Ch1 Introduction 2009
Transcript of Ch1 Introduction 2009
-
8/11/2019 Ch1 Introduction 2009
1/32
2140201 Electrical Circuits2140201 Electrical Circuits
Chedsada ChinrungruengThavatchai Tayjasanant
Department of Electrical Engineering
Chulalongkorn University
-
8/11/2019 Ch1 Introduction 2009
2/32
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
-
8/11/2019 Ch1 Introduction 2009
3/32
-
8/11/2019 Ch1 Introduction 2009
4/32
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.
-
8/11/2019 Ch1 Introduction 2009
5/32
1.1.1 Electrical Circuits1.1.1 Electrical Circuits
An electrical circuit consists of various typesof circuit elements connected in closed
paths by conductors.
-
8/11/2019 Ch1 Introduction 2009
6/32
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).
-
8/11/2019 Ch1 Introduction 2009
7/32
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.
-
8/11/2019 Ch1 Introduction 2009
8/32
Current Notations:
Use arrow to indicate the direction.
Use double-subscripts to define thereference direction, e.g. from a to b.
-
8/11/2019 Ch1 Introduction 2009
9/32
DC versus ACDC versus AC
Direct Current (DC)Constant magnitude with time.
Alternating Current (AC)Magnitude varies with time.
-
8/11/2019 Ch1 Introduction 2009
10/32
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).
-
8/11/2019 Ch1 Introduction 2009
11/32
Voltage PolaritiesVoltage Polarities
Polarities: direction of energy flow.
Positive charge moves from + through theelement toward ,
indicating that the element absorbs energy.
-
8/11/2019 Ch1 Introduction 2009
12/32
Voltage Notations:Voltage Notations:
Use double-scripts to definethe reference polarities,
e.g. between points a and b.
-
8/11/2019 Ch1 Introduction 2009
13/32
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.
-
8/11/2019 Ch1 Introduction 2009
14/32
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:
-
8/11/2019 Ch1 Introduction 2009
15/32
1.3 Kirchhoff1.3 Kirchhoffs Lawss Laws
1. Kirchhoffs Current Law (KCL)
2. Kirchhoffs Voltage Law (KVL)
-
8/11/2019 Ch1 Introduction 2009
16/32
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.
-
8/11/2019 Ch1 Introduction 2009
17/32
i i i+ =1 2 3 i i=3 4 i i i+ + =5 6 7 0
Kirchhoffs Current Law (KCL)
-
8/11/2019 Ch1 Introduction 2009
18/32
Find current values using KCL
-
8/11/2019 Ch1 Introduction 2009
19/32
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.
-
8/11/2019 Ch1 Introduction 2009
20/32
Voltages are added or subtracted depending on
their reference polaritiesrelative to the direction of travel around the loop.
-
8/11/2019 Ch1 Introduction 2009
21/32
a b cv v v + + = 0
c d ev v v + = 0
a b d ev v v v + = 0
-
8/11/2019 Ch1 Introduction 2009
22/32
Find voltage values using KVL
-
8/11/2019 Ch1 Introduction 2009
23/32
1.4 Ideal Circuit Elements1.4 Ideal Circuit Elements
1. Conductors
2. Voltage sources
3. Current sources
4. Resistors
-
8/11/2019 Ch1 Introduction 2009
24/32
-
8/11/2019 Ch1 Introduction 2009
25/32
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
-
8/11/2019 Ch1 Introduction 2009
26/32
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.
-
8/11/2019 Ch1 Introduction 2009
27/32
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.
-
8/11/2019 Ch1 Introduction 2009
28/32
1.4.3 Current Sources1.4.3 Current Sources
An ideal independent current source can force
a specified current to flow through itself.
-
8/11/2019 Ch1 Introduction 2009
29/32
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.
-
8/11/2019 Ch1 Introduction 2009
30/32
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).
-
8/11/2019 Ch1 Introduction 2009
31/32
-
8/11/2019 Ch1 Introduction 2009
32/32
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.