Basic Electrical Engineering – Introduction to Electrical

Basics of Electricity

Electricity is a form of energy in which electron flows in a conductor from higher to lower potential in the same manner as water flows from higher to lower level. The flow of electricity can be detected by one or more of the following effects:-

(a) Heating EffectA flow of electricity will generate heat in the conductor through which it flows. This property is madeuse in lamps, Electric heaters etc.

(b) Chemical EffectA flow of electricity through certain solution will split the solution into its various constituents. This property is made use of electroplating, Electrical batteries.

(c) Magnetic EffectA flow of electricity through a conductor causes a magnetic field to be setup around the conductor. This property is made use in generators, motors, relays, solenoids and actuators.

(d) Physical EffectOur body is a good conductor. Current can easily pass through it.  When the body touches a conductor carrying current, current flows through our body to ground (Earth) and body experiences a shock.

The following are few sources of electricity:

(a) BatteryThis stores electrical energy in the form of chemical energy and when required it gives off electrical energy. Batteries are used as a power source for various electrical appliances like radios, cell phones etc.

(b) GeneratorIt is a machine, which converts mechanical energy into electrical energy i.e., when the machine is driven by a prime mover, it produces electrical energy. This is one of the ways of generating commercial power.

(c) Thermo CoupleTwo dissimilar metallic wires joint together at one end develop an EMF when there is temperature difference between their junction points. This is used to read temperature of jet engine and furnaces. e.g., hot wire meter.

(d) Solar EnergyThe energy obtained from the sun is called solar energy. It is a natural source of energy available in abundance.  Solar cell or a photo voltaic cell is used to convert solar energy into electrical energy by making use of photo voltaic effect. Devices which use solar energy to produce electricity are solar cell which are used commonly in calculators, watches, etc.

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Current. The free movement of electrons in a wire of conducting material like copper is known as electric current. An electric current usually termed as ‘current’ is nothing but flow of electrons or a flow of charge from higher potential to lower potential. If a charge Q passes across a conductor in time T, and current I is said to have flown then:

I           =          Q/T

Where,

Q         –           Charge measured in Coulombs

T          –           Time in sec

Hence the current I is measured in Coulombs per second

Thus, Ampere = Coulombs/ second

ResistanceThe resistance of a conductor is the opposition offered to the flow of current through the conductor.  The unit of resistance is Ohm. The temperature remaining constant, the current flowing through a conductor is directly proportional to the potential difference between the two ends of the conductor.  V / I = R where R is a constant, known as resistance of the conductor.

(a) Ohm.  It is that resistance, which permits a current of one ampere to flow when a potential difference of one volt is applied.

(b) Resistance of a conductor is directly proportional to the length of the conductor and inversely proportional with its cross sectional area and specific resistance of the conductor.

Voltage. Electrical pressure, which causes an electric current to flow through a complete electrical circuit or the force which results from the conversion of heat, chemical or mechanical energy into electrical energy is known as the Electro motive force (EMF).Unit of EMF or potential difference (PD) is volt.  The PD between two points in an electric field is said to be one volt if one joule of work done in moving a coulomb of charge from one point to the other. Relationship between Q and V is:

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V         =          Q/C

where C is the constant known as Capacity and V is the potential difference.

Capacitance.  It is the ratio of charge to potential. It is defined as the charge required raising its potential to unity.  Unit of capacitance is Farad. A capacitor is a device, which increases the capacity without increasing the dimensions in an apparatus for condensing a large quantity of electricity on a comparatively small surface. It consists of two conducting surface divided by a layer of an insulating medium by dielectric. The conducting surface may be in the form of either circular (or rectangular) plates or spherical or cylindrical shapes. The purpose of capacitor is to store energy by electrostatic stress in di-electric.

InductanceThe property of the coil due to which it opposes any increase or decrease of an alternating current or flux through it is known as inductance. The unit of inductance is Henry and denoted by capital letter “L”.

Resistance in SeriesWhen two or more resistors are connected in series, the total resistance of the circuit will be the algebraic sum of individual resistors. Let three resistors R1, R2 & R3 are connected in series. The total resistance is:-

R         =          R1+R2+R3

Resistance in ParallelWhen the resistors are connected in parallel, the inverse of their effective resistance is equal to the sum of the inverses of individual resistance. Let R1, R2and Rare connected in parallel as shown in image below.

Then,     1/R  =          1/R1+1/R2+1/R3

Series and Parallel Resistance in CombinationA circuit in which some resistances are joined in series while others are joined in parallel is known as series parallel combination.

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Transformer

Transformer is a static piece of device by means of which electric power in one circuit is transformed to electric power of same frequency in another circuit. A transformer is either step-up or step down type. It consists of three main parts:

(a) An Iron core which provides a circuit of low reluctance for an alternating magnetic field created by

(b) Primary winding which is connected to the main power source.

(c) A secondary winding which receives electrical energy by mutual induction from the primary winding and delivers it to the secondary winding.

When an alternating voltage is applied to the primary winding an alternating current will flow and by self-induction it will establish a voltage in the primary winding, which is opposite and is almost equal to the applied voltage. The difference between these two voltages will allow just enough current to flow in the primary winding to setup an alternating magnetic flux in the core. The flux cuts across the secondary winding and by mutual induction a voltage is established in secondary winding.

Mutual Induction.   It is the ability of a coil or circuit to produce EMF in near by coil by induction, when the flux in the circuit of the first coil changes. This can be reciprocal transformer.

The following are the types of transformer:

(a) Step up

(b) Step down

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