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Transcript

Electrical Principles ENGG DE4401 Topic 1 : INTRODUCTION TO ELECTRICAL PRINCIPLES © Unitec New Zealand 1 Topic overview • Physics and physical quantities • Engineering approach – Lumped Circuit Abstraction • Current, Voltage, Resistance, Power – difference between electron flow and conventional current flow • Measurements – Units, Metric conversion, Scientific notation, Graphs and tables • Resistors • Ohm’s Law © Unitec New Zealand 2 Electricity • We are interested in electricity: a phenomena related to the charged particles, the forces between them and their movement. – Chapter 1 Schaum’s Basic Electricity book © Unitec New Zealand 3 Engineering problems... • We want to answer this question: Will this light bulb glow? • We cannot see the electrons, but we can measure their movement in the form of electric current (I) and we can measure the potential energy that initiate that current flow. © Unitec New Zealand 4 ..require engineering approach: Abstraction • We do not care about – Length of the wire in the light bulb – Light bulb filament – The temperature of the light bulb, etc. • We replace physical item with a discrete element as if the physical property (resistance of the light bulb, R, battery voltage V) is concentrated in a single point and we can access it across its terminals (A and B in the Fig below). • Now we observe only the key issue: the power delivered to the load. © Unitec New Zealand 5 Lumped Circuit Abstraction (LCA) • We are working with discrete elements (components) and each has a physical quantity describing it. © Unitec New Zealand 6 Electric circuit • An electric circuit is formed when a closed conductive path is created to allow free electrons to continuously move. • This continuous movement of free electrons through the conductors of a circuit is called a current • The electromotive force which “motivates” electrons to "flow" in a circuit is called voltage or emf. © Unitec New Zealand 7 Basic Definitions: Current • The movement or the flow of electrons (charge) is referred to as current. • Current is represented by the letter symbol I ( it stands for “intensity”). • Current is the rate of flow of electrons through a conductor. The basic unit in which current is measured is the ampere (A). – One ampere of current is defined as the movement of one coulomb ( quantity of charge) past any point of a conductor during one second of time. • An instrument called an ammeter is used to measure current flow in a circuit. © Unitec New Zealand 8 Basic Definitions: Voltage • An electric charge has the ability to do the work of moving another charge by attraction or repulsion. The ability of a charge to do work is called its potential. • Voltage is a measure of potential energy , always relative between two points (potential difference). – The symbol for voltage is V, (emf can be e or E). – The basic unit for voltage or emf is the volt ( V ). • Remember: Voltage is always relative between two points: – What is the meaning of a battery voltage output of 6 V? – A voltage output of 6V means that the potential difference between the two terminals of the battery is 6V. © Unitec New Zealand 9 EMF vs Voltage Drop • EMF (ElectroMotiveForce) (Volts) - forces Current to flow through a circuit with• Resistances in it • Current flowing through Resistances in the circuit causes Voltage Drops (Volts) across each Resistance. • The SUM of the voltage drops around a circuit is equal to the EMF applied to the circuit from the ‘source’ © Unitec New Zealand 10 Analogy –water in pipes © Unitec New Zealand 11 We adopt symbols and conventions © Unitec New Zealand 12 Electron and Conventional current flows • Electric current flow is the movement of ‘free’ electrons along a conductor. Electrons are negative charges. Negative charges are attracted to positive charges. Electrons move from the negative terminal of a battery to the positive terminal. This is called electron current flow. • Another way to look at electric current flow is in terms of charges. Electric charge movement is from an area of high charge to an area of low charge. A high charge can be considered positive and a low charge negative. With this method, an electric charge is considered to move from a high charge (positive or +) to a low charge (negative or -). This is called conventional current flow. © Unitec New Zealand 13 We choose conventional flow! • Conventional current flow is a standard adopted in NZ industry and we will use it from now on. © Unitec New Zealand 14 Resistance • Free electrons tend to move through conductors with some degree of friction, or opposition to motion. • This opposition to motion is called resistance. • Resistance R is measured in ohms: Ω • Opposite of the resistance is conductance G: G=1/R • Conductance G is measured in Si (siemens) , but sometimes the unit used is mho (opposite of ohm, used for R) © Unitec New Zealand 15 Calculating Resistance Resistance depends on : • Type Material of which the conductor is made (a constant ρ called Specific Resistance or resistivity) • Dimensions of the conductor • Shape of the conductor For a piece of material with cylindrical shape: © Unitec New Zealand 16 Resistors • Special components called resistors are made for the express purpose of creating a precise quantity of resistance for insertion into a circuit. • Two common schematic symbols for a resistor are © Unitec New Zealand 17 Resistors value A resistor colored Yellow-Violet-Orange-Gold would be 47 kΩ with a tolerance of +/- 5%. © Unitec New Zealand 18 Resistors in circuits... © Unitec New Zealand 19 Don’t confuse them with inductors • These are resistors: the • This is an inductor (see standard beige/brown ones are L201 written on the side?) carbon film and metal film resistors are often blue. © Unitec New Zealand 20 Surface mount resistors © Unitec New Zealand 21 How to get a law? • Measurements • Using an instrument (multimeter) we can measure voltage, current, resistance. © Unitec New Zealand 22 Units International standard of units is called SI (systeme internationale). There