Transcript Unit 51: Electrical Technology

Unit 51: Electrical Technology
The Properties of Conductors
Course Aims
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At the end of this course the learner will
be able to describe…
1. Describe the electrical properties of
conductors: conductivity; resistivity
2. Describe the mechanical properties of
conductors: tensile strength; rigidity
3. Compare and contrast different conducting
materials
4. Describe the conducting properties of liquids
and gases
So what is a
Conductor?
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A conductor is a materials that contains
moveable electrical charges.
In a metal such as copper or aluminium
these moveable electrical particles
(charges) are electrons
Materials that don’t allow electrons to
flow freely are termed insulators
Conductivity v Resistivity
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Conductivity (σ) is a parameter that distinguishes the
physical character of a material with respect to its
ability to allow electrons to ‘flow’ i.e. the greater the
conductivity the easier the electrons will flow.
Conversely another way of looking at this is to
consider how much resistance a material has with
respect to a flow of electrons. This physical property is
termed resistivity (ρ). Thus the greater the resistivity
the harder it is for the electrons to flow.
Thus conductivity and resistivity are ‘two sides of the
same coin’.
Thus σ = 1/ρ
Conductivity v Resistivity
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For a given material, the resistance (R) that that
material offer to the flow of electrons (i.e. an
electric current) depends upon…
– The length of the material (l)
– The cross sectional area of the material (A)
Thus R is proportional to the length (l) of the
material i.e. the longer the material the more the
electrons will have to ‘battle’ to get through the
material
R is inversely proportional to the area (A) of the
material i.e. the smaller the area the harder it will
be for the electrons to ‘squeeze’ into the area
Conductivity v Resistivity
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Thus R is proportional to l / A
In mathematics we can replace a
proportionality sign with an equal sign and a
constant, thus…
R = ρl/A
ρ the constant of proportionality is known as
the resistivity
Rearranging give ρ = RA/l
ρ is measured in Ω.m
Conductivity v Resistivity
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σ = 1/ρ
Thus σ = l/RA
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The unit of conductivity is 1/(Ω.m) i.e. S/m or S.m-1
S (siemens) is the inverse of Ω (resistance)
This resistance (R) is measured in Ohms (Ω)
Conductance (G) is measured in siemens (S)
Thus R = 1/G
Thus G = σA/l
Problems
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• Given the conductivity of silver is 6.30 x
107 S.m-1 at 200C what is the resistance of
the conductor whose length is 3 mm and
diameter 0.5 mm?
• Given the resistivity of lithium is 9.28 x 10-8
Ω.m at 200C what is the conductance of a
length of material is 3 cm long with a cross
sectional area of 5 mm x 12mm?
Linear Thermal
Expansion
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• The linear thermal expansion coefficient
relates the change in a material's linear
dimensions to a change in temperature. It is
the fractional change in length per degree
of temperature change. Ignoring pressure,
we may write:
– αL=(1/L)(dL/dT)
• where L is the linear dimension (e.g. length) and
dL/dT is the rate of change of that linear
dimension per unit change in temperature.
Linear Thermal
Expansion
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• The change in the linear dimension can be
estimated to be:
– ΔL/L = αLΔT
• This equation works well as long as the linear
expansion coefficient does not change much
over the change in temperature. If it does, the
equation must be integrated.
• Thus if lo is the initial length and l is the length
recorded after the change in temperature then
ΔL = l – lo
Linear Thermal
Expansion
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• Thus ΔL/L = (l – lo)/lo
• Thus ΔL/L = αLΔT becomes…(l – lo)/lo = αLT where T is
the difference in temperature
• Rearranging gives l = lo(1+αLT)
• Given the coefficient of linear expansion of aluminium is
23 x 10-6 (/oC) at 20oC calculate the increase in the
length of the 50 m length of the material used as a
overhead power line if the heat radiated from the sun
causes the daytime temperature to rise to 24oC. If the
cross sectional area of the power line is 5 cm and its
resistivity is 2.82 x 10-8 Ω.m calculate its conductance
The effect of
temperature on conductance
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• In general when the temperature of a conductor
increases the resistance increases.
• If the resistance of a conductor increases then
its conductance decreases.
• (Note please don’t confuse thermal conductivity
with electrical conductivity – thermal conductivity
(W/(m.K) – watts per metre-Kelvin) is a measure
of the ‘flow’ of heat through a conductor!)
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Tensile Strength
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• Ultimate tensile strength (UTS), is the
maximum stress that a material can withstand
while being stretched or pulled before necking,
which is when the specimen's cross-section
starts to significantly contract.
• Tensile strength is the opposite of compressive
strength
• The UTS is usually found by performing a tensile
test and recording the stress versus strain; the
highest point of the stress-strain curve is the
UTS.
