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Tuesday, 30 July 2019

Applications of Superconductors in Physics and Chemistry

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SUPERCONDUCTIVITY:


The resistance of material increases with increase in temperature. Similarly as the temperature is reduced its resistance also decreases. If the temperature is brought down to nearly -273°C, the resistance of some materials becomes zero. A state at which a material attains zero resistivity is called superconductivity. The temperature at which this transition takes place from normal conductivity to superconductivity is called transition temperature. Transition takes place almost suddenly and it has not been accounted for satisfactorily. It is found experimentally that if a current is induced in a mercury ring at a temperature of 4.5 degree Kelvin, it will continue to flow for years without taking any power from the source of supply. Similarly a lead ring carries a current of several hundred amperes over a year with no change.



Superconductivity has been observed to occur in poorer metallic conductors like tin, lead to rather than in better conductors like silver, copper etc. The biggest temperature at which superconductivity has been observed to occur is 20 degree Kelvin for a compound consisting of Niobium, Aluminium and Germanium. The transition temperature of some of the materials is given in Table.

Transition Temperature of Elements

Material
Transition temperature (oK)
Aluminium
1.14
Cadmium
0.6
Indium
3.77
Lead
7.26
Mercury
4.16
Tin
3.72
Uranium
0.8
Zinc
0.786
Pb 2 Au
7
Sn Sb
39
Copper Sulphide
1.6
Zinc Chloride
2.3

APPLICATIONS OF SUPERCONDUCTIVITY


On cooling a tin cylinder in a magnetic field and then measuring the field near its surface, it was found that the flux was suddenly affected as the cylinder became superconducting. It means as the metal became superconductor, the flux within it was thrust out. The magnetic field inside a metal in the superconducting state is zero regardless of its value before entering this state. The occurrence of unique magnetic state in a superconductor corresponding to B = 0 is of great importance because it means that the behaviour of a superconductor in a magnetic field is reversible.

FIELD APPLICATIONS OF SUPERCONDUCTORS IN PHYSICS


Superconductivity as it is does not find much application. This is on account of the difficulty involved in reaching a low temperature of -273°C and maintaining the equipment in that state but it is likely to find application in the following cases:

(a) Electrical Machines: The electrical generators and transformers can be easily manufactured by using superconductivity in exceptionally small size. These manufactured generators and transformers have efficiency nearly equal to 100%.

(b) Power Cables: A 200 kV cable with superconductivity material will enable transmission of power over long distances using thin conductors without any significant power loss and voltage drop.

(c) Electromagnets: Superconducting solenoids have been made in such a way that it do not produce any heat during operation. It acts as a substance with zero magnetic permeability. Therefore the external magnetic field in the superconductor repels each other. The property can be utilized in developing frictionless bearings with magnetic lubrication. These frictionless bearings are most common in gyroscopes and electrical machines.

APPLICATIONS OF SUPERCONDUCTORS IN CHEMISTRY


Helium, an expensive gas is now used to attain low temperatures, required for superconductivity. Efforts are being made to develop compound which are superconductors at temperatures possible to obtain by easily available hydrogen gas.


Monday, 29 July 2019

Materials used for Lamp Filament

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Usually carbon, tantalum and tungsten are used for manufacture of filament. Besides the properties of carbon mentioned above, its commercial efficiency of carbon filament lamp is 4.5 lumens per watt or 3.5 watts per candle power. To prevent the blackening of the bulb, the working temperature is only 1800°C. Tantalum is a material whose resistivity is 12.4 micro ohm-cm. Its temperature co-efficient of resistance is 0.0036 per degree and its melting point is 2900°C. Its specific gravity is 16.6. Its efficiency is about 1.6 watts per candle power. Due to its low efficiency it is not much in use. Tungsten has a high melting point and besides the properties mentioned previously, the efficiency of tungsten filament at a working temperature of 2700°C in an evacuated bulb is 12 lumens per watt.



The steps followed for the preparation of tungsten filament is :

(a) Chemically pure tungsten oxide is reduced at red heat in an atmosphere of hydrogen to metallic tungsten in the form of grey powder.

(b) The powder is pressed in steel moulds into small bars under hydraulic pressure.

