20. Which of the following material has the highest
resistivity?
(A) Copper
(C) Plastics
(B) Selenium
(D) Iron
Answers
PLASTIC FRIEND
AS IT IS NOT AFFECTED BY ATMOSPHERE OR SURROUNDING
PLEASE MARK ME BRAINLIST PLEASE
Answer:
1. Tungsten:
(i) Hard metal.
(ii) Resistivity twice that of aluminium.
ADVERTISEMENTS:
(iii) Can be drawn into very thin wires (for making filaments). The thinner the tungsten wire, the greater is its tensile strength.
(iv) In the atmosphere or inert gas (Nitrogen, Argon etc.) or in vacuum, tungsten can be readily worked at temperatures like 2000°C and even higher.
(v) Oxidises very quickly in the presence of oxygen even at temperature of a few hundred degree centigrade.
Uses:
ADVERTISEMENTS:
I. This material is used in electronic and vacuum engineering. Used as filament in lamps, electrodes, heaters, springs etc. It is also used in electron, X-ray and other kinds of tubes.
II. The tungsten filament is made in straight, coiled or coiled-coil form. The straight filament has tendency to sag if the lamp is operated in horizontal position. Coiled filament concentrates the light. Coiled-coil filament is used when an extremely concentrated light source is needed like in projectors.
III. The great hardness and high boiling and melting point of tungsten coupled with its resistance to abrasion establish this metal as an outstanding material for electrical contacts in certain applications. It is extremely resistant to the destructive acting forces.
Properties of Tungsten Conductor:
ADVERTISEMENTS:
(i) Specific weight … 20 g/cm3
(ii) Melting point … 3410°C
(iii) Boiling point … 5900°C
(iv) Thermal coefficient of expansion … 4.4 x 10-9 per degree
ADVERTISEMENTS:
(v) Resistivity … ρ = 0.055 Ω mm2/m
(vi) Temperature coefficient of resistance … α = 0.005
The tungsten is produced by very complicated processes from rare ores or from a tungstic acid.
2. Carbon:
(i) ρ = 1000 to 7000 μ ohm cm, α = – 0.0002 to – 0.0008
ADVERTISEMENTS:
(ii) Melting point = 3500°C
(iii) Density 1.7 to 3.5.
Uses:
1. Carbon is used in automatic voltage regulators for making the pressure sensitive pile resistors.
2. It is used in the manufacture of welding electrodes, fixed and variable resistors for light current and contacts of certain classes of D.C. switch gear.
Carbon brushes reduce considerably the severity of sparking and rate of commutation wear in electrical machines.
3. Nichrome or Brightray B:
Composition:
Ni = 60%, Cr = 15%, Fe = 25%.
Properties:
(i) Resistivity … ρ = 1.10 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.0002 per degree
(iii) Specific gravity … 8.24
(iv) Melting point … 1350°C
Uses:
It is used in making of tubular heaters and electric irons.
4. Nichrome V or Brightray C:
Composition:
Ni = 80%, Cr = 20%
Properties:
(i) Resistivity … ρ = 40 m ohm cm
(ii) Temperature coefficient of resistance … α = 0.0001 per degree
(iii) Specific gravity … 8.4
(iv) Melting point … 1400°C
(v) Possess high resistance to oxidation and temperature effects.
Uses:
It is employed in the making of heating elements and furnaces.
5. Manganin:
Composition:
Cu = 84%; Mn = 12%; Ni = 4%.
Properties:
(i) Resistivity … ρ = 44 μΩ cm
(ii) Temperature coefficient of resistance … α = ± 0.0015 per degree
(iii) Specific gravity … 8.19
(iv) Melting point … 1020°C.
Uses:
It is used in instrument shunts and standard resistance coils.
6. Constantan or Eureka:
Composition:
Ni = 40%; Cu = 60%.
Properties:
(i) Resistivity … ρ = 44 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.00002 per degree
(iii) Specific weight … 8.9
(iv) Melting point … 1300°C
(v) Heat resisting properties – poor.
Uses:
It is used for making starters, field regulator resistances; wires for resistance boxes and thermocouples.