**Basic Electrical Definitions and Laws for Electrical Students**

**Basic electrical definitions and laws…**

**Introduction**

An electric circuit means a source of supply of electric current. We can generate electricity through batteries and generators. We use power from these sources and illuminate our house, office, in the industry to drive motors, heating of furnace, and many other applications.

In an electric circuit voltage, current and frequency are the major quantities. In this topic, we will study **Basic electrical definitions and laws** which is very helpful for students to revise them.

**Conductor**

Conductors are substances that offer a flow of electricity or current (or flow of electrons) very easily. It is the length of wire. In these substances, there is very small resistance (almost negligible amounts) which opposes the flow of electricity.

Examples- Copper, Aluminum, Silver, Gold, Iron, Electrolyte, etc.

**Insulator**

Those substances have very high resistance and there is practically no electricity to flow through the circuit.

Examples- Rubber, Asbestos, Bakelite, Mica, Ebonite, etc.

**Coil**

It has one or more turns usually circular or cylindrical form which carries current and produces a magnetic field.

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**Dielectric strength**

It is the property of the substances which gives the ability of substances to withstand electricity at high voltages. It is measured in terms of the potential difference per unit area (kV/mm).

High withstanding voltage means when we applied voltage to the thickness (say 1 mm) of insulating material. it will not break down its insulation. But if we further increase voltage beyond its operating limits its insulation will break and a spark is produced.

A material having high dielectric strength can withstand high operating voltages.

Example- The breakdown voltage of air, mica, rubber, porcelain, and Teflon are 3.2 kV/mm, 12 kV/mm, 20 kV/mm,15 kV/mm, and 50 kV/mm respectively.

**Alternating Current(AC)**

AC means alternating current. It is defined as the voltage or current wave which alternates its direction and magnitude during positive and negative half-cycles in a certain period of time. Its direction and magnitude change in positive and negative half-cycles. One complete cycle of the waveform is known as the full ac waveform.

It reaches its peak value in the positive and negative half cycle of the complete waveform in the ac circuit.

**Also Read:- what is electricity**

**Direct Current(DC)**

It means direct current which is further defined as that current that has uniform magnitude and flows in one direction. It does not break or reach zero value like ac current.

**Also Read:- what is battery**

**Waveform**

The electrical quantities (voltage or current) are represented in the cycle in graphical form which shows the amount of variation in amplitude over a certain period of time.

**Peak value**

The maximum amplitude of ac quantity reaches its maximum value during a given period of time.

**Instantaneous value**

The value of the alternating quantity at any instant is known as the instantaneous value. Its value is i1, i2, i3….

**Average value**

The average instant value of instantaneous current in one-half cycles is known as an average value.

Suppose, the instant value of current during half cycle is i1, i2, i3….In

Then average value=i1+i2+i3+…..i _{n}/n

Its value is 0.636 I max.

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**RMS value **

It’s meant for root to mean square value. The value ac current is expressed in terms of RMS value. It is defined as the steady dc current (direct current) which produces the same amount of heat as that produced by an alternating current.

It is also called the virtual effective value of ac quantity.

I _{rms}=0.707 I _{max}.

V _{rms}=0.707 V _{max.}

**Form factor**

It is the ratio of RMS voltage to the average value of ac circuit.

Form factor= rms value/ average value=1.11

**Phase **

It is the angular displacement between two or more alternating quantities.

**In phase**

If two or more two alternating quantities reach their maximum or minimum value simultaneously in the same direction and at the same time is called in the phase of ac quantities.

**Out of phase**

If two or more than two alternating quantities reach their maximum or minimum value at different times but the phase angle between them is the same is called out of phase

In three phases balanced system voltage or current waveform are equal in magnitude and displaced from each other by a phase angle of 120 degrees apart.

**Balanced AC system (3 phase)**

A three-phase ac **balanced** supply consists of three-phase voltages or currents that are equal in magnitude and displaced from one another by 120 degrees apart.

In this balanced load (current) system impedance in each phase is equal so that their phase current will be equal in magnitude and displaced by 120 degrees.

Moreover, a balanced three-phase ac supply has equal values of active power and reactive power in each of the three-phase as well as load.

In **unbalanced** three phases, the magnitude of voltage and current waveform in each -phase may not be equal or the phase may not be 120 degrees.

**The phase sequence of AC waveform (3 phase)**

It is the order in which three-phase of voltages (or currents) gain their maximum value in the positive and negative half cycle is called phase-order or phase-sequence.

Let us suppose the voltage of phase “A” attains its maximum value first, after 120 degrees the voltages of phase “B” reach its maximum value and after another 120 degrees further reaches its maximum value. So, the phase sequence is ABC.