are seven “base” units from which all other units are derived: © Unitec New Zealand 23 Physical quantities and units of measure All of these symbols are expressed using capital letters. However, if a quantity is changing in time , we use small letter (called an "instantaneous" value). Direct-current (DC) values will be in capital letters, for AC (alternate current) values we use small letters. © Unitec New Zealand 24 Scientific notation • Sometimes we work with very small or very large values. To avoid writing large number of zeros, we introduce Scientific notation, using powers of number 10. © Unitec New Zealand 25 Metric prefixes • We go step further, and introduce code words for frequently used scientific notations (multiples of 3). We use these words as prefixes to our Units © Unitec New Zealand 26 For practice: • Book Schaum’s Outline of BASIC ELECTRICITY • Chapter 2 , pages 15-27 © Unitec New Zealand 27 MEASUREMENT • Introducing a Measuring Instrument into a circuit should NOT AFFECT the quantity being measured! • Ammeters connected in SERIES (so the Current Flows through them), must have very LOW Resistance, so they don’t alter the Current in the circuit (by adding extra resistance. • Voltmeters in parallel (“ACROSS” parts), must have very HIGH resistance, so they don’t alter the operation of the circuit (by drawing extra current through the circuit). • A LOW resistance Ammeter connected to measure a voltage across a part will cause a DAMAGING SHORT ! © Unitec New Zealand 28 Your multimeter • Make sure your leads are connected to the right plug: – You may damage your mulitmeter if you are not using it properly!(BLACK in COM) • Rotating switch must be on the right field: chose between DCV, AC V, A or Ohm – Chose higher range for current and than reduce it, if needed. • Do not touch the tip of the probe while measuring! © Unitec New Zealand 29 Measuring resistance • Your multimeter is now an Ohmmeter • Make sure your ohmmeter range is correct. © Unitec New Zealand • Important: measuring resistance must only be done on de-energized components! (disconnected from other parts). When the meter is in "resistance" mode, it uses a small internal battery to generate a tiny current through the component to be measured. If there is any additional source of voltage in the loop, faulty readings will result. In a worse-case situation, the meter may even be damaged by the external voltage. 30 Exercise 1: Measuring resistance • For all three offered resistors, do the following: – Select a resistor from the assortment – Set your multimeter to the appropriate resistance range – Measure the resistance using your multimeter: • Be sure not to hold the resistor terminals when measuring resistance, or else your hand-to-hand body resistance will influence the measurement! – Record measured resistance value in the table. – Confirm the value by reading the color code from the chart. © Unitec New Zealand 31 Measuring voltage and current • Current : – Always measured with multimeter connected in series. – Connecting in series means you must break the circuit to insert the multimeter (so the current flowing in circuit goes through meter). • Voltage – measured with multimeter connected in parallel to the component. • Series or parallel? Clue: the current will split in two paths for parallel circuit. In the series circuit, there is only one current path) © Unitec New Zealand 32 Measuring current- DANGER – SHORT CCT • Multimeter is now working as an Ammeter. • An Ammeter is very LOW resistance –will SHORT OUT anything you probe • It must be connected in series, • Make sure the plug is in Amp hole, not in VΩ hole! • Choose DC or AC, as needed: we measure DC current © Unitec New Zealand 33 Measuring voltage • Multimeter is now working as an Voltmeter(High R). • It must be connected in parallel • Make sure the plug is in VΩhole, not in Amp hole! • Be careful not to touch the bare probe tips together while measuring voltage, as this will create a short-circuit! © Unitec New Zealand 34 Measured value: in Table or Graphs © Unitec New Zealand 35 Exercise 2: Measuring voltage and current • Aim: to observe the change of the current through a 1kΩ resistor when the voltage on the resistor is varied. © Unitec New Zealand 36 IV characteristics for an ideal resistor © Unitec New Zealand 37 Formula • Graph is good for representing unknown relationships, but sometimes relationship between two values is simple and easier to describe using a mathematical formula. • That is true for our example with current and voltage across the resistor and the formula is called Ohm’s law: © Unitec New Zealand 38 Ohm’s Law • Ohm's Law describes relationship between current, voltage and resistance. • Georg Simon Ohm discovered that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage impressed across it, and (inversely proportional to the Resistance), for any given temperature. • That constant of proportionality is called resistance. © Unitec New Zealand 39 Using Ohm’s Law in circuit analysis • Ohm’s law is expressed in the form of a simple equation: V=IR • If we know the values of any two of the three quantities (voltage, current, and resistance) in this circuit, we can use Ohm's Law to determine the third. V=RI I=V/R R=V/I © Unitec New Zealand 40 Solution: how to find current I © Unitec New Zealand 41 Find R... • What is the amount of resistance (R) offered by the lamp? © Unitec New Zealand 42 Solution for R © Unitec New Zealand 43 Find E... • In the last example, we will calculate the amount of voltage supplied by a battery, given values of current (I) and resistance (R): What is the amount of voltage provided by the battery? © Unitec New Zealand 44 Solution for E... © Unitec New Zealand 45 Maths revision: Algebra • How to use a formula • Manipulate the formula to find unknown value • Fractions • Indices © Unitec New Zealand 46 • If you find examples challenging, read the notes in this presentation and practice the examples from Schaum’s Basic Electricity. • If it is still not clear, write it down and bring it up first thing next class. © Unitec New Zealand 47