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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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