(c) The bars are then heated in the furnace upto 1100°C in the presence of hydrogen whereby the articles sinter together. This imparts mechanical strength to the bars.

(d) The mechanical strength is improved further by heating (electrically) the bars almost to melting point.

(e) The bars are then hammered or rolled at red heat to make them ductile and finally drawn into filaments.
To improve the efficiency of the bulb, it is filled with an inert gas, orgon with a small percentage of nitrogen. To minimise the convection currents produced by molecules of the gas in bulbs, the filament is wound into close spiral and suspended horizontally in the form of a circular arc. In high wattage bulbs necks are provided. This is done to avoid blackening of the bulbs as the convection currents carrying particles from the filaments blacken the neck only without impairing the candle power in the direction below the horizontal. The efficiency of gas filled coiled coil tungsten filament lamps is about 25 lumens per watt or 0.6 watts per candle power and thus these lamps are called half watt lamp. Figure shows the straight and coil form of tungsten filament.

Saturday, 27 July 2019

Carbon - Properties and Uses

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Carbon is mostly available at the earth in the form of graphite. This contains about 90% carbon. The armorphous carbon is found in the form of coal coke, charcoal, petroleum etc. The manufacturing process of electrical carbon products consists of grinding of the raw carbon materials, With the binding agent, mix the powered carbon (coal tar) moulding of requisite components and baking them. Copper or bronze powder is mixed with carbon moulding compound to increase the conductivity. Carbon has very high resistivity 4600 x 10-8 ohm-m in graphite form. It has negative temperature co-efficient of resistance, pressure sensitive resistance, and low surface friction. The main application of carbon is brush for DC motors. Some important properties of carbon are listed below:



(a) Pure carbon is a semi-conductor.
(b) Carbon has negative temperature co-efficient of resistance.
(c) It has conductivity slightly less than that of metals and their alloys.
(d) Its melting point is 3900°C
(e) Its density is 1.7 to 35.
(f) Its electrical resistance of carbon contact decreases as the pressure increases.
(g) It has a low surface friction.
(h) It is not affected by moisture, acids and bases.
(i) Its resistivity is 1000 to 70000 micro ohm-cm.
(j) It temperature co-efficient is — 0.0002 to — 0.0008.
(k) It gets oxidised in air beyond 300°C.
(I) It possesses good thermal conductivity.
(m) It can withstand arcing very nicely.
(n) Powdered carbon in conjunction with binding material can be used for making moulds of suitable shapes.

The Uses of Carbon are:

(a) It is used extensively as brushes as it possesses mirror smooth surface, sufficient strength and adequate conductivity.
(b) Used as electrodes for electric arc furnaces.
(c) Used in electrolytic baths and arc welding.
(d) Can be used in arc lamp and as carbon resistors film type and solid type,
(e) Applicable in battery cells.

Table gives the properties of different forms of carbon.

Properties of different forms of Carbon

Type of Brushes
Commutator brushes at velocity of 25 m /sec current density of 20 A/sq. cm and pressure of 0.8 kg/sq. cm
Kind of Brush
Hardness kg/sq.mm
Resistivity x ohm-m
Contact voltage drop / brush pair V
Maximum wear in 20 hrs mm
Max. Friction, co-efficient
Graphite
8 - 22
800 - 2200
0.6 – 1.8
0.4 - 0.5
0.3
Electro- graphitised
3- 50
6- 50
1.1- 2.5
0.4 - 0.6
0.23 - 0.3
Carbon graphite
1 - 42
40 - 50
1.5 – 2.5
0.1
0.3
Copper graphite
4 - 25
0.0 - 15
0.1 – 2.2
0.15 - 3 8
0.2 - 0.26


Friday, 26 July 2019

Classification of High Resistivity Materials based on uses

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Requirements of High Resistivity Conducting Materials :

Some of the essential requirements of high resistivity conducting materials are :

(a) High specific resistance.
(b) Ductility and malleability.
(c) Should not be brittle.
(d) Should have low temperature co-efficient of resistance.
(e) Should have high melting point.
(f) Should not get easily oxidised on heating.
(g) Should be able to dissipate much heat per unit volume.

