02 introduction to electricity.ppt
TRANSCRIPT
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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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Soln of Example Problem-1
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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Soln of Example Problem-1
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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Soln of Example Problem-1
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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Soln of Example Problem-2
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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Soln of Example Problem-2
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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Soln of Example Problem-2
We could have alsofound this using
conservation of energy:
10 SP P
10 10W P 10P W