S^-2'.
- Its unit is 'coulomb' and the symbol is 'Q' or 'C'.
- Dimension is denoted as 's*A'.
Emf or Potential or Potential Difference. (V)
- Its unit is 'volts' and the symbol is 'V'.
- Dimension is denoted as 'm^2.kg.S^-2 A^-1'.
- Its unit is 'Hertz' and the symbol is 'Hz'.
- Dimension is 'S^-1'.
Main practical units in electrical engineering and its symbol.
- Amperes (A) - A unit of electric current (motion of charge 1 coulomb per sec is 1 ampere).
- Joule (J) - A unit of energy.
- Watt (W) - A unit of power.
- Coulomb (C) - A unit of electric charge.
- Volts (V) - The unit of voltage or potential difference.
- Farad (F) - A unit of capacitance.
- Weber (wb)- A unit of Magnetic flux(Ψ).
- Henry (H) - A unit of inductance(L).
- Ohm (Ω)- A unit of reactance, resistance, impedance.
- Tesla(T) - A unit of magnetic flux density(B).
- Watt-hour (Kwh) - A unit of Active power.
- Volt ampere (Kva) - A unit of Apparent power.
- Voltamp reactive(Kvar) - A unit of Reactive power.
- Hertz(Hz)= A unit of frequency.
Explanation of the electrical unit.
- The SI unit of current is 'Ampere' and it is denoted as 'A'.
Ampere (A):- Used to measures the electron or current flow in the conductor.
For example, the current flowing in a circuit is 1A.
- 1A=flow of One coulomb per second
Energy
- The SI unit of electrical energy is Joule.
Joule (J):- Used to measures energy and work.
For example, 1 Joule of work is done by a body of mass 1 Kg when force is applied and 1 joule of energy is spent to complete a work.
- One joule = One watt-sec
Electric Power
- The SI unit of electrical power is the watt.
Watt(W):- Used to denote the rate of doing work (Joules per second). In electrical it is used to measure electrical power.
For example, an electric heater utilizes 2000 joules per second then it means the power utilized by the device is 2000 Watts.
Electrical power(P)=V x I
Electric charge
- The SI unit of electric charge is the coulomb
Coulomb (C):- The standard unit of measuring the quantity of charge that crosses the point in the conductor within 1 sec.
- 1 Coulomb = 6.25x 10^18 charges.
Electric Potential
- The SI unit of potential difference is volt and it is denoted by V.
Volt(V):- The standard unit of measuring potential difference or emf between two charges.
- 5 volt means the energy required to circulate a 1-coulomb charge to the whole circuit.
Electric Capacitance
- The SI unit of capacitance is the farad.
- The value of capacitance is C=2 farads.
Electric or magnetic flux
- The SI unit of flux is weber.
- Change in the flux of one wb/s induces the potential difference of one 1 volt.
Electric Inductance
- The SI unit of electric inductance is henry.
- The capacity of self-inductance of the coil is represented in henry.
Electric resistance
- The SI unit of electric resistance is the ohm.
- A conductor has one ohm(Ω) to oppose the flow of 5 Ampere electric current.
Flux density
- 1 tesla = 1 Wb\sqm
Active power
- The SI unit of active power is Watt-hour.
- The power which actually consumed by a device concerning time is measured. P=V x I x t
Apparent power
- The SI unit of apparent power is Volt-ampere.
- This measures the total power generated at the generating station.
Reactive power
- The SI unit of reactive power is volt amp reactive.
- It simply used to measure a power that doesn't do any real work (current which induces a magnetic field around the coil) or (current that charges a capacitor).