Chapter 1 Basic Electrical Concepts

AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Chapter 1 Basic Electrical Concepts © Copyright 2001 Basic Electrical Concepts 1 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Learning Outcome At the end of this chapter, you should be able to • Explain the basic and derived quantities (voltages, currents and resistance) and state their units of measurement • Write values and perform calculations using scientific notation, metric prefixes, engineering notation and significant figures • State the difference between conventional and electron current flow • Compare the electrical characteristics of common types of conductors, insulators and semiconductors and state typical applications © Copyright 2001 Basic Electrical Concepts 2 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Atoms • All matter is composed of atoms • An atom consists of a central nucleus (with positively-charged protons and uncharged neutrons) surrounded by orbiting negativelycharged electrons 1 valence electron 29 electrons Nucleus with 29 protons and 29 neutrons The Copper Atom © Copyright 2001 Basic Electrical Concepts 3 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Atoms • Electrons in outermost shell are known as valence electrons • When valence electrons gain enough energy from the surrounding medium, they can break away from the parent atom, becoming free electrons Current Flow • Free electrons drift randomly from atom to atom in all directions within the conductor A © Copyright 2001 Basic Electrical Concepts 4 B AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Electrical Charge ( Q) • An atomic structure is charged when the no. of electrons is NOT equal to no. of protons • If no. of protons > no. of electrons, then the structure is a +VE charge and becomes a +VE ion • If no. of electrons > no. of protons, then the structure is a -VE charge and becomes a -VE ion • E.g. static electricity is due to presence of +VE or –VE charge • Unit of charge is Columns (C) © Copyright 2001 Basic Electrical Concepts 5 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Electrical Charge ( Q) • One single electron has a charge of 1.6x10 -19 C • Hence, 1 C is the total charge possessed by 6.25x10 18 electrons Current (I) • If point A is relatively more positively-charged than point B, then A is denoted as “+” and B is denoted as “-” A + © Copyright 2001 Basic Electrical Concepts 6 B _ electron flow conventional current direction AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Current (I) • The free electrons will move towards the more positively-charged end • • Like charges repel, unlike charges attract The flow of these free electrons constitutes the electric current Direction of conventional current is opposite to the direction of electron flow Conventional current can be measured using meter Unit of current is Ampere (A) • • • © Copyright 2001 Basic Electrical Concepts 7 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Definition of Current • Current is defined as the amount of charge that flows past a point per unit time • In short, current is the rate of flow of charge I = DQ / Dt … (1) DQ Dt where I = current in Ampere (A) DQ = amount of charge in Coulomb (C) Dt = time interval in second (s) • from equation (1), it can be seen that current is proportional to amount of charge and inversely proportion to time interval * © Copyright 2001 Basic Electrical Concepts 8 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Voltage (V) Voltage A B _ + • • • electron flow conventional current direction Point A is said to be at higher potential (+) while point B is at lower potential (-) The difference in potentials between point A & B causes current to flow This potential difference is also called the voltage or electromotive force (EMF) * © Copyright 2001 Basic Electrical Concepts 9 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Voltage (V) • Note that the conventional current always flows from the higher potential point to the lower potential point • E.g. sources of voltage are battery, power supply, solar cell, generator Definition of Voltage • Voltage is defined as the electrical force, or “pressure” that causes current to flow in a circuit • Unit of voltage is volt (V) * © Copyright 2001 Basic Electrical Concepts 10 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Analogy of Voltage and Current using Water Flow Water pipe (Conductor) Water Analogy of voltage and current with water pipe Water flow (Current) Friction (Internal Resistance) Force (Voltage) * © Copyright 2001 Basic Electrical Concepts 11 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Resistance (R) • Free electrons collide with atoms and loss energy, flow is restricted • This results resistance in current • Unit of resistance is ohm (W) Analogy of Resistance using Water Flow These vertical plates offer resistance (R) to the water flow, ie. restrict the water flow (current) Force (Voltage) Water pipe (Conductor) © Copyright 2001 Basic Electrical Concepts 12 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Work (w) • Work is resulted from moving an object over a distance • Electrical work is performed when heat or mechanical action is generated • Unit of work is Joule (J) Energy (E) • Energy is the ability to do work • Energy appears in many form, e.g. heat, light, sound, mechanical, … • Unit of work is also Joule (J) © Copyright 2001 Basic Electrical Concepts 13 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Power (P) • Consumers usage of public power is measured in watt-hour (W-h) or kilowatt-hour (kW-h) using meter Questions • In an electrical circuit, energy comes from load/supply and is consumed by load/supply • The current is opposed by __________ • An electrical circuit is driven by __________ • If the resistance in a circuit is increased, the current flow will __________ • If the supply voltage is increased, the current flow will __________ © Copyright 2001 Basic Electrical Concepts 14 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Summary • An electrical circuit requires a supply and load • Supply and load are connected by wires • Supply’s EMF (voltage) forces current around the circuit and through the load • Opposition to current flow is resistance © Copyright 2001 Basic Electrical Concepts 15 AIT TAFE Centre, Singapore Electrical Principles School of Electronics Engineering Electrical Quantities Summary Quantity It measures … unit symbol Charge, C No. of electrons Coulomb C Conductance How easily current can ,G flow Siemen S Current, I Amount of electrons flowing Ampere A Energy, E The ability to do work Joule J Power The rate of work Watt W Resistance, R The opposition to current flow Ohm W Voltage, V Electrical force (EMF) Volt V Work, w Amount of energy used joule J © Copyright 2001 Basic Electrical Concepts 16