Transcript Electrical systems - University of KwaZulu

SYSTEMS AND CONTROL
B. M Thabethe
4th TERM
Electrical and Electronics
Hydraulics and Pneumatics
Combining Systems
Work to be covered this term:
 Electrical and electronic systems
 Pneumatics and Hydraulics
 Pascal’s law and MA
 Combining systems
Electrcal and electronics
 to understand how electrical circuits,
including those with switches, can be
used to achieve functional results;
 to use electrical switches to control
devices;
 to analyse the performance of an
electric system in order to check if it
is working effectively to design, use
and interconnect simple electrical
and electronic systems
Electrical systems
Involve the use of circuits
Outcomes for today
 to understand what is electricity and how it works
 to understand how electrical circuits, including those
with switches, can be used to achieve functional
results;
 to use electrical switches to control devices;
 to know that systems have inputs, processes and
outputs, and to recognise these in products they have
made
 to understand the importance of feedback and how it
can be used to ensure the correct functioning of an
electrical or electronic system
 to analyse the performance of an electric system
in order to check if it is working effectively to
design, use and interconnect simple electrical
and electronic systems
Electricity
 But what is electricity?
 Where does it come from?
 How does it work?
 Before we understand all that, we need to
know a little bit about atoms and their
structure.
Conduction of electricity
 Conductors
 Semi-conductors
 Insulators
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silver
copper
gold
aluminum
iron
steel
brass
bronze
mercury
graphite
dirty water
concrete
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glass
rubber
oil
asphalt
fiberglass
porcelain
ceramic
quartz
(dry) cotton
(dry) paper
(dry) wood
plastic
air
diamond
pure water
ATOMS
 All matter is made up of atoms, and atoms are made
up of smaller particles.
 The three main particles making up an atom are the
proton, the neutron and the electron.
 Electrons spin around the center, or nucleus, of
atoms, in the same way the moon spins around the
earth.
 The nucleus is made up of neutrons and protons.
 Electrons contain a negative charge, protons a
positive charge. Neutrons are neutral -- they have
neither a positive nor a negative charge.
Atoms
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There are many different kinds of atoms, one for each type
of element.
An atom is a single part that makes up an element. There
are 118 different known elements that make up every
thing!
Some elements like oxygen we breathe are essential to life.
Each atom has a specific number of electrons, protons
and neutrons.
But no matter how many particles an atom has, the number
of neutrons usually needs to be the same as the number of
protons.
If the numbers are the same, the atom is called balanced,
and it is very stable.
If not, the atom is considered to be unstable.
Quick look at the periodic table
 So, if an atom had six protons, it should also have six
electrons.
 The element with six protons and six electrons is
called ……..
 Carbon is found in abundance in the sun, stars,
comets, atmospheres of most planets, and the food
we eat.
 Coal is made of carbon; so are diamonds.
Moving electrons from one atom to
another!
 Electrons can be made to move from one atom to
another.
 When those electrons move between the atoms, a
current of electricity is created.
 The electrons move from one atom to another in a
"flow" - called the “bucket brigade method”
 This method is similar to the fire fighter's bucket
brigades in olden times.
Moving electrons and circuits
 Electrons with a negative charge, can't "jump"
through the air to a positively charged atom.
 They have to wait until there is a link or bridge
between the negative area and the positive area.
 We usually call this bridge a "circuit."
 When a bridge is created, the electrons begin moving
quickly.
 Depending on the resistance of the material making
up the bridge, they try to get across as fast as they
can.
Electric systems
 How do we construct a circuit?
 What are the principal parts to be found in
every circuit?
 What happens to bring about the action of
electric circuit?
– the principal activity involves electric
charge – when we arrange for electric charge
to move in a pre-determined way, we achieve
an electric current.
- to produce this effect, we require to enlist
the aid of an e.m.f. (electromotive force)
Electric systems
 George Ohm related e.m.f. to the current in his
simple law – Ohm’s law, which states:
 the ratio of voltage to current is constant,
provided other physical factors such as
temperature remain unchanged; meaning,
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the amount of current in a circuit is
directly proportional to voltage across
the circuit, and is inversely proportional
to the resistance of the circuit
 By applying Ohm’s Law we can find out about
resistance which is an important physical
property associated with all circuits;
Ohm’s Law
 A conductor has a resistance of 1 Ω (ohm)
if a potential difference (pd) of 1 V (volt)
across its ends causes a current of 1 A
(ampère) to flow through it.
R 
V
I
Current refers to the movement of charges
 In an electrical circuit electrons move from
the negative pole to
the positive.
 If you connected the
positive pole of an
electrical source to the
negative pole, you
create a circuit.
Current (I)
 The number of electrons we are willing to let across
the circuit at one time is called "current".
 We measure current using amperes, or "Amps".
 One AMP is defined as:
18
6
.
25

10
625,000,000,000,000,000,000
electrons moving across your circuit every
second!
I 
Q
t
 Since no one wants to remember such a big number,
that big number is called a "coulomb," after the
scientist Charles A Coulomb who helped discover
what a current of electricity is.
One coulomb (C) is defined as the amount of charge (Q) that
passes a point in a conductor in one second (s) when the
current in the conductor is one ampère: Q = It
Symbol
Meaning
Unit
Q
Charge moving in conductor
C (coulomb)
I
Current in conductor
A (ampère)
t
Time during which current passes a
point in a conductor
s (seconds)
Task
1. How long will it take a charge of 80C to
pass a point in a conductor if the current
flowing is 2A?
2. What is the charge in coulomb that
subsists in a circuit while a current of 2A
flows through it for 2mins?
Voltage (V)
 The amount of charge between the sides of the circuit
is called "voltage” or potential difference (p.d.)
