Electronics Technology Fundamentals

Transcription

1 Lindem 11. jan 09 Electronics Technology Fundamentals Chapter 1 Principles of Electricity 1

2 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 2

3 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 3

4 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 4

5 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 elektroner / cm3 Figuren viser et kopperatom. 5

6 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 6

7 1.2 Current P2 Elektrisk strøm Represented by the letter I (for intensity) Measured in charge per unit time where I = Q 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 electrons Unit of Current Ampere (A) = 1 coulomb/second 3 coulombs of charge pass a point in a wire every two seconds. Calculate current. I = t Q = 3 C 2 s = 1.5 C/s = 1.5 A 7

8 1.2 Current P4 Electron Flow Versus Conventional Current Insert Figure

9 1.2 Current P5 Direct Current - DC Versus Alternating Current - AC Direct Current (dc) unidirectional Alternating Current (ac) - bidirectional 9

10 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. 10

11 1.4 Resistance and Conductance P1 - Motstand og ledningsevne 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 Ohms lov Tradisjonelt bruker vi ofte U som betegnelse for spenning. Boka har imidlertid konsekvent valgt å benytte V ( U = R I ) 11

12 1.4 Resistance and Conductance P2 - Motstand og ledningsevne Conductance ( Ledningsevne ) a measure of the ease which current will pass through a component G = 1 R Unit of Conductance siemens (S) Examples where G = conductance R = resistance 1. Calculate the conductance of a 10 KΩ resistor. 1 G = = R 1 10 kω = 100 µs 2. Calculate the resistance of a circuit that has a conductance of 25 ms. 1 R = = G 1 25 ms = 40 Ω 12

13 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 13

14 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) 14

15 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-Ω/ft Circular-mil ohms per foot Ω-cm Ohm-centimeters Length Cross-Sectional Area ρ l A l R = ρ A = resistivity (greek letter, rho) = length = cross-sectional area Example Calculate the resistance of a 25 cm length of copper that has a crosssectional area of 0.04 cm 2. l R = ρ = A = ( X 10 Ω cm) X 10 3 Ω = 1.08mΩ 25cm 0.04cm 2 15

16 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 ohm Motstand i ohm 20,00 15,00 10,00 5,00 0,00 5,00 4,00 3,00 2,00 1,00 0,00 Motstand i 12volt 10watt lyspære Spenning over lyspæra i volt Motstand i 12v 10watt lyspære ( v) 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. 16

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