Basic Electrical Theory | Electrical Law | Instruments | ETO


    What is the Electricity?

    The invisible energy which constitutes the flow of electrons through a circuit to do work is called electricity.

    What is the Charge?

    It is the smallest particles of an atom which has some property of attraction and some property of repulsion.

    What is Current?

    The flow of electron is known as current or in another word rate of flow of charge is known as current
    It is written by
    I = dq/dt = q/t c/sec ampere (Amp)

    What is the Direction of current?

    In source, the current direction is from lower potential to higher potential and in an element the current direction is from higher potential to potential.

    What is Kirchhoff’s current law?

    Accordingly, the total incoming current to a junction is equal to total outgoing current from that junction of an electric circuit or
     In other word the net current flowing at a junction of a circuit is zero.

    What is the Electric Circuit?

    For communicating or transferring energy from one point to other, we require interconnection of electrical devices
    An electric circuit is an interconnection of electrical elements

    Comparison of series and parallel circuit of the resistor

    Current remains the same through all resistance
    Voltage remains the same across all resistance
    The voltage across each resistance is different
    The current through each resistance is different
    The sum of the voltages across all the resistance is the supply voltage
    The sum of the resistances is the supply current
    The equivalent resistance is
    R1 + R2 +………+Rn
    The equivalent resistance is
    1/R1 + 1/R2 +……..+1/Rn
    The equivalent resistance is the largest than each of the resistances in series
    The equivalent resistance is the smallest of all the resistances in parallel

    What is the Voltage or potential difference?

    Voltage is the energy required to move a unit charge through an element.
    It is denoted by ‘v’.
    Its unit is volt.
    1 volt = 1 joule/coulomb = 1 newton meter/coulomb
    It is also known as absolute potential at the point.

    What is Power?

    Power is the time rate of expanding or absorbing energy
    Unit of power is watts-W

    What is the Potential drop or voltage drop?

    When current flows through an element the voltage produced across that element is known as voltage drop.

    Faraday’s First Law - Electromagnetic Induction

    Whenever a conductor is placed in a varying magnetic field, an EMF is induced. A current is induced when the conductor circuit is closed. This current is named an induced current.

    Faraday’s Second Law - Electromagnetic Induction

    The induced electro motive force EMF in a coil is equal to the rate of change of the flux linkage.

    What is mutual induction?

    When an electro motive force (EMF) is produced in a coil due to the change in current in a coupled coil, the effect is known as mutual inductance.

    What is the Electro motive force (EMF)?

    The force which motivates the electrons to flows in a circuit known as electro motive force.
    Its unit is volt.

    Fleming’s right hand rule

    direction of the movement of conductor
    Direction of the magnetic field
    Direction of the induced current
    Electric Generators

    Fleming’s left hand rule

    Direction of thrust (force) on the conductor
    Direction of the magnetic field
    Direction of the current
    Electric motors

    What is Kirchoff's voltage law?

    Accordingly, sum of voltages in a closed circuit on a closed path is equal to zero.

    What is the Polarity of voltage?

    The polarity of the voltage is determined with respect to the direction of flow of current.
    In a source, if the current direction is from lower potential to higher potential then the source voltage is positive.
    If in a source the current direction is from higher potential to lower potential then the source voltage is negative.
    But in an element the current is always from higher potential to lower potential, so the element voltage is negative.

    What are the Application of Kirchoff's voltage law (KVL) and Kirchhoff’s current law (KCL)?

    KVL and KCL are used to analyze a circuit completely i.e. to determine each branch current, voltage, and power, etc.
    During KCL we consider each and every junction point of a circuit. The junction is a common point where 3 or more elements are joined together in a circuit.
    During the application of KVL, we start from any one point of the circuit and end at the same point.

    Sign convention of power

    In case of source, the power given is always positive
    But in case of element power absorbed by the element is positive but power given by the element is negative.

    Circuit elements
    An element is the basic building block of a circuit. There are two types of elements
    Active element
    Passive element

    What are the types of circuit element?


