02 INTRODUCTION TO ELECTRICITY.ppt

8/10/2019 02 INTRODUCTION TO ELECTRICITY.ppt

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Basic Concepts ofElectricity


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The Atom

An Atom Consists of

very small small

particles called

Nucleus. WhichConsists of

Protons

Neutrons

Electronsmoves

around the nucleus in

a path called orbit .


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Charge

If an atom has more protons than

electrons, it is Positively charged

If an atom has more electrons than

protons, it is Negatively charged


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Charge

Charge is an electrical property of

atomic particles

Measured in Coulombs


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Charge

Particle Designation Charge (C)

Proton Positive 1.6 X 10-19C

Neutron Neutral 0 C

Electron Negative -1.6 X 10-19C


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Electrons

A very small, elementary particle

Electrons are called charge carriers

Mass of 9.11 1031kg

Charge of -1.6 X 10-19C


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Holes

Holes are also considered charge

carriers

Holes are locations of positive charge

Rather than being moving protons,however, they are basically the absence of

electrons


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Static Electricity

Electrons may be able to move around

between atoms.

When atoms are either stationary or

moving very slowly relative to each

other, we call this electrostaticsor StaticElectricity.


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Static Electricity can be

Produced by:-

Rubbing your feet on a carpet.

Rubbing your hair with a balloon.

Rubbing your hair with a Cumb.

Lightning in rainy seasons.


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Static Electricity Rule:-

Objects with like charge tend to repel each

other.

Objects with different charge tend to

attract each other.


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Like Charges Repel each

other. In which direction the rod on the string will

move?


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Opposite Charges Attract each

other.

In which direction will the rod on the string

swing?


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Static Electricity

Piece of Metal


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Moving Electrons

Electrons can move around between

atoms in one object or two

Static Electricity

Electrons can move in a constant direction

at a constant rate

Current


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Current and voltage are the twobasic variables in electric circuits

Electrical Variables:-


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Current

Electric currentis the flow of electronic

charge in a conductor


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Current

Electrons can only move freely around

some atoms.

Their movement depends on the type of

the atoms.


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Conductors

What is a Conductor??

Substance in which electrons move freely is called

Conductors.

Example of Conductors.

Metals: silver, copper, aluminum.

Salt water.


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Insulators

What is an Insulator???

substance which prevents electrons from

moving.

or allow electrons to flow in small amounts.

Example of an Insulator???

Wood.

Plastic.

Rubber.


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Semiconductors

What is a Semiconductor??

Substance which allows electrons to flow

under some certain circumstances. Or In a special Case

Example of Semiconductors.

Silicon Germanium


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Current

Current can only flow when a circuitis

completed

An electric circuitis an interconnection of

electrical elements


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Current

The time rate of change of charge

Number of electrons (or holes) that pass a

single point in one second

dq qI

dt t


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Current

Measured in Amperes (A) = C/s

Note that we dont talk about electrons/sec

this would be a reallylarge number

Is represented by the variableIor i


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Example Problem

If 465 C of charge pass through a wire in

2.5 minutes, find the current in Amperes.


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Current Direction

Current direction can be taken in thedirection that holes are moving.


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Current Direction

Wire


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Current Direction

Holes


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Current Direction

Holes Electrons


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Current Direction

Holes Electrons

CurrentDirection


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Current Direction

Current is actually the averageforward

movement of all electrons


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Direct Current

If current does not change with time, but

remains constant, we call it direct current

(dc)

A current that varies with time is called

alternating current (ac)


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Direct Current

Alternating Current

When a current is constant with time,

we say that we have direct current,

abbreviated as dc. On the other hand,

a current that varies with time, reversing

direction periodically, is called

alternating current, abbreviated as ac.


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.


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Direct Current (DC)

Electron

Flow

Hole

Flow10 5 0 5 10

6

4

2

0

2

4

66

6

I

1010 t


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Alternating Current (DC)

10 5 0 5 106

4

2

0

2

4

66

6

I t( )

1010 t


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Directions for Current


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Voltage

The voltageassociated with a circuitelement ,is the energy transferred per

unit of charge that flows through theelement.

