Introduction and classification of conducting materials

1. Conducting Materials

2. • Resistance: It is the property of the material that oppose the flow of
electric current.
• Effect of material dimensions
• Effect of temperature
• Effect of Material
• Why thick wires are used for greater current capacity?
Electrical Properties of Conducting materials
𝑅 = 𝜌
𝐿
𝐴

3. Electrical Properties of Conducting materials
• Conductivity (σ) : The conductivity (σ) is the reciprocal of electrical
resistivity of the material.
• Units: mhos/cm
• It is the property of a material due to which the electric current flows
easily through the material. In other words it provides ability to the
material to flow electric current through the material.
𝜎 =
1
𝜌

4. • Temperature
The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher
the temperature the more violently the atoms vibrate. The vibration of the atom due to the collision
of electrons produces heat. The collision hinders the path of electron flow and thus the resistance
of the conductor increase with an increase in the temperature. It has Positive temperature
coefficient.
α is the temperature coefficient of resistance.
Q: The resistance of a platinum resistance
thermometer at 0 C temperature is 3Ω and at
100 C it is 3.75Ω. Find the resistance at 20 0C.
Factors affecting resistance
𝑅1 = 𝑅0(1 + 𝛼∆𝑇)
At Room Temp
At elevated Temp

5. Alloying
• Alloying is the process of combining more than two material by heating and formation of
another material.
• By adding some impurities (a small percentage of some other material) to a metal its
resistivity/resistance can be increased.
• The increase in resistance can be due to formation of irregular arrangement of atoms due
to diffusion of material.
• Alloys have higher resistivity than the pure base metal.
• By alloying copper with zinc its resistivity is increased i.e. conductivity is decreased by about
4 times.
• But the strength of brass is much more than that of copper and therefore may be used for
making structural products such as rods, shafts, heavy plates, plug points, socket outlets,
knife switches etc. where high strength and hardness are usually desirable
Factors affecting resistance

6. Effect of mechanical stressing on resistance:
The resistivity of a material changes also under the influence of mechanical treatment.
The fabrication of conductor to the final stage comprises initially hot working and finally cold
drawing. Cold working operation (stressing)distorts the crystal structure of the metal.
This generally tends to harden the material, increase its tensile strength and increase slightly
its resistivity.
The increase in tensile strength is very useful for many purposes such as overhead conductor. T
hat is why many types of conductors are finally drawn in cold stage in which case they are
identified as hard drawn.
Although mechanical stressing increase the resistivity i.e. decreases the conductivity, annealing
(heat treatment process) restores the electrical conductivity by establishing regularity in crystal
structure.
Factors affecting resistance

7. Classification of Conducting materials
Conducting materials
High resistivity Materials
• Heating element
• Starters
• Resistances
• Filaments
Low resistivity Materials
• House Wiring
• Power transmission
• Windings
All application where power loss and
voltage drop should be low.
Eg: Transmission of power from bhakhra
to Bathinda
Materials: Copper, Aluminum, Silver
All such applications where a large
value of resistance is required .
• if low resistivity materials were used:
the length of the wire would be too
large. Eg: Manganin, Nichrome etc.

8. Classification of Conducting materials
Low Resistivity Materials:
Apart form low resistivity, the materials should also have following properties:
• Low temperature coefficient: The change of resistance with change in temperature should be low. This is necessary
to avoid variation in voltage drop and power loss with changes in temperature.
• Eg: the resistance of transmission lines which are very long will increase when exposed to hot summer sun. this
will cause increase in voltage drop and power loss in the transmission line.
• The windings of electrical machines andapparatus become hot when loaded. This causes temperature rise and if
theconducting material of the winding has high temperature coefficient of resistance, the voltage drop and
power loss in the winding will be high
• Mechanical Strength: To bear the mechanical loadings eg. Overhead wires: wind loading; windings
• Ductility:The conducting material should be ductile enough to enable itself being drawn into different sizes and
shapes
• Solderability: The joint should offer minimum contact resistance. A simple joint would be to twist the conductors with
the material to which it is to be jointed. But this gives high contact resistance. Minimum contact resistance results if
the joint is soldered. All materials do not lend themselves to proper soldering. So while selecting a conducting
material, this point should be kept in view.
• Resistance to corrosion: the conducting material should be such that it is not corroded when used in out door
atmosphere.