    What is the Active Element?

    These are the elements that activate on produces current in a circuit.
    A voltage source (battery, shell) 
    And current source (semiconductor devices)

    What is the Practical voltage source and current source?

    These are the sources having some internal resistance practically.
    The internal resistance of a voltage source is connected in series with the source but internal resistance of a current source is connected across the source.

    What is Source Conversion?

    A practical voltage source can be converted to its equivalent practical current source and vice versa.

    What is the Dependent voltage source and current source?

    These sources are also known as controlled sources which depend upon other sources present in a circuit.

    What is the Passive Element?

    These are the receiving element on parameters which help to carry current in a circuit.

    What is the Resistor?

    It is a device which opposes the flow of current.

    What is the Resistance?

    It is the property of a resistor by virtual of which it opposes the flow of current.
    The mathematical expression of resistance is given by ohm’s law.

    Ohm’s law

    Accordingly at constant temperature and pressure the current through a conductor is directly proportional to the voltage across the conductor.
    I V
    Constructional resistance is directly proportional to length of conductor and inversely proportional to the cross-sectional area of the conductor.

    What is Resistivity?

    It is defined as the total resistance offered by a conductor per unit length.

    What is the Inductor?

    It is a device which opposes any change inflow of current.

    What is Inductance?

    It is the property of an inductor by which it opposes any change the inflow of current.
    Unit of inductance is henry (H).
    It is represented by L.
    Symbol is
    Mathematically it is given by L = NФ/I
    N = number of turns of the coil
    Ф = Flux of the coil
    I = current in the coil

    What is the Inductive Reactance?

    It is the property of an inductor by which it opposes the flow of current.
    It is given by; XL = 2πfL = wL
    f = natural frequency
    w = angular frequency

    What is the Node?

    It is the point on point in a circuit where 2or more elements join together.

    What is the Junction?

    It is a point in a circuit where 3 or more elements join together.
    Note: Each junction can be considered as a node but each node cannot be considered as a junction.
    Note 2: Nodal analysis of a circuit is based on the number of junction

    What is the Branch?

    A branch is that part of the network which lies between two junctions

    What is the Loop?

    A loop is any closed path of a network

    What is the Mesh?

    It is a loop and can’t be further divided into other loops

    What is the linear circuit?

    If the parameters of the circuit are independent of voltage and current then the circuit is called linear circuit

    What is the Non-linear Circuit?

    If the parameters of the circuit change with voltage and current then the circuit is called nonlinear circuit

    What is the bilateral circuit?

    A circuit whose characteristics are the same in either the direction of current flow is called a bilateral circuit.

    What is the Super position theorem?

    Accordingly, if two or more sources are acting simultaneously in a circuit, then-current in any branch is equal to the net current following through that branch due to individual sources.
    Note: During consideration of individual sources all other sources in that circuit is considered as zero

    What is the Maximum power transfer theorem?

    Accordingly in an electric circuit the load resistance or required resistance will supply maximum power only when the load resistance is equal to the internal resistance.

    What is Norton’s theorem?

    Any linear bilateral circuit can be converted into an equivalent circuit consist of a current source (Isc = short circuit current) in parallel with the equivalent resistance of the circuit across the required load terminal.
    Note: Thevenin’s circuit and Norton’s circuit are equivalent to each other because using source conversion method Thevenin’s circuit can be converted to Norton’s circuit and vice versa

    What is the Transient Circuit?

    When a circuit condition changes from ON to OFF condition and OFF to ON condition is known as a transient circuit.

    What is the Transient Time?

    The time between ON to OFF condition and OFF to ON condition is known as transient time.

    AC circuit

    The circuit which operates with alternating supply having some frequency with some energy stored element like inductor and capacitor
    Note: DC circuit consists of resistance only but AC circuit consists of all the three elements either in series or in parallel.

    What is Power Factor?