The units of voltage are volts (V), whichare equivalent to joules per coulomb(J/C).


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Voltage

That makes current flow?

Voltageis the push to move electrons Also called potential difference

Measured in volts


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Voltage

Potential difference can be a result of:

Voltage sources, like batteries

Build up of static electricity


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Voltage

The voltage is measuredacross an element

Voltage is represented bythe variable Vor v

The plus (+) and minus (-)

signs are used to define areference direction orvoltage polarity


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Voltage

The vab

can be interpreted

in two ways:

1. Point ais at a potential ofv

abvolts higher than point b

2. The potential at pointawith respect to point bis v

ab


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Voltage

Inverting the voltage polarity

changes the sign of the

voltage

ab bav v


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Voltage

Both (a) and (b) represent the same

voltage


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Voltage

In (a) point ais +9V above point b

In (b) point bis -9V above point a


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Voltage

There is a 9-V voltage dropfrom ato b

There is a 9-V voltage risefrom bto a


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Voltage

A voltage drop from ato bis equivalent to

a voltage rise from bto a


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Voltage Sources

Convert some other form ofenergy into electrical energy

Potential Difference existsacross the two terminalsregardless of whether they areconnected

Current is induced only whenthey are connected

+

-


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Power

For practical purposes, we need to knowhow much powera device can handle

A 100-watt bulb gives more light than a60-watt bulb

Our bills to the utility companies charge usfor the electric energyconsumed over themonths time


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Energy

Energy is the capacity to do work

There are lots of different kinds ofenergy

Electrical

Chemical Heat

Nuclear


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Energy

Is always conserved

No energy is ever created it is only converted

from a different type of energy

No energy is ever destroyed, it is converted

into a different type of energy


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Energy

Motors and generators convert energy

Mechanical

Energy

Electrical

Energy

Generator

Motor


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Power

Poweris the rate of expending or

absorbing energy

Measured in Watts (W)


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Power

Depends on the current and voltagethrough a circuit element

P = I X V

The power absorbed or supplied by an

element is the product of the voltageacross the element and the currentthrough it


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Power

Voltage and current sources supplyor putenergy into a circuit

Circuit elements (resistors) absorbenergyand transform the energy into anotherform


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Power

In general,

+ Power absorbed = - Power supplied

The Law of Conservation of Energymust

be obeyed in any electric circuit

The sum of power in a circuit must be zero


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Summary

An electric circuit consists of electrical

elements connected together

Current is the rate of charge flow

Voltage is the energy required to move

charge through an element

Power is the energy supplied or absorbed

per unit time and is equal to I X V


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PROBLEMS


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Example Problem-1

The circuit below has five elements. If p2= 100

W, p3= -20 W, p4= -60 W, p5= 20 W. Calculate

the power p1received or delivered by element 1.


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Soln of Example Problem-1

p2= 100 W, p3= -20 W, p4= -60 W, p5= 20 W

We must approach this problem withconservation of energy.


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p2= 100 W, p3= -20 W, p4= -60 W, p5= 20 W

Sum of individual powers must be equal to zero.


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p2= 100 W, p3= -20 W, p4= -60 W, p5= 20 W

1 2 3 4 5 0p p p p p


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p2= 100 W, p3= -20 W, p4= 60 W, p5= 20 W

3 40p W


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Example Problem-2

For this voltagesource, how much

power is supplied to

the circuit?

What is the power

absorbed by the

circuit element?


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Power absorbed by thevoltage source:

According to passive sign convention,

should the power be positive or

negative?

P I V

5 2P A V

10P W

10P W


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Follow the same processto find power absorbedby the element (P1):

According to passive sign convention,should the power be positive ornegative?

P I V

5 2P A V

10P W

10P W


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We could have alsofound this using

conservation of energy:

10 SP P

10 10W P 10P W

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02 INTRODUCTION TO ELECTRICITY.ppt

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