basic electrical, unit-xi

7/28/2019 Basic Electrical, Unit-XI

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Electrical Basics

Charge

Is a fundamental property of matter. Electric charges are of two general types: positive and

negative. Many fundamental, or subatomic, particles of matter have the property of electric charge.

For example, electrons have negative charge and protons have positive charge, but neutrons have

zero charge.

Electric Potential Difference (Voltage)

By definition, the electric potential difference is the difference in electric potential (V) between the

final and the initial location when work is done upon a charge to change its potential energy. In

equation form, the electric potential difference is

V = VBVA =

The standard metric unit on electric potential difference is the volt, abbreviated V and named in

honor of Alessandra Volta.

Work

Char e


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Electric current

An electric current is a flow of microscopic particles called ELECTRONS flowing through wires and

electronic components. It can be likened to the flow of water through pipes and radiators etc. As

water is pushed through pipes by a pump, electric current is pushed through wires by a battery.

Resistance

The electrical resistance of a circuit component or device is defined as the ratio of the voltage

applied to the electric current which flows through it.

The resistivity, and thus the resistance, is temperature dependent. Over disable ranges of

temperature, this temperature dependence can be predicted from a temperature coefficient of

resistance.


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Capacitor

A capacitor or condenser is a passive electrical component consisting of an insulating, or dielectric,

layer between two conductors. When a voltage potential difference occurs between the conductors,

an electric field occurs in the insulator. This field can be used to store energy, to resonate with a

signal, or to link electrical and mechanical forces. Capacitors are manufactured as electronic

components for use in electrical circuits, but any two conductors linked by an electric field alsodisplay this property. The effect is greatest between wide, flat, parallel, narrowly separated

conductors.

Inductor

An inductor is a passive electrical component that can store energy in a magnetic field created by

the electric current passing through it, An inductors ability to store magnetic energy is measured

by its inductance, in units of Henries.

DC Electric Power

The electric power in watts associated with a complete electric circuit or a circuit component

represents the rate at which energy is converted from the electrical energy of the moving charges

to some other form, e.g., heat, mechanical energy, or energy stored in electric fields or magnetic

fields. For a resistor in a DC Circuit the power is given by the product of applied voltage and the

electric current.

P = V I

Power = Voltage Current

Voltage divider circuits

Lets analyze a simple series circuit, determining the voltage drops across individual resistors.


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From the given values of individual resistances, we can determine a total circuit resistance,

knowing that resistances add in series.

R = r1 + r2 + r3

We can use Ohms Law (I= E/R) to determine the total current, which we know will be the same as

each resistor current, currents being equal in all parts of a series circuit.

Current divider circuits

Let s analyze a simple parallel circuit, determining the branch currents through individual

resistors

Knowing that voltages across an components in a parallel circuit are the same, we can fill in our

voltage/current/resistance table with 6 volts across the top row knowing that branch currents add

up in parallel circuits to equal the total current, we can arrive at total current by summing 6 mA, 2mA, and 3 mA.


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Alternating Current

An alternating current (AC) is an electric current of which magnitude and direction vary. That is the

difference to direct current, whose direction remains constant. The usual waveform of an AC power

circuit is a sine wave, because this leads to the most efficient transmission of energy. But in certain

applications there are used different waveforms, such as triangular or square waves.

AC Power

As in the case with DC power, the instantaneous electric power in an AC circuit is given by P= VI,

but these quantities are continuously varying. Almost always the desired power in an AC circuit is

the average power, which is given by:

Pavg = VI Cos

Power Factor

For a DC circuit the power is P= VI, and this relationship also holds for the instantaneous power in

an AC circuit. However, the average power in an AC circuit expressed in terms of the rms voltage

and current is:

Pavg =VI Cos

Where

is the phase angle between the voltage and current. The additional term is called the power factor.

Power Factor = cos =

Power factor is characteristic of alternating current (AC) circuits. Always a value between (0.0) and

(1.0), the higher the number the greater/better the power factor. Circuits containing only heating


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elements (filament lamps, strip heaters, cooking, stoves, etc) have a power factor of 1.0 . Other

circuits containing inductive or capacitive elements (ballasts, motors, personal computer, etc.)

Faradays Law

Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be induced

in the coil. No matter how the change is produced, the voltage will be generated. The change could

be produced by changing the magnetic field strength, moving a magnet toward or away from the

coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet.

Lenzs Law

When an emf is generated by a change in magnetic flux according to, the polarity of the induced

emf is such that it produces a current whose magnetic field opposes the change which produces it.

The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop

constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it.


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If it is decreasing, the induced field acts in the direction of the applied field to try to keep it

constant.

Transformer

A transformer makes use of Faradays aw and the ferromagnetic properties of an iron core to

efficiently raise or lower AC voltages.

It of course cannot increase power so that if the voltage is raised, the current is proportionally

lowered and vice versa.

REVIEW:

Mutual inductance is where the magnetic flux of two or more inductors are linked so that voltage

is induced in one coil proportional to the rate-of-change of current in another.


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A transformer is a device made of two or more inductors, one of which is powered by AC, inducing

an AC voltage across the second inductor. If the second inductor is connected to a load, power will

be electromagnetically coupled from the first inductors power source to that load.

STEP-DOWN TRANSFORMER

The step-up/step-down effect of coil turn ratios in a transformer is analogous to gear tooth ratios

in mechanical gear systems, transforming values of speed and torque in much the same way.

A transformer designed to increase voltage from primary to secondary is called a step-up

transformer. A transformer designed to reduce voltage from primary to secondary is called a step-

down transformer.


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basic electrical, unit-xi

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