Phase sequence in the electrical system is mostly denoted by the color code of R (RED), Y (yellow), and B (blue).

**Magnet**

The substance having the property of attraction or repulsion of iron and its alloy is called a magnet. It has two poles north (N) and South Pole(S). The same pole of two magnets repel each other and different poles attract each other.

**Magnetic pole**

It is that region, where the external magnetic effects of a magnet are concentrated at a point or the point where the strength of the magnet has a maximum value.

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**Electric circuit**

The closed path for electric current has switches, resistance ( in ac circuits also have inductance and capacitance), and the power source is called an electric circuit.

**Magnetic circuit**

The closed path for magnetic flux is called a magnetic circuit. It is the continuous path followed by magnetic lines of force or magnetic flux.

**Electric field**

The region around a charged conductor within which a force is exerted on another charged conductor is called an electric field.

**Magnetic field**

The area around a magnetic coil which behaves like a magnet and produces forces that attract or repel the metals towards or away from it is called the magnetic field.

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**Line of force in a magnetic field**

It indicates the existence of a magnetic field in the neighborhood of the magnetic pole. These are considered to emanate from N (north) pole, traversing the surrounding medium and re-enter the S (south) pole, and again passes through the magnet from the S pole to the N pole.

*These lines of force must form a closed loop*.

**Magnetic Flux **

In a magnetic field, there is a number of line of force which is taken together and is given the name flux or magnetic flux. It is represented by the symbol ф. Magnetic flux is analogous to current in an electric circuit. Its unit is Weber.

ф= MMF/reluctance Wb

**Magnetic flux intensity**

If a magnetic circuit has a homogeneous and uniform cross-section area; the magnetic force per unit length of the magnetic circuit is related to the terms magnetic field intensity. It is represented by the term H. Its unit is ampere-turns/meter or ATs/m

H=I N/l, where I is current in ampere, N is no. of turns of the coil is the length of wire.

**Magnetic flux density**

It is defined as the ratio of the number of lines of magnetic flux to the cross-section area of the core of the magnet. It is denoted by B. its unit is Weber /m² or tesla (T).

B= ф /A, where ф is magnetic flux, A is the cross-section are of the core of the magnet.

**Magnetic induction**

The magnetic pole is produced in a piece of metal by the influence of another magnet in contact or at a certain distance is called an induced poles and this phenomenon occurs is known as magnetic induction.

One thing is noted is that the end of metals of the nearest pole has a polarity opposite to that of the magnet pole.

**Leakage current**

The electric current through a circuit that does not follow the designed path is called leakage current. This leakage current is ineffective for the desired use.

**Leakage flux**

The flux in a magnetic circuit that does not follow the designed path which is ineffective for the desired use of it is called leakage flux or magnetic leakage.

**Inductance**

It is the property of the magnetic circuit which produces an induced emf in-circuit itself by changing current.

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**Self-induced emf**

This phenomenon is felt when there is changing in current in the coil. When we give supply to the coil, flux is produced in it which changes with time. This changing of flux causes an induced emf in the coil itself according to Faraday’s law of electromagnetic induction. This emf induced is always opposed to the applied voltage (or supply). This induced emf is called counter emf or self-induced emf.

**Mutual induction**

It is the property of electromagnetic induction produced by one circuit to another nearby circuit due to changing the flux of the first circuit.

Or,

When two coils or circuits are kept near to each other and supply is given to anyone circuit, it is seen that current is flowing in the first coil which causes changing of flux. If the second coil is placed near to it, it causes flux linkage to it due to the first coil. This causes induced emf or induced current in the second coil also.

The creation of emf in the second coil is due to variation of flux and hence current in the first coil. This phenomenon of induction in the second coil due to the first coil is known as mutual induction.

**Reluctance**

The opposition offered by the magnetic circuit which opposes the magnetic flux is called reluctance. It is just like resistance in an electric circuit.

It is represented by S or Rm =L/(μ.A) **AT/Wb or ampere-turns per weber or (henry) ^{-1}**

Where L= length of the magnetic path

A= Cross-section area is normal to flux, in m².

μ=μ_{0 }μ_{r}= permeability (or absolute permeability) of magnetic material.

μ_{r}=relative permeability of magnetic material.

μ_{0}=permeability of free space=4π x 10-7 H/m.

Moreover,

We know resistance, R=ρ L/A= L/ (1/ρ). A

Where ρ is the specific resistance or resistivity of the material, L is the length of the cylinder and A is the cross-section area.

**Reciprocal of resistivity(ρ) is known as conductivity (σ)**

So Reluctance Rm=Rm =L/(μ.A) and Resistance R= L/ (1/ρ). A

Comparing these two we find that, μ=1/ρ

This means permeability is analogous to the reciprocal of resistivity or conductivity (1/ρ).

