ch1 introduction 2009

8/11/2019 Ch1 Introduction 2009

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2140201 Electrical Circuits2140201 Electrical Circuits

Chedsada ChinrungruengThavatchai Tayjasanant

Department of Electrical Engineering

Chulalongkorn University


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Chapter 1: IntroductionChapter 1: Introduction

1.1 Circuits, Current, and Voltage

1.2 Power and Energy

1.3 Kirchhoffs Laws

1.4 Circuit Elements

1.5 Summary


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What Happens in Headlight Circuit?What Happens in Headlight Circuit?

1. Chemical forces in the battery cause electricalcharges to flow.

2. The charges flow in the wires until they reachthe headlamps.

3. The tungsten filaments in the headlampsbecome hot and emit the light.

4. The switch is used to control the flow ofcurrent.


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1.1.1 Electrical Circuits1.1.1 Electrical Circuits

An electrical circuit consists of various typesof circuit elements connected in closed

paths by conductors.


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1.1.2 Current1.1.2 Current

Current is defined as the flow rate of chargesthrough a conductor or a circuit element.

( )( )

dq ti t

dt

=

The units are amperes (A).


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Reference Direction of CurrentThe actual direction is not known,

and the reference direction is arbitrarily

selected.

After current values are calculated,the actual direction will be known.

If the value is negative,the assigned direction is wrong.

That is, the actual direction is opposite tothe reference initially selected.


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Current Notations:

Use arrow to indicate the direction.

Use double-subscripts to define thereference direction, e.g. from a to b.


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DC versus ACDC versus AC

Direct Current (DC)Constant magnitude with time.

Alternating Current (AC)Magnitude varies with time.


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1.1.3 Voltage1.1.3 Voltage

Voltage is defined as the energy transferredper unit of charge that flowsthrough the element.

The units are volts (V),or joules per coulomb (J/C).


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Voltage PolaritiesVoltage Polarities

Polarities: direction of energy flow.

Positive charge moves from + through theelement toward ,

indicating that the element absorbs energy.


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Voltage Notations:Voltage Notations:

Use double-scripts to definethe reference polarities,

e.g. between points a and b.


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1.2 Power and Energy1.2 Power and Energy

Current: Rate of change of charges.

Voltage: Energy transferred per unit of charges.

Current Voltage = Rate of energy transferred.

The units of power are joules/second or watts.


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1.2 Power and Energy1.2 Power and Energy

Positive power means energyis being absorbed by the element.

Negative power means the elementis supplying energy.

( )t

tw p t d t =

2

1

Energy (w) can be calculated by integrating

power:


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1.3 Kirchhoff1.3 Kirchhoffs Lawss Laws

1. Kirchhoffs Current Law (KCL)

2. Kirchhoffs Voltage Law (KVL)


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1.3.1 Kirchhoff1.3.1 Kirchhoffs Current Laws Current Law

The algebraic sum of the net currententering a node = 0.

The algebraic sum of the net currentleaving a node = 0.

The algebraic sum of the currentsentering a node equals the sum of thecurrents leaving a node.

I = 0

Node: a point at which two or more

circuit elements are joined together.


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i i i+ =1 2 3 i i=3 4 i i i+ + =5 6 7 0

Kirchhoffs Current Law (KCL)


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Find current values using KCL


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1.3.2. Kirchhoffs Voltage Law

V = 0

For any closed path (loop) in an electrical circuit,the algebraic sum of the voltages along the path

equals zero.

Loop: a closed path starting at a node andproceeding through circuit elements, eventually

returning to the starting point.


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Voltages are added or subtracted depending on

their reference polaritiesrelative to the direction of travel around the loop.


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a b cv v v + + = 0

c d ev v v + = 0

a b d ev v v v + = 0


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Find voltage values using KVL


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1.4 Ideal Circuit Elements1.4 Ideal Circuit Elements

1. Conductors

2. Voltage sources

3. Current sources

4. Resistors


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1.4.1 Conductors1.4.1 Conductors

The voltage between the ends of an idealconductor is zero regardless of the currentflowing through the conductor.

Ideal conductor = Short circuit

All points that are connected by idealconductors = Single node


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1.4.2 Voltage Sources1.4.2 Voltage Sources

An ideal independent voltage source canmaintain a specified voltage across itsterminal.

Voltage can be constant or a function oftime.


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1.4.2 Voltage Sources1.4.2 Voltage Sources

An ideal dependent or controlled voltage sourcehas the voltage across the source terminals as

a function of other voltages or currents.


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1.4.3 Current Sources1.4.3 Current Sources

An ideal independent current source can force

a specified current to flow through itself.


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1.4.3 Current Sources1.4.3 Current Sources

The current flowing through a dependent orcontrolled current source is determined by

a current or voltage elsewhere in the circuit.


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1.4.4 Resistors and Ohm1.4.4 Resistors and Ohms Laws Law

The voltage across an ideal resistoris proportional to the current through the resistor,

as defined by Ohms Law:

v i R=

The constant R is the resistance, with unitof ohm (

).

The reciprocity of the resistance isconductance (G), with unit of Siemen (S).


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1.5 Summary1.5 Summary

Current is the time rate of flow of electricalcharges. Units are ampere (A).

Voltage is the energy transferred per unit ofcharges that flows through the element.Units are volt (V).

Current reference direction and voltagereference polarity.

Kirchhoffs laws: I = 0 & V = 0.

Circuit elements: conductors, voltagesources, current sources and resistors.

ch1 introduction 2009

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