9. Classification of Conducting materials
High Resistivity Materials:
Apart form low resistivity, the materials should also have following properties:
• Low temperature coefficient: high resistivity materials are often used as shunts in electrical measuring instruments,
in making wire wound precision resistance and resistance boxes. For such precision applications an important
requirement is that the material of the element should have negligible temperature coefficient of resistance as
otherwise the accuracy of measurements will be reduced.
• High melting point: in application like loading rheostats and starters for electrical motors the material of the resistance
element should be able to withstand high temperature for a long time without melting.
• No tendency for oxidation: materials used as high resistance elements in heating appliances should be able to
withstand high temperatures for a long time without oxidation.
• Ductility: The conducting material should be ductile enough to enable itself being drawn into different sizes and
shapes
• Mechanical Strength: used for applications where the wire must be very thin are required to have high tensile
strength as otherwise they may break during the drawing of the wire or during the assembly and subsequent
operation.

10. Low resistivity Material: copper alloys
Copper
• Due to its high conductivity and reasonable cost, copper is most widely used metal for electrical purposes
• It is a crystalline, non-ferrous, nonmagnetic (diamagnetic), reddish colored metal.
Advantages:
• It is a ductile metal having a ductility of more than 15%. By virtue of this property, it can be easily drawn into thin bars
• and wires. Hence, it is very useful for making cables, strands, and conductors.
• Its ultimate tensile strength is high enough (300–350 MPa) which makes it substantially strong to sustain mechanical
loads.
• Its melting point is sufficiently high (1083°C) that makes it suitable for use at high temperatures also.
• When exposed to atmospheric environment, it forms a protective layer of copper oxide (CuO). Thus, the copper is
highly resistant to corrosion which is a desired property for bare/open overhead conductors.
• It can be easily brazed (a kind of welding) which is a necessary requirement in electrical wiring and other connections.
Types:
1. Annealed copper, and
2. Hard drawn copper.

11. Annealed copper is more ductile than the hard drawn copper. It can withstand severe bending and forging stresses
without failure. It is used as power cables, winding wires for electrical machines and transformers, and in making coils.
Hard drawn copper possesses high mechanical strength. It is suitable for overhead transmission wires etc.
Description Annealed Copper Hard drawn Copper
Conductivity Higher Lower
Tensile strength Less More
Hardness Less More
Resistivity 1.72x10-8 1.77x10-8
Applications Low voltage Power cables,
Insulated conductors, coils,
flexible wires, transformers
Overhead conductors,
high-voltage cables, under
ground cables
Low resistivity Material: copper alloys

12. 1. Brass = Cu + Zn
2. Bronze = Cu+Sn
Both are available with different compositions so have different properties.
Brass:
• It is an alloy of copper containing 40% Zn.
• Its conductivity is lower than that of copper.
• It has a high tensile strength and is fairly resistant to corrosion.
• It can be easily pressed into a desired shape and size, can be drawn into wires, and can be easily brazed.
• Brasses are widely used in the following applications: plug-points • socket-outlets • lamp holders • fuse holders •
switches • knife switches • sliding contacts for rheostats and starters, etc.
Low resistivity Material: copper alloys

13. Low resistivity materials: copper alloys
Bronze:
• It has a composition of 10% Sn in 90% Cu.
• Its conductivity is lower than that of pure copper.
• Bronze components are generally made by forging process.
• It is corrosion resistant and possesses high strength.
• Different types of bronze are generally used in the following applications.
• Beryllium bronze for making current carrying springs, sliding contacts, knife-switch blades etc.
• Phosphor bronze for making springs, bushings etc.
• Cadmium bronze for making commutator segments.

14. High resistivity materials: Nickel
• It is a crystalline, non-ferrous, ferromagnetic metal of silvery-white colour.
• Its hardness matches with the hardness of soft steel but ductility is less than that.
• It is capable of high quality polishing, thereby provides luster to the products on which it is polished.
• It is reasonably malleable and can also be rolled provided the carbon content is in small amount (upto 0.05% or less).
• It is resistant to acidic attacks, but dissolves readily in nitric acid.
• Its electrical resistivity at 20°C is 1.05 10–7 ohm-m and thermal conductivity is 54 W/m-K.
Applications:
Nickel is extensively used for nickel-plating of metals to provide protective coating against corrosion. Carbonized nickel is
used to make anodes of power tubes for rapid conduction of heat.

