Download Electronics Technology Fundamentals

Lindem 14 jan.07
Electronics Technology
Fundamentals
Chapter 1
Principles of Electricity
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1.1 The Starting Point
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Atomic Structure
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‰
Atom – smallest particle of matter that retains the physical
characteristics of an element
Bohr Model
„
„
„
Simplest model of an atom
Central core (nucleus) – contains protons and neutrons
Electrons revolve around nucleus
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1.1 The Starting Point: Elements, Atoms and Charge – P3
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Atomic Structure (Continued)
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Atomic Number of an Atom – number of protons
Atoms contain an equal number of protons and electrons
Electrons travel in orbital paths (shells)
Valence Shell
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Outermost shell
Cannot hold more than eight electrons
Complete shell contains eight electrons
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1.1 The Starting Point: Elements, Atoms and Charge – P6
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Attraction and Repulsion - Like charges repel each other
and opposite charges attract each other
Ions
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Outside force can cause an electron to leave its orbit -atom is
referred to as a positive ion
Outside force can cause an atom to gain an electron -atom is referred
to as a negative ion
Free Electrons
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An electron that is not bound to any particular atom
Can neutralize a positive ion
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1.1 The Starting Point: Elements, Atoms and Charge – P7
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1.2 Current – P1
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Elektrisk strøm
Current – the directed flow of charge through a conductor
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Thermal energy (heat) is sufficient to free electrons in copper
Free electron motion is random unless outside force is applied
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1.2 Current – P2
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„
Elektrisk strøm
Represented by the letter I (for intensity)
Measured in charge per unit time
where
Q
I=
t
I = the intensity of the current (Ampere)
Q = the amount of charge ( coulomb )
t = the time (in seconds) required for the
charge (Q) to pass
Coulomb (C) – represents the total charge of
approximately 6.25 x 1018 electrons
„ Unit of Current – Ampere (A) = 1 coulomb/second
3 coulombs of charge pass a point
Q 3C
I
=
=
= 1.5 C/s = 1.5 A
in a wire every two seconds.
t 2s
Calculate current.
„
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1.2 Current – P4
„
Electron Flow Versus Conventional Current
Insert Figure 1.10
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1.2 Current – P5
Direct Current Versus Alternating Current
Direct Current (dc)
– unidirectional
„
Alternating Current
(ac) - bidirectional
„
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1.3 Voltage – P1
„
Voltage – a “difference of potential” that generates the
directed flow of charge (current) through a circuit
„
„
„
Often referred to as
electromotive force
(EMF)
Unit of Voltage – volt
(V) = 1 joule/coulomb
Volt – the difference of
potential that uses one
joule of energy to move
one coulomb of charge.
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1.4 Resistance and Conductance – P1
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„
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Resistance – opposition to current
Unit of Resistance – ohm (Ω - Greek letter omega)
Ohm – the amount of resistance the limits current to one
ampere when one volt is applied
V = R⋅ I
Insert Figure 1.13
Ohms lov
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1.4 Resistance and Conductance – P2
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Conductance ( Ledningsevne ) – a measure of the ease
which current will pass through a component
1
G=
R
where
G = conductance
R = resistance
„
„
Unit of Conductance – siemens (S)
Old Unit of Conductance – mhos (upside down – omega
symbol)
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1.4 Resistance and Conductance – P3
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Examples
1. Calculate the conductance of a 10 KΩ resistor.
1
1
G= =
= 100 µS
R 10 kΩ
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2. Calculate the resistance of a circuit that has a
conductance of 25 mS.
1
1
R= =
= 40 Ω
G 25 mS
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1.5 Conductors, Insulators and Semiconductors – P1
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Conductors – materials that provide little opposition to the
flow of charge (current)
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Example: copper
Few valence shell electrons, one valence shell electron per atom
makes the best conductor
Insulators – materials that normally block current
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Example: rubber
Complete valence shell
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1.5 Conductors, Insulators and Semiconductors – P2
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Semiconductors – materials that are neither good
conductors nor good insulators
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Example: graphite (used to make resistors)
Half-complete valence shells (four valence electrons)
4 valenselektroner
Silisium (Si)
Germanium (Ge)
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1.5 Conductors, Insulators and Semiconductors – P3
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Other Factors that Affect Resistance
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Resistivity – the resistance of a specified volume of an element or
compound
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CM-Ω/ft – Circular-mil ohms per foot
Ω-cm – Ohm-centimeters
Length
Cross-Sectional Area
l
R=ρ
A
where
ρ = resistivity (greek letter, rho)
ℓ = length
A = cross-sectional area
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1.5 Conductors, Insulators and Semiconductors – P5
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Example
Calculate the resistance of a 25 cm length of copper that has a
cross-sectional area of 0.04 cm2.
(
l
R = ρ = 1.723 X 10 −6 Ω − cm
A
)
⎛ 25cm ⎞
⎜
2 ⎟
⎝ 0.04cm ⎠
= 1.08 X 10 −3 Ω = 1.08mΩ
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1.5 Conductors, Insulators and Semiconductors – P6
The Effects of Temperature on
Resistance
Positive Temperature Coefficient
„
„
‰
Resistance increases as
temperature increases and vice
versa
Example: most conductors
Motstand i ohm
‰
Motstand i 12volt 10watt lyspære
20,00
15,00
10,00
5,00
0,00
0
Negative Temperature
Coefficient
„
„
Resistance decreases as
temperature increases and vice
versa
Example: most semiconductors
and insulators
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Spenning over lyspæra i volt
Motstand i 12v 10watt lyspære (0.1 - 1.0v)
Motstand i ohm
„
5,00
4,00
3,00
2,00
1,00
0,00
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Spenning 0.1v trinn
End 1.
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