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Transcript
```Electrical Principles
ENGG DE4401
Topic 1 :
INTRODUCTION TO ELECTRICAL
PRINCIPLES
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
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
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.
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.
5
Lumped Circuit Abstraction (LCA)
• We are working with
discrete elements
(components) and each
has a physical quantity
describing it.
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.
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.
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.
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’
10
Analogy –water in pipes
11
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.
13
We choose conventional flow!
• Conventional current flow is a standard adopted in NZ
industry and we will use it from now on.
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)
15
Calculating Resistance
Resistance depends on :
• Type Material of which the
ρ called Specific Resistance or
resistivity)
• Dimensions of the conductor
• Shape of the conductor
For a piece of
material with
cylindrical shape:
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
17
Resistors value
A resistor colored Yellow-Violet-Orange-Gold
would be 47 kΩ with a tolerance of +/- 5%.
18
Resistors in circuits...
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.
20
Surface mount resistors
21
How to get a law?
• Measurements
• Using an instrument
(multimeter) we can
measure voltage,
current, resistance.
22
Units
International standard of units is called SI (systeme
internationale). There are seven “base” units from
which all other units are derived:
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.
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.
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
26
For practice:
• Book Schaum’s Outline of BASIC ELECTRICITY
• Chapter 2 , pages 15-27
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 !
28
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!
29
Measuring resistance
• Your multimeter is now an
Ohmmeter
range is correct.
• 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
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.
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)
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
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!
34
Measured value: in Table or Graphs
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.
36
IV characteristics for an ideal resistor
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:
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.
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
40
Solution: how to find current I
41
Find R...
• What is the amount of resistance (R) offered by the lamp?
42
Solution for R
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?
44
Solution for E...
45
Maths revision: Algebra
• How to use a formula
• Manipulate the formula to find unknown value
• Fractions
• Indices