15. High resistivity materials: Alloys
• Nichrome, a nickel-chromium alloy, having a composition of about 79-80% Ni + 19-20% Cr + 1-1.5% Mn + some Fe.
• Constantan, a copper-nickel alloy, having a composition of about 60% Cu + 40% Ni.
• Manganin., a copper-manganese alloy, having a composition of about 86% Cu + 12% Mn + 2% Ni.
Applications:
• Shunts in electrical measuring instruments
• Wire-wound precision resistances
• Filaments for incandescent lamps.
• Starters for electric motors
• Loading rheostats
• Heating elements for heaters, ovens, starters etc
Parameter Nichrome Constantan Manganin
Resistivity 110 x 10-8 52 x 10-8 48 x 10-8
Melting Pt. 1540 oC 1300 oC 1020 oC
Permissible
working
temperature
1100°C 850°C 700°C
Important
applications
Heating
elements for
electric furnaces
and ovens,
room heaters,
electric Iron
Resistance
elements for
rheostats,
starters of
electric
motors
Wire-wound
shunts and
precision
resistances, coils
for precision
measuring
instruments

16. SuperConductivity
• The phenomenon of superconductivity, in which the electrical resistance of
certain materials completely vanishes at low temperatures.
• In 1911 Kamerlingh Onnes and one of his assistants discovered the
phenomenon of superconductivity while studying the resistance of metals at
low temperatures. They studied mercury because very pure samples could
easily be prepared by distillation.
• As in many other metals, the electrical resistance of mercury decreased
steadily upon cooling, but dropped suddenly at 4.2 K, and became
undetectably small. Soon after this discovery, many other elemental metals
were found to exhibit zero resistance when their temperatures were
lowered below a certain characteristic temperature of the material, called
the critical temperature, Tc.

17. The Meissner effect
• In 1933, Walter Meissner and Robert Ochsenfeld discovered a
magnetic phenomenon that showed that superconductors are not
just perfect conductors.
• Imagine that both the ideal conductor and superconductor are above
their critical temperature, Tc.
• It is found that the superconductor expels the magnetic field from
inside it, while the ideal conductor maintains its interior field. Note
that energy is needed by the superconductor to expel the magnetic
field. This energy comes from the exothermic superconducting
transition.
• Switching off the field induces currents in the ideal conductor that
prevent changes in the magnetic field inside it – by Lenz’s law.
However, the superconductor returns to its initial state, i.e. no
magnetic field inside or outside it.

18. Type-I and Type-II Superconductors
• High magnetic fields destroy superconductivity and restore the normal conducting
state.
• Depending on the character of this transition, we may distinguish between type I
and II superconductors.
• It is found that the internal field is zero (as expected from the Meissner effect) until
a critical magnetic field, Bc, is reached where a sudden transition to the normal
state occurs. This results in the penetration of the applied field into the interior.
Superconductors that undergo this abrupt transition to the normal state above a
critical magnetic field are known as type I superconductors.
• Type II superconductors, on the other hand, respond differently to an applied
magnetic field, as shown in Figure 5. An increasing field from zero results in two
critical fields, Bc1 and Bc2.
• At Bc1 the applied field begins to partially penetrate the interior of the
superconductor. However, the superconductivity is maintained at this point. The
superconductivity vanishes above the second, much higher, critical field, Bc2.

19. Applications of superconductors
• The first large scale commercial application of superconductivity was in magnetic resonance imaging (MRI).
• This is a non-intrusive medical imaging technique that creates a two-dimensional picture of say tumors and other
abnormalities within the body or brain. This requires a person to be placed inside a large and uniform electromagnet
with a high magnetic field.
• Although normal electromagnets can be used for this purpose, because of resistance they would dissipate a great deal
of heat and have large power requirements.
• Superconducting magnets on the other hand have almost no power requirements apart from operating the cooling.
• Once electrical current flows in the superconducting wire, the power supply can be
switched off because the wires can be formed into a loop and the current will persist indefinitely as long as the
temperature is kept below the transition temperature of the superconductor.

20. Insulating materials
• Electrical insulators are materials with a high resistivity (resistivity is a property of the material) so they can make
objects with a high resistance. This allows insulators to prevent electric current from flowing where it's not wanted.
Insulators are useful for coating wires, or acting as dielectrics in capacitors.
Electrical Properties: Volume resistivity, Surface resistance, Dielectric loss, Dielectric strength (breakdown voltage),
Dielectric constant

21. Volume Resistivity
• The volume resistivity of a polymer material is its ability to oppose the flow of electric current through a volume of the
cubic specimen. The SI unit of volume resistivity is ohm-meter (Ohm-m).
• Volume resistivity is also known as:
• electrical resistivity,
• bulk resistivity,
• specific electrical resistance, or
• specific volume resistance.

22. Factors affecting Restivity
• Temperature: The insulation resistance falls off with an increase in temperature. For example, PS has high insulation
resistance. It becomes unsatisfactory above 80°C (176°F). Under these conditions, polymers like PTFE and PCTFE are
more suitable.
• Relative humidity: The insulation resistance falls off with an increase in humidity. Plastics with high water resistance
are less affected by high humidities.
• Voltage: The longer the application of voltage the higher the volume resistivity.
• Presence of fillers: The presence of fillers in the polymer affects the volume resistivity. The type and amount of filler
change the volume resistivity.