    It is the cosine angle between voltage and current of a circuit
    Power factor = cos Ф
    Power factor is also defined as the ratio of resistance to the total impedance of the circuit

    What is the Single Phase?

    One conductor is considered a single phase

    What is the Three phase?

    If three conductors then considered as three-phase.
    In three-phase AC system the phase displacement between voltage or current of all conductors is given by 360/n = 360/3 = 120 degree

    Principle of single phase AC generation

    The single-phase AC generation is based on the principle of Faraday's law of electromagnetic induction.
    Accordingly when a conductor rotates within a magnetic field and circuit and cut the magnetic flux an EMF is induced on that conductor and is given by
    E = N(dФ/dt)
    Where Ф = Flux link with the conductor
    N = number of turns

    Lenz law

    Accordingly whatever may be the cause of production the induced EMF opposes its initial cause

    What is the Form Factor?

    It is the ratio of RMS value to the average value of an alternating quantity
    Vavg = 2Vm /π
    VRMS = Vm/√2
    Form factor = VRMS/ Vavg

    What is the Crest or Peak or the amplitude factor?

    It is the ratio of RMS value to maximum values of alternating quantity
    Maximum value = Vm
    RMS value = Vm/√2
    Peak factor = 1.414

    What are the advantages of AC?

    AC voltage can be raised or lowered effectively with the help of a device called transformer for different applications but it is not easy for DC
    AC machines are simple in construction and cheaper as compared to DC
    AC sources are easily available

    What is the energy storing element?

    Energy storing elements are inductor and capacitor

    Three-phase star connection

    Line current IL = phase current Iph
    Line voltage VL = √3 Vph

    Three-phase Delta connection

    Line voltage VL = Vph Phase voltage
    Line current IL = √3 Iph


    Power P = √3 VL IL
    Real power = √3 VL IL cos Ф
    Reactive power = √3 VL IL sin Ф


    The production of electrical energy in generating stations using renewable and non-renewable energy sources is known as generation

    What is Transmission?

    The supply of generating electrical energy from generating stations to substations are known as transmission
    It is divided into two stages
    Primary transmission – from generating station to Grid
    Secondary transmission – from the grid to area substations

    What is Distribution?

    The supply of electrical energy from area substations to the consumer is known as distribution
    Distribution is also divided into two types
    Primary Distribution – From area substation to area transformer
    Secondary Distribution – From area transformer to consumers

    What is the Apparent power (S)?

    It is defined as the product of RMS value of voltage V and current I 
    it is denoted by S
    S = VI > VA

    What is the Reactive power (Q)?

    It is defined as the product of the applied voltage and the reactive component of the current
    It is also defined as an imaginary component of the apparent power
    It is represented by Q and it is measured in volt-ampere reactive VAR
    Q = VI sin   VAR

    What is the necessity and advantages of three-phase systems?

    • The output of the three-phase machine is always greater than that of a single-phase machine of the same size. Hence for a given size and voltage, a three-phase machine occupies less space and has less cost compared to a single-phase machine having the same rating
    • To transmit and distribute a given amount of power a three phase system requires less copper than single-phase line
    • Three-phase motors have uniform torque whereas single phase motors have pulsating torques
    • Single-phase motors are not self-starting whereas three phase motors are self-starting
    • The three-phase system give a steady output
    • Single-phase supply can be obtained from three-phase but three-phase cannot be obtained from single-phase

    What is the necessity of the protective device?

    • In an electrical circuit, many types of faults occur which may damage the entire equipment and other devices connected to the faulty equipment’s
    • Excessive high current can cause damage to equipment or to the operator
    • Failure of insulation can cause severe shock to the operator
    • Due to all the reasons, there is a need for various protection schemes and protective devices

    What is the Fuse?

    The fuse is a device which consists of a small piece of metal, which is connected in series with the circuit. When the current through it increases some predetermined value, the metal melts to interrupt the circuit current which protects the circuit from excessive-high current

    What is the Fuse Element?