Tensile Strength
Force pulling
material apart
‘necking’
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Force pulling
material apart
Tensile Strength
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 It is an intensive property; therefore its value
does not depend on the size of the test
specimen. However, it is dependent on other
factors, such as the preparation of the specimen,
the presence or otherwise of surface defects,
and the temperature of the test environment and
material.
• Tensile strengths are rarely used in the design
of ductile members, but they are important in
brittle members. They are tabulated for common
materials such as alloys, composite materials,
ceramics, plastics, and wood
Tensile Strength
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 Tensile strength is defined as a stress, which is
measured as force per unit area. For some nonhomogeneous materials (or for assembled
components) it can be reported just as a force or
as a force per unit width.
 In the SI system, the unit is pascal (Pa) or,
equivalently, newtons per square metre (N/m²).
The customary unit is pounds-force per square
inch (lbf/in² or psi), or kilo-pounds per square
inch (ksi), which is equal to 1000 psi; kilopounds per square inch are commonly used for
convenience when measuring tensile strengths.
Tensile Strength
 Convert 14.5 psi to Pa. (N.m-2)
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Tensile Strength
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Tensile Strength
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Many materials display linear elastic behavior, defined by a linear stress-strain
relationship, as shown in the figure up to point 2, in which deformations are
completely recoverable upon removal of the load; that is, a specimen loaded
elastically in tension will elongate, but will return to its original shape and size when
unloaded. Beyond this linear region, for ductile materials, such as steel, deformations
are plastic. A plastically deformed specimen will not return to its original size and
shape when unloaded. Note that there will be elastic recovery of a portion of the
deformation. For many applications, plastic deformation is unacceptable, and is used
as the design limitation.
After the yield point, ductile metals will undergo a period of strain hardening, in which
the stress increases again with increasing strain, and they begin to neck, as the
cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently
ductile material, when necking becomes substantial, it causes a reversal of the
engineering stress-strain curve (curve A); this is because the engineering stress is
calculated assuming the original cross-sectional area before necking. The reversal
point is the maximum stress on the engineering stress-strain curve, and the
engineering stress coordinate of this point is the tensile ultimate strength, given by
point 1.
The UTS is not used in the design of ductile static members because design
practices dictate the use of the yield stress. It is, however, used for quality control,
because of the ease of testing. It is also used to roughly determine material types for
unknown samples.
Conduction in Liquids
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• An electrolyte is any substance containing free
ions that make the substance electrically
conductive
• The most typical electrolyte is an ionic solution,
but molten electrolytes and solid electrolytes are
also possible
• Commonly, electrolytes are solutions of acids,
bases or salts. Furthermore, some gases may
act as electrolytes under conditions of high
temperature or low pressure.
Conduction in Liquids
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• Electrolyte solutions are normally formed when a
salt is placed into a solvent such as water and
the individual components dissociate due to the
thermodynamic interactions between solvent
and solute molecules, in a process called
solvation. For example, when table salt, NaCl, is
placed in water, the salt (a solid) dissolves into
its component ions, according to the dissociation
reaction
– NaCl(s) → Na+(aq) + Cl−(aq)
Electrolysis
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• Video: Electric Circuit Experiments: Water
as a Conductor | eHow.com
Electrolysis - History
• 1800 • 1807 • 1875 -
• 1886 • 1886 • 1890 -
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William Nicolson and Johann Ritter decomposed water
into hydrogen and oxygen.
Potassium, sodium, barium, calcuim and magnesium
were discovered by Sir Humphrey Davy using
electrolysis.
Paul Emile Lecoq de Boisbaudran discovered gallium
using electrolysis.
Fluorine was discovered by Henri Moissan using
electrolysis.
Hall-Heroult process developed for making aluminium
Castner-Kellner process Castner-Kellner process
developed for making sodium hydroxide
Source: Wikipedia
Electrolysis
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• Electrolysis is the passage of a direct electric current
through an ionic substance that is either molten or
dissolved in a suitable solvent, resulting in chemical
reactions at the electrodes and separation of materials.
• The main components required to achieve electrolysis
are :
– An electrolyte : a substance containing free ions which are the
carriers of electric current in the electrolyte. If the ions are not
mobile, as in a solid salt then electrolysis cannot occur.
– A direct current (DC) supply : provides the energy necessary to
create or discharge the ions in the electrolyte. Electric current is
carried by electrons in the external circuit.