**Permeability**

It is the ratio of the magnetic flux produced by a given magnetic force of the medium or material to the magnetic flux that is produced by the same magnetic force in a perfect vacuum is called permeability.

Or it is the ratio of flux density to flux intensity. It is denoted by symbol μ

Or, μ= B/H, it has no unit.

**Residual magnetism**

It is the magnetism which remains in the material even after the removal of an external magnetic field is called residual magnetism.

**Magnetomotive****force (Mmf)**

As we know that current is produced in electric circuits due to the presence of electromotive force. Similarly in the magnetic circuit, the magnetic flux is produced due to the presence of magnetomotive force.

This MMF is created in the magnetic circuit by a current flowing through one or more turns.

Mmf=N .I ampere-turns.

**Faraday’s law of electromagnetic induction**

**First law**

Faraday’s law of electromagnetic induction tells that an emf is induced in a coil (or conductor) due to changing of magnetic flux linking with this coil with changing of time. It is measured in volt.

This law concluded that an emf is induced in a coil or conductor can be obtained by any of the following methods:-

(I)Magnetic flux is constant, but the coil or conductor is set to move within magnetic flux. In this case, the conductor cuts the magnetic flux which varies with time. The induced emf generated is known as motional emf or rotational emf.

(ii)Coil or conductor is stationary but dc electromagnet (or permanent magnet) is rotated. In this case, flux cutting action is generating induced emf which is also moving or rotational emf.

(iii) Coil is stationary but the flux passing through the coil changes with time and this flux is linked with the other coil of the same core. The induced emf generated is used transformer from this principle seen in the transformer.

**Second law**

This law states that the magnitude of the induced emf generated is directly proportional to the rate of changing of flux linkage with the coil.

So, E =N dф/dt= dΨ/dt

Where E= emf induced in volt, N= number of turns in the coil, Ψ= N.ф= flux linkage with the coil in Wb-turns, t= time in second.

**So, overall we can define Faraday’s law as:-**

Whenever any conductor is set to rotate in the varying magnetic fields, it cuts the magnetic line of forces or flux, then emf will be induced in that conductor. If the conductor is closed, there is an induced current in the conductor.

Or, an emf is induced in the conductor when the magnetic flux linking with the coil changes with time.

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**Lenz’s law**

This law states that the induced emf in an electromagnetic circuit causes an induced current when the circuit is closed and this induced current or induced emf always opposes its cause. Here the cause changes in flux linking with the coil which cause induced emf or current according to Faraday’s law.

So, E= (**–**) N dф/dt= (-) dΨ/dt

Here, the negative sign shows the opposition to the cause produced.

**Fleming left-hand rule**

If we stretched the thumb, middle finger, and forefinger of our left hand, and these fingers are kept perpendicular (90 degrees) to each other and the current-carrying conductor is placed in a magnetic field then the conductor experience a force which is known as a magnetic force.

Note- Here, the thumb points in direction of magnetic force, the middle finger in direction of the current, and the forefinger points in the magnetic field respectively.

This rule is applied to electric motors.

**Fleming right-hand rule**

If we stretched the thumb, middle finger, and forefinger of our left hand, and these fingers are kept in perpendicular (90 degrees) to each other and the conductor is forcefully brought into the magnetic field, there will be an induced current in that conductor.

Note- Here, the thumb points in direction of motion of the conductor, the middle finger in direction of induced current, and the forefinger points in magnetic field respectively.

This rule is applicable to an electric generator.

**Coulomb’s law**

This law is also called coulomb’s inverse square law. This law gives the amount of force between two stationary point charges.

According to this law, if two stationary point charges are placed at certain distances, the force of attraction or repulsion is directly proportional to the product of point charges and inversely proportional to the distances between them.

F= 1/4πε.Q1.Q2/r^{2}

Where 1/4πε=9.0x 10 ^{9} N-m^{2}/C^{2}

**Gauss’s law**

This law states that total electric flux through a closed surface is equal to the total charges enclosed by that surface divided by the permittivity.

Ф_{E}=Q/ε, where, Ф_{E} is electric flux, Q is total charge enclosed, and ε is permittivity.

Also, ε= ε_{0} ε_{r} for air, ε_{r}=1 and ε_{0}=8.854x 10^{-12} F/m (farads/meter)

∫E. ds=Q/ε

The electric flux through an area is defined as an integral part of the multiplication of electric field intensity and area of a closed surface.