    The part of the fuse which melts when excessive current flows through it is called fuse element or fuse wire

    What is the Rated current of fuse?

    • It is that maximum current which fusing element can normally carry without any undue overheating or melting.
    • It depends on
    • The temperature rise of fuse contacts of the fuse holder
    • Fusing element material
    • Deterioration of fuse due to oxidation

    What is the Fusing Current?

    The minimum value of the current at which the fuse element melts to interrupt the circuit current is called fusing current. It is always more than the current rating of the fuse.

    What is the Fusing Factor?

    The ratio of minimum fusing current and the rating of the fuse is called fusing factor. Fusing factor is always greater than one

    What is the fusing elements?

    Various fusing elements
    Tin-lead alloy

    What are the Advantages of fuse?

    • The simplest and cheapest form of protecting device
    • Requires no maintenance
    • Operation is automatic
    • Minimum operating time can be made much smaller than that of circuit breaker
    • Inverse time-current characteristic
    • The capacity of clearing high values of short circuit currents
    • Consistent performance

    What are the Disadvantages of fuse?

    • Replacement is necessary after every operation
    • Replacement takes a lot of time
    • They are prone to high-temperature rise

    What is the necessity of earthing?

    • To protect the human being from disability or death from shock in case the human body comes into contact with the frame of any electrical machinery/appliances which are electrically charged due to leakage current or fault
    • To maintain the line voltage constant

    What is the Magnetic Hysteresis?

    The lagging of magnetic flux density (B) behind magnetic field (H) intensity is known as magnetic hysteresis

    What is the Electrical measuring instruments?

    • Indicating instrument – voltmeter, ammeter
    • Integrating instrument – energy meter
    • Recording instrument

    What are the Essential the torque of indicating instrument?

    • Deflecting torque
    • Controlling torque
    • Damping torques

    What are the types of indicating instrument?

    • Moving coil type instrument
    • Moving iron type instrument
    • Dyna momentum type instrument
    • Electrostatic instrument
    • Induction instrument

    What is the Luminous Flux?

    It is the rate of energy radiation in the form of light waves
    Luminous flux = energy radiated/sec
    Unit = Lumen

    What is the Lumen?

    It is defined as the candle power as the luminous flux emitted in a unit solid angle by a source of candle power
    Lumen = candle power x solid angle

    What is Illumination?

    It is the luminous flux received by a surface per unit area

    What is the Transformer?

    It is an electrostatic device which transfers electricity from one AC circuit to another AC circuit at a constant frequency

    Why transformer is an electrostatic device?

    During the operation of the transformer since it remains stationary and no rotating part, therefore, it is known as an electrostatic device.

    Principle of operation of the transformer

    The principle of transformer is based on Faraday's law of mutual induction

    What is the Machine?

    The elements which have moving part or rotating part is known as machine - motor

    What is the Device?

    The element having no rotating part is known as a device - transformer

    What is the Electromagnetic Induction?

    Whenever a conductor cuts magnetic flux a dynamically induced EMF induced in it. The magnetic of induced EMF is directly proportional to the rate of flux linkages.

    What is the Flux?

    The amount of magnetic field around the magnet is represented through the lines of force is called flux. Units are webers represented by 0

    What is the Flux Linkages?

    The extent of interaction between flux and the conductor or conductor and flux

    What is the Difference between AC and DC?

    It is the current of magnitude varying with time
    It is the current of constant magnitude
    It's reverse its direction while following in a circuit
    It flows in one direction in the circuit
    Electrons keep switching directions-forward and backward
    Electrons move steadily in one direction
    It is obtained from AC generator
    It is obtained from a cell or battery or rectifier
    The frequency of AC is 50 Hz or 60 Hz
    The frequency of DC is zero
    The passive parameter is the impedance
    The passive parameter is the resistance only
    Power factor lies between 0 to 1
    Power factor is always 1
    Types: sinusoidal, triangular, square
    Types: pure and pulsating

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