 Voltage is the measure of electrical "push" ready to
motivate electrons to move through a conductor.
 We measure Voltage in Volts. The word volt was
named after another scientist, Alexander Volta, who
built the word's first battery.
 You'll remember that we defined energy as the "ability
to do work." Well, one volt is defined as the amount
of electrical charge needed to make one Coulomb
(625,000,000,000,000,000,000 electrons) do one a
specific amount of work -- which is labeled one joule.
Joule is also named after a scientist, James Prescott
Joule.
Voltage (V)
 In scientific terms, it is the specific energy per unit
charge, mathematically defined as joules per
coulomb.
 It is analogous to pressure in a fluid system: the force
that moves fluid through a pipe, and is measured in
the unit of the Volt (V).
Resistance (R)
 We can limit the number of electrons crossing over
the "circuit," by letting only a certain number through
at a time.
 And we can make electricity do something for us while
they are on their way.
 For example, we can "make" the electrons "heat" a
filament in a bulb, causing it to glow and give off
light.
 When we limit the number of electrons that can cross
over our circuit, we say we are giving it
"resistance.". We "resist" letting all the electrons
through. Copper wire is just one type of bridge we use
in circuits.
Effects of resistance
 Before electrons can move far, however, they can
collide with one of the atoms along the way.
 This slows them down or even reverses their
direction.
 As a result, they lose energy to the atoms. This
energy appears as heat, and the scattering is a
resistance to the current.
The 4 constituents of a basic
electrical system
1. The source: The function of the source is to
provide the energy for the electric system. A
source may usually be thought of as a
battery or a generator
2. The Load: The function of the load is to
absorb the electric energy supplied by the
source. Most domestic electric equipment
constitutes loads. Common examples include
lamps, heaters, kettles, stoves etc.
The 4 constituents of a basic
electrical system
3. The transmission system: This
conducts energy from the source to
the load. Typically the transmission
system consists of insulated wire.
4. The control apparatus: As the name
suggests, its function is to control. The
most simple control is a switch which
permits the energy to flow or else
interrupts the flow.
Simple circuit
 Draw a diagram
Systems with increased loads
 Electrical technology is very complex,
thus our e.g. of a simple circuit seldom
exists except in battery torches;
 In this regard we look to systems with
an increased number of loads;
 To handle 2 or more loads we need to
be adept at recognizing seriesconnected and parallel-connected loads
Series and parallel connected loads
 Loads are series connected when the
same current flow passes through each
of them;
 Loads are connected in parallel when
the same potential difference is applied
to each of them.
Characteristics of Series Connections
1. Current has a single pathway through the circuit.
2. Total resistance is the sum of the individual resistances along
the circuit path.
3. The current in the circuit is equal to the voltage supplied by
the source divided by the total resistance of the circuit.
4. The total voltage impressed divides among individual devices
in the circuit so that the sum of the “voltage drops” across the
resistance of each individual device is equal to the total voltage
supplied by the source.
5. The voltage drop across each device is proportional to its
resistance.
Characteristics of Parallel Connections
1. Each device connects the same two points A and B of the
circuit. The voltage is therefore the same across each device.
2. The total current in the circuit divides among the parallel
branches. The current in each branch is inversely
proportional to the resistance of the branch.
3. The total current in the circuit equals the sum of the
currents in its parallel branches.
4. As the number of parallel branches is increased, the overall
resistance of the circuit is decreased.
Use of A-meter and V-meter
 A-meter is connected in series with
the load
 V-meter is connected in parallel
(across) to the load or energy source
Digital communication: Binary system
 Logic gates: AND-gate and OR-gate
 use circuits to model how these two gates
operate;
 determine the rule governing the operation
of these gates;
 use the computer to simulate how these
gates operate;
 apply what you have learnt to solve a real
life situation problem.
Boolean addition
 Boolean addition is equivalent to the
OR function and basic rules are:
0+0=0
0+1=1
1+1=1
Boolean Multiplication
 Boolean multiplication is equivalent to the
AND operation and the basic rules are:
 0.0=0
 0.1=0
 1.1=1
 Note: The dot (.) implies multiplication.
 A product term is equal to 1 if only all the
variables are 1.
 A product term is equal to 0 when one or
more of the variables are equal to 0.
Series and parallel connected controls
(switches)
 A logic gate is one that behaves like a
switch, i.e. two position device with on
and off states.
 This is termed a binary device, in which
the on state is represented by 1 and the
off by state by 0.
The OR function
 Either A or B should happen
 F = A OR B which in logic terms is
represented as:
 F= A+B
 NB. (+ not additive function but means
OR in logic)
The AND function
 Both A and B should happen;
 F = A AND B which in logic terms is
represented as:
F  AB
Task
 Ms Dube, being an eager teacher, had
planned to demonstrate how simple
electronic circuits and devices are
used to make an output signal
respond to an input signal. In her
planning she assumed certain prior
knowledge and competency. She was
terribly disappointed when her
learners didn’t demonstrate the
necessary prior knowledge and
competency in the following areas:
Homework
 understanding of electrical circuits with more than
one output device in the circuit (series and
parallel) and represent them using systems diagrams;
 understanding of how electrical circuits with more
than one input or control device will work based
on different logic conditions (‘AND’ and ‘OR’ logic) and
represent them using circuit diagrams, systems
diagrams and truth tables
 To do:
 Discuss how you plan a way forward for Ms Dube. In
other words, how would you help her teach this
section?
Power
 The rate at which energy is delivered to a load / the rate of
energy transfer / rate of doing work is called electric power.
 Electric power can be measured by the product of current and
voltage (power = current x voltage).
 Electric power is measured in watts (W).
 Thus one watt of power is the result of one ampere of current
driven by one volt of force through a circuit.