Download - 1 Electronics Technology Fundamentals Chapter 1 Principles of Electricity Lindem 11. jan 09
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Electronics Technology Fundamentals
Chapter 1
Principles of Electricity
Lindem 11. jan 09
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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
Det enslige elektronet i ytterste ”skall” er svakt bunnet til kjernen.Ved ”normal” temperatur har vi ca 1 fritt elektron pr. atom -ca 1023 elektroner / cm3 Figuren viser et kopperatom.
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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 I = the intensity of the current (Ampere)Q = the amount of charge ( coulomb ) t = the time (in seconds) required for the
charge (Q) to passt
QI
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 in a wire every two seconds. Calculate current.
A 1.5C/s 1.5s 2
C 3
t
QI
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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 - DC Versus Alternating Current - AC
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- Motstand og ledningsevne
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
IRV Ohms lov
Tradisjonelt bruker vi ofte Usom betegnelse for spenning. Boka har imidlertid konsekvent valgt å benytte V ( U = R · I )
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Conductance ( Ledningsevne ) – a measure of the ease which current will pass through a component
Unit of Conductance – siemens (S)
μS 100kΩ 10
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RG
RG
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whereG = conductanceR = resistance
Examples
1. Calculate the conductance of a 10 K resistor.
2. Calculate the resistance of a circuit that has a conductance of 25 mS.
Ω 40mS 25
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GR
1.4 Resistance and Conductance – P2- Motstand og ledningsevne
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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)
Silisium (Si) Germanium (Ge)
4 valenselektroner
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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
AR
= resistivity (greek letter, rho)ℓ = lengthA = cross-sectional area
ExampleCalculate the resistance of a 25 cm length of copper that has a cross-sectional area of 0.04 cm2.
1.08mΩΩ10X1.08
0.04cm
25cmcmΩ10X1.723
3
26
A
lR
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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 Negative Temperature Coefficient
Resistance decreases as temperature increases and vice versa
Example: most semiconductors and insulators
Motstand i 12volt 10watt lyspære
0,00
5,00
10,00
15,00
20,00
0 1 2 3 4 5 6 7 8 9 10 11 12 13
Spenning over lyspæra i volt
Mo
tsta
nd
i o
hm
Motstand i 12v 10watt lyspære (0.1 - 1.0v)
0,00
1,00
2,00
3,00
4,00
5,00
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
Spenning 0.1v trinn
Mo
tsta
nd
i o
hm
End 1.
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