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**Synchronous speed**

It is the speed of the rotating magnetic field which is produced when polyphase supply is applied to polyphase winding. Synchronous speed is represented as N _{s}

So, N s= 120f/P rpm.

f= frequency of supply, P=number of pole

So, synchronous speed depends on the frequency of supply and the number of poles of polyphase winding.

**Transformer**

The transformer is a static device that transforms electric power (voltage and current) from one circuit to another circuit without changes in frequency.

**Working principle**

It is based on mutual induction between two circuits linked by the common magnetic flux of the same magnetic core. A transformer consists of two inductive coils (also primary and secondary winding) which are electrically separated but magnetically linked through a path of low reluctance.

If we are given the ac (alternating) source to one side of the coil, an alternating flux is set up in the laminated core, which is mostly linked with another coil in which an emf is induced according to Faraday’s law of electromagnetic induction.

If the circuit is closed, the current is flowing through the circuit and so electrical energy is transferred from the first coil (winding) to the second coil (winding).

The main function of a transformer is to increase or decrease the electrical quantities (voltage and current) from one circuit to another circuit. It works on the principle of mutual induction between two circuits linked by a common magnetic flux.

**It has two types according to transformation:-**

**Step up transformer:-**When the output supply (voltage) is greater than the input supply (voltage), it is called a step-up transformer.

**Step down transformer: –** When the output supply (voltage) is lower than the input supply (voltage), it is called a step-down transformer.

**Note-** If HV winding is connected to an ac (alternating) supply, it becomes a step-down transformer in which HV winding acts as the primary winding.

Similarly, if the LV winding is energized from an ac (alternating) supply, it acts as a step-up transformer having the LV side as the primary winding.

Transformer ratio:-

V1/V2=N1/N2=E1/E2=I2/I1=K (voltage transformer ratio), where V1, I1, N1, and E1 are voltage, current, number of turns, and induced emf respectively on primary sides.

Similarly, V2, I2, N2, and E2 are voltage, current, number of turns, and induced emf respectively on the secondary side.

The RMS value of induced emf on the primary and secondary side of wingding is

E=4.44 f N ф=4.44 f N B A, where N, f, B, and A are the number of turns, frequency, flux density, and area.

**Induction motor**

Induction motors are most commonly used today. It is mostly used in industries, homes,s, and other fields. The induction motor is also called the asynchronous motor.

It also works on the principle of mutual induction according to Faraday law.

**Working principle**

When three-phase stator winding is given by a three-phase ac supply, a magnetic flux of constant magnitude but rotating at synchronous speed is set up in the stator. This flux passes through an air gap between the stator and rotor of the induction motor and cuts the rotor conductor which is still stationary.

Due to relative motion between rotating flux and stationary conductor, an emf is induced in rotor according to Faraday law of electromagnetic induction. As rotor bars or conductor is short-circuited at the end ring.

Current is induced in it whose direction is given by Lenz’s law.

According to this law induced voltage or current opposes the cause produced. In this case, the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductor. Hence to reduce the relative speed, the rotor starts rotating in the same direction as that of flux.

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**Alternator or Generator**

The machine uses in power plants to generate power alternating or sinusoidal in nature) is called an alternator or generator.

It has two winding i.e. field winding and armature winding.

**Working principle**

It also works on the principle of Faraday’s law of electromagnetic induction. When dc (direct current) supply is given to the exciter of the field winding (stator), a magnetic field is generated in it.

As the rotor (armature on it) on the generator shaft rotates under it, an emf (electromagnetic force) is induced in it according to Faraday’s law. So induced current and voltage generated on it.

This induced emf (ac) is converted into dc (direct current) through a rotating diode. This dc voltage is then given to the field winding of the rotor on the same shaft of the generator rotor which is known as the main field winding of the generator. Now it produces a magnetic field on the field winding.

Now the situation is reversed and the armature conductor (on the generator stator) cuts the magnetic field of the field winding under the main pole of the generator. So an emf is induced in the armature (stator) of the generator. This induced voltage causes induced current which is used for electricity.

The power in the generator per phase, P=1.737 V _{ph} I _{ph} cosф. This induced emf is given as:-

E=4.44 f N_{ph} ф K_{w}

Where, E_{ph}= no-load generated emf per phase or excitation emf E_{f,} f=rotational frequency, N_{ph}=series turns per phase= flux per pole, K_{w}=winding factor=K_{c.} k_{d} (Kc is pitch factor) and K_{d} is distribution factor.

In generator, E_{f}= V+ Ia (r_{a}+j X_{s})

Where V**t**= Terminal voltage, a = armature current, r_{a} = armature resistance, X_{s}= synchronous reactance.

*The main purpose of writing on this topic of some basic electrical definitions and laws for the students to make revisions and get benefits from it.*

*Also Read:- what is power factor*