Download Basic Concepts

Introduction to Circuit Theory
Basic Concepts
2012-09-12
Jieh-Tsorng Wu
National Chiao-Tung University
Department of Electronics Engineering
Outline
1.
2.
3.
4.
5.
Introduction
Systems of Units
Current and Voltage
Power and Energy
Circuit Elements
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A Simple Electric Circuit
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A Radio Receiver
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International System of Units (SI)
Quantity
Basic unit
Symbol
Length
meter
m
Mass
gram
g
Time
second
s
Electric current
ampere
A
Thermodynamic
temperature
kelvin
K
Luminous
intensity
candela
cd
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Units Commonly Used in Circuit Theory
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Electric Charges
 Charge is an electrical property of the atomic particles of which matter consists,
measured in coulombs (C).
 The charge e on one electron is negative and equal in magnitude to 1.602  10-19 C
which is called as electronic charge. The charges that occur in nature are integral
multiples of the electronic charge.
 The law of conservation of charge states that charge can neither be created or
destroyed, only transferred. Thus the algebraic sum of the electric charges in a
system does not change.
e  1.602 1019 C
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Current
i
dq
dt
 Current is the amount of charge passing through a cross section in one unit time.
 Electric current i = dq/dt. The unit of ampere can be derived as 1 A = 1C/s.
 The charge transferred between time t0 and t is
t
Q   idt
t0
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Direction of Current Flow
Negative Charges
Positive Charges
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DC and AC
 A direct current (dc) is a current that
remains constant with time.
 An alternating current (ac) is a current
that varies sinusoidally with time.
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Current Example 1
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Current Example 2
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Voltage
vab 
dw
dq
 Voltage (or potential difference) is the energy required to move a unit
charge through an element, measured in volts (V).
 w is energy in joules (J) and q is charge in coulombs (C).
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Voltage Polarity
 Electric voltage, vab, is always across the circuit element or between two
points in a circuit.
 vab > 0 means the potential of a is higher than potential of b.
 vab < 0 means the potential of a is lower than potential of b.
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Power and Energy
 Power is the time rate of expending or absorbing energy, measured in
watts (W).
p
dw dw dq


 vi
dt dq dt
i
i
+
+
v
v
_
_
P = +vi
Absorbing Power
1. Basic Concepts
p = –vi
Supplying Power
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Power and Energy
 The law of conservation of energy.
p0
 Energy is the capacity to do work, measured in joules (J).
t
t
t0
t0
w   pdt   (v  i )dt
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Power Example
p  4V  3A
p  4V  3A
=  4V  3A
 12W
1. Basic Concepts
 4V  3A
 12W
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Energy Example
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Circuit Elements
Active Elements
Independent
sources
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Passive Elements
Dependant
sources
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Dependent Voltage Source and Current Source
There are four possible types of dependent sources, namely:
1. A voltage-controlled voltage source (VCVS).
2. A current-controlled voltage source (CCVS).
3. A voltage-controlled current source (VCCS).
4. A current-controlled current source (CCCS).
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Example
Calculate the power supplied or absorbed by each element.
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