– Two electrodes : an electrical conductor which provides the
physical interface between the electrical circuit providing the
energy and the electrolyte
Laws of Electrolysis
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• Faraday's 1st Law of Electrolysis - The mass of a
substance altered at an electrode during electrolysis is
directly proportional to the quantity of electricity
transferred at that electrode. Quantity of electricity refers
to the quantity of electric charge, typically measured in
coulomb
• Faraday's 2nd Law of Electrolysis - For a given
quantity of electricity (electric charge), the mass of an
elemental material altered at an electrode is directly
proportional to the element's equivalent weight. The
equivalent weight of a substance is its molar mass
divided by an integer that depends on the reaction
undergone by the material
Laws of Electrolysis
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• Faraday 1st law can be summarized by
–
• where:
– m is the mass of the substance liberated at an electrode in
grams
– Q is the total electric charge passed through the substance
– F = 96,487 C mol−1 is the Faraday Constant
– M is the molar mass of the substance
– z is the valency number of ions of the substance (electrons
transferred per ion).
• Note 1: M/z is the same as the equivalent weight of the
substance altered.
• Note 2: m is proportional to Q
Laws of Electrolysis
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• For Faraday's second law, Q, F, and z are
constants, so that the larger the value of
M/z (equivalent weight) the larger m will
be.
• In the simple case of constant-current
electrolysis, Q = It leading to
Application of Electrical
Conduction in Liquids
1. Electroplating of metals.
2. Electro-refining of metals.
3. Extraction of metals or Electrometallurgy.
4. Battery
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Electroplating of
Metals
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Electroplating is a process whereby a thin coating of
desired material is applied on a required material. This is
mostly done on stainless steel to prevent rusting, or on
some decorative items, so that they look attractive. On
stainless steel, generally nickel-chromium plating is done.
On decorative items, such as spoons, plates, jewelry items,
silver, gold or other plating is done. Electroplating is cheap
and cost effective. It enhances the life of the object and
makes it look better in appearance.
Electroplating of
Metals
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First the item to be electroplated is smoothened and cleaned thoroughly.
It should not have any oily or dirt marks on it.
An electrolyte is selected whose ions are required to be deposited on the
item.
Direct current is preferred to alternating current, as alternating current
may result in non-smooth deposit.
The item to be electroplated forms the anode or cathode of the
electrolytic cell. This is the drawback of the electroplating process. The
item has to be electrically conducting, or has to be made electrically
conducting.
For a smooth coating, the electrolytic process has to be optimized for
time, temperature and current in the cell.
Extraction of Metals
(Electro-metallury)
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Extraction of metals by the process of electrolysis is
known as electro-metallurgy. This process is used in
case highly reactive metals such as sodium. An ore
containing sodium is used in a molten form. This forms
the electrolyte. Anode and cathodes are generally
carbon rods or steel. The Na atoms get attracted to the
cathode of the cell and then the entire cathode with its
coating is stored for further use.
Battery
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All batteries that we come across in our day to day use, including car
batteries, dry cells used in torches, calculators, hand-sets, etc. are all
examples of an electrolytic cell. But in this case the reverse of an actual
electrolytic process is being used. The chemicals inside the cells produce
current (and voltage) which is utilized.
In a car battery, for example, two grids are used as anode (Pb) and
cathode (PbO2). The solution is H2SO4 of generally about 6 M in
concentration.
Conduction in a Gas
(or Plasma)
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In air and other ordinary gases below the breakdown field, the
dominant source of electrical conduction is via a relatively small
number of mobile ions produced by radioactive gases, ultraviolet
light, or cosmic rays.
Since the electrical conductivity is low, gases are dielectrics or
insulators. However, once the applied electric field approaches
the breakdown value, free electrons become sufficiently
accelerated by the electric field to create additional free electrons
by colliding, and ionising, neutral gas atoms or molecules in a
process called avalanche breakdown.
The breakdown process forms a plasma that contains a significant
number of mobile electrons and positive ions, causing it to behave
as an electrical conductor. In the process, it forms a light emitting
conductive path, such as a spark, arc or lightning.
Plasma
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Plasma is the state of matter where some of the
electrons in a gas are stripped or "ionized" from their
molecules or atoms.
A plasma can be formed by high temperature, or by
application of a high electric or alternating magnetic
field.
Due to their lower mass, the electrons in a plasma
accelerate more quickly in response to an electric field
than the heavier positive ions, and hence carry the
bulk of the current.
Applications of Electrical
Conduction in Gases
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Electrical conduction in gases have a few but significant
commercial and scientific applications. These include…
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…thyratrons, gaseous rectifiers, ignitrons, glow tubes, and
gas-filled phototubes. These tubes are used in power supplies,
control circuits, pulse production, voltage regulators, and
heavy-duty applications such as welders.
…In addition, there are gaseous conduction devices widely
used in research problems. Some of these are ion sources for
mass spectrometers and nuclear accelerators, ionisation
vacuum gauges, radiation detection and measurement
instruments, and thermonuclear devices for the production of
power