Transcript Electronics Technology Fundamentals

Lindem 14 jan.07
Electronics Technology
Fundamentals
Chapter 1
Principles of Electricity
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1.1 The Starting Point

Atomic Structure


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

Atomic Structure (Continued)




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



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

Attraction and Repulsion - Like charges repel each other and
opposite charges attract each other

Ions



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


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

Elektrisk strøm
Current – the directed flow of charge through a conductor


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


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



Resistance – opposition to current
Unit of Resistance – ohm ( - 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
U=R·I
Denne læreboka skiller
ikke mellom U og V
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1.4 Resistance and Conductance – P2

Conductance ( Ledningsevne ) – a measure of the ease
which current will pass through a component
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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

Examples
1. Calculate the conductance of a 10 K resistor.
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1
G 
 100 μS
R 10 kΩ

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

Conductors – materials that provide little opposition to the
flow of charge (current)



Example: copper
Few valence shell electrons, one valence shell electron per atom
makes the best conductor
Insulators – materials that normally block current


Example: rubber
Complete valence shell
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1.5 Conductors, Insulators and Semiconductors – P2

Semiconductors – materials that are neither good
conductors nor good insulators


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

Other Factors that Affect Resistance

Resistivity – the resistance of a specified volume of an element or
compound




CM-/ft – Circular-mil ohms per foot
-cm – Ohm-centimeters
Length
Cross-Sectional Area

R
A
where
 = resistivity (greek letter, rho)
ℓ = length
A = cross-sectional area
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1.5 Conductors, Insulators and Semiconductors – P5

Example
Calculate the resistance of a 25 cm length of copper that has a
cross-sectional area of 0.04 cm2.

l
R    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
1
2
3
4
5
6
7
8
9
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Spenning over lyspæra i volt
Negative Temperature Coefficient


Resistance decreases as
temperature increases and vice
versa
Example: most semiconductors
and insulators
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
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Fargekode for motstander
End 1.
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