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Transcript
Electrical Principles
ENGG DE4401
Topic 1 :
INTRODUCTION TO ELECTRICAL
PRINCIPLES
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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.
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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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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’
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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
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:
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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
A resistor colored Yellow-Violet-Orange-Gold
would be 47 kΩ with a tolerance of +/- 5%.
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Resistors in circuits...
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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.
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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
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.
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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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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 !
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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!(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!
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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!
(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.
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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- 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
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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!
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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, and
(inversely proportional to the Resistance), 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=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
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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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• 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.
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