Basic Electrical Theory | Electrical Law | Instruments

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

SERIES CIRCUIT	PARALLEL CIRCUIT
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

P=VI

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

THUMP	direction of the movement of conductor
FORE FINGER	Direction of the magnetic field
MIDDLE FINGER	Direction of the induced current
APPLICATIONS	Electric Generators

Fleming’s left hand rule

THUMP	Direction of thrust (force) on the conductor
FORE FINGER	Direction of the magnetic field
MIDDLE FINGER	Direction of the current
APPLICATIONS	Electric motors

What is Kirchoff's voltage law?

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

∑v=0

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.

Example

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

Where

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

Where

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

1.11

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

Power P = √3 VL IL

Real power = √3 VL IL cos Ф

Reactive power = √3 VL IL sin Ф

Generation

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

Copper

Tin-lead alloy

Silver

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?

ALTERNATING CURRENT AC	DIRECT CURRENT 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