Lindem 14 jan 07 Electronics Technology Fundamentals Chapter

Lindem 14 jan. 07 Electronics Technology Fundamentals Chapter 1 Principles of Electricity 1

1. 1 The Starting Point n Atomic Structure q q Atom – smallest particle of matter that retains the physical characteristics of an element Bohr Model n n n Simplest model of an atom Central core (nucleus) – contains protons and neutrons Electrons revolve around nucleus 2

1. 1 The Starting Point: Elements, Atoms and Charge – P 3 n Atomic Structure (Continued) q q 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 n n n Outermost shell Cannot hold more than eight electrons Complete shell contains eight electrons 3

1. 1 The Starting Point: Elements, Atoms and Charge – P 6 n Attraction and Repulsion - Like charges repel each other and opposite charges attract each other n Ions q q n 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 q q An electron that is not bound to any particular atom Can neutralize a positive ion 4

1. 1 The Starting Point: Elements, Atoms and Charge – P 7 5

1. 2 Current – P 1 n Elektrisk strøm Current – the directed flow of charge through a conductor q q Thermal energy (heat) is sufficient to free electrons in copper Free electron motion is random unless outside force is applied 6

1. 2 Current – P 2 n n 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 pass n n 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. 7

1. 2 Current – P 4 n Electron Flow Versus Conventional Current Insert Figure 1. 10 8

1. 2 Current – P 5 Direct Current Versus Alternating Current Direct Current (dc) – unidirectional n Alternating Current (ac) - bidirectional n 9

1. 3 Voltage – P 1 n Voltage – a “difference of potential” that generates the directed flow of charge (current) through a circuit n n n 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

1. 4 Resistance and Conductance – P 1 n n n 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 Insert Figure 1. 13 Ohms lov U=R·I Denne læreboka skiller ikke mellom U og V 11

1. 4 Resistance and Conductance – P 2 n Conductance ( Ledningsevne ) – a measure of the ease which current will pass through a component where G = conductance R = resistance n n Unit of Conductance – siemens (S) Old Unit of Conductance – mhos (upside down – omega symbol) 12

1. 4 Resistance and Conductance – P 3 n Examples 1. Calculate the conductance of a 10 K resistor. n 2. Calculate the resistance of a circuit that has a conductance of 25 m. S. 13

1. 5 Conductors, Insulators and Semiconductors – P 1 n Conductors – materials that provide little opposition to the flow of charge (current) q q n Example: copper Few valence shell electrons, one valence shell electron per atom makes the best conductor Insulators – materials that normally block current q q Example: rubber Complete valence shell 14

1. 5 Conductors, Insulators and Semiconductors – P 2 n Semiconductors – materials that are neither good conductors nor good insulators q q Example: graphite (used to make resistors) Half-complete valence shells (four valence electrons) 4 valenselektroner Silisium (Si) Germanium (Ge) 15

1. 5 Conductors, Insulators and Semiconductors – P 3 n Other Factors that Affect Resistance q Resistivity – the resistance of a specified volume of an element or compound n n q q CM- /ft – Circular-mil ohms per foot -cm – Ohm-centimeters Length Cross-Sectional Area where = resistivity (greek letter, rho) ℓ = length A = cross-sectional area 16

1. 5 Conductors, Insulators and Semiconductors – P 5 n Example Calculate the resistance of a 25 cm length of copper that has a cross-sectional area of 0. 04 cm 2. 17

1. 5 Conductors, Insulators and Semiconductors – P 6 n The Effects of Temperature on Resistance q Positive Temperature Coefficient n n q Resistance increases as temperature increases and vice versa Example: most conductors Negative Temperature Coefficient n n Resistance decreases as temperature increases and vice versa Example: most semiconductors and insulators 18

Fargekode for motstander End 1. 19