1 © Unitec New Zealand Electrical and Electronic Principles ENGG DE4401 Topic 1 : I NTRODUCTION TO...

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Electrical and Electronic Principles

ENGG DE4401

Topic 1 : INTRODUCTION TO ELECTRICAL AND ELECTRONICS PRINCIPLES

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

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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

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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.

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..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.

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Lumped Circuit Abstraction (LCA)

• We are working with discrete elements (components) and each has a physical quantity describing it.

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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.

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.

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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.

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Analogy –water in pipes

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We adopt symbols and conventions

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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.

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We choose conventional flow!

• Conventional current flow is a standard adopted in NZ industry and we will use it from now on.

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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)

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Calculating Resistance

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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:

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

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Resistors value

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A resistor colored Yellow-Violet-Orange-Gold would be 47 kΩ with a tolerance of +/- 5%.

Resistors in circuits...

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Don’t confuse them with inductors

• This is an inductor (see L201 written on the side?)

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• These are resistors: the standard beige/brown ones are carbon film and metal film resistors are often blue.

Surface mount resistors

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How to get a law?

• Measurements

• Using an instrument (multimeter) we can measure voltage, current, resistance.

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Units

International standard of units is called SI (systeme internationale). There are seven “base” units from which all other units are derived:

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Physical quantities and units of measure

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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.

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.

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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

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For practice:

• Book Schaum’s Outline of BASIC ELECTRICITY

• Chapter 2 , pages 15-27

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Your multimeter

• Make sure your leads are connected to the right plug:– You may damage your

mulitmeter if you are not using it properly!

• 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!

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Measuring resistance

• Your multimeter is now an Ohmmeter

• Make sure your ohmmeter range is correct.

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• Important: measuring resistance must only be done on de-energized components! 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.

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.

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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)

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Measuring current

• Multimeter is now working as an Ammeter.

• 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

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Measuring voltage

• Multimeter is now working as an Voltmeter.

• 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!

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Measured value: in Table or Graphs

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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.

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IV characteristics for an ideal resistor

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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:

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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, for any given temperature.

• That constant of proportionality is called resistance.

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Using Ohm’s Law in circuit analysis

• Ohm’s law is expressed in the form of a simple equation:

V = I R

• 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.

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V = R I I = V / R R = V / I

Solution: how to find current I

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Find R...

• What is the amount of resistance (R) offered by the lamp?

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Solution for R

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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?

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Solution for E...

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Maths revision: Algebra

• How to use a formula

• Manipulate the formula to find unknown value

• Fractions

• Indices

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Moodle quiz 1

• Please work on Moodle quiz 1

• If you find questions 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.

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