Classification of High Resistivity Materials based on uses:

High resistivity conducting materials are mostly alloys of different metals. Manganin, Eureka, and Nichrome are examples for this type of material. Based on their practical use they can be divided into three groups.

(a) The first group consists of materials used in precisions electrical measuring instruments, standard resistances and resistance boxes.

(b) The second group consists of materials used for all types of rheostats and similar control devices.

(c) The third group consists of the materials used for high temperature heating elements, loading rheostats etc.

The first group of materials should have least temperature co-efficient of resistivity and good stability in resistance value over long period of time. The thermo electro motive force of this material with copper should be minimum because the e.m.f. affect the accuracy of measurement. The most important material of this group is Manganin.

Bimetal Relay and Thermocouples Working Principle

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Bimetal Relay Working Principle :


A thermostatic bimetal element is based on the theory that metals expand on heating and contract on cooling. Two strips of different metals with different co-efficients of expansion are welded together lengthwise and when heated it bends due to difference in co-efficient of linear expansion of the two metals and regains back it shape when the temperature is removed. Commonly used materials for bimetallic strips are :

(a) High co-efficient of expansion materials Nickel, Iron, Constantan.
(b) Low co-efficient of expansion materials Alloy of iron and nickel.



When two metals are joined together, it is called bi-metals. When these two metals have different co-efficient of linear expansions, it gives an advantage of using it in the field as a thermostat. On application of heat to those bi-metals, each of the metal in the bimetal expands depending upon its co-efficient of linear expansion and since they are fused together, the net result will be bending. The bimetal will bend towards the side of least co-efficient of linear expansion. This phenomenon is used in thermostat to cut off the supply when excessive current flows through bimetals. Figure shows a thermostat using bimetals.

Thermocouples Working Principle :


Thermocouples are used for the measurement of temperature. Depending on the range of temperature to be measured, proper materials are chosen for a thermocouple. When two wires of different metals are jointed at their ends and the junction ends are maintained at different temperatures then an e.m.f. exists across the junctions. The e.m.f. causes a current to flow from hot junction to cold junction upto a certain maximum temperature. If the temperature of hot junction is increased beyond the maximum value, then the current decreases and becomes zero at a particular temperature. If the temperature of the hot junction is still increased, then the current reverses and increases. Current produced in this way by heating the junctions of different metals are known as thermo electric current. The e.m.f. are called thermo e.m.f.

Thermo e.m.f. depends upon the types of metals or alloys and on the difference of temperature between the junctions. The e.m.f. produced by a thermo couple is very small and is measured by a sensitive moving coil multi voltmeter. The temperature of the hot junction, at which maximum current flows is a constant for a given couple, and is known as the neutral temperature for the couple.

The materials used for thermocouple should have the following properties

(a) The e.m.f produced should not change rapidly with time.
(b) Should have high resistance to corrosion and should not oxidise.
(c) The e.m.f. generated per degree should be large enough.
(d) The couple should be used through a broad range of temperatures.
(e) Cost should be less.

Table gives the list of thermocouples with the temperature range and e.m.f. at 500°C.

List of Thermocouples

Material
Temperature range (°C)
e.m.f. at 500 ° C (mV)
Copper / Constantan
200 to 400
27.6
Iron / Constantan
0 to 900
26.7
Nickel Chromium / Nickel
0 to 1100
10
Platinum / Platinum rhodium
500 to 1400
4.5

The thermo electrical e.m.f. has an important application in certain measuring instruments. In Figure the cold junction A is maintained at constant temperature and the temperature of junction B is slowly increased, high resistance milli voltmeter indicates a definite potential difference for any given temperature of junction B.
Circuit for Thermocouple EMF

The instrument can be calibrated to read the junction B temperature directly and employed to measure the temperature of a hot body with which the junction B is placed in contact. This forms a very convenient method of measuring temperatures at remote points or in positions where ordinary thermometers cannot be introduced. The instrument may be joined upto the junctions by quite long conductors, provided they are of low resistance.

The same principle can be applied to the measurement of electric currents. The current to be measured is passed through a short length of resistance of wire or strip. A thermocouple is fixed permanently in contact with or in close proximity to it, and is connected to a high resistance multi voltmeter. The instrument gives a deflection depending on the temperature of resistance or strip, which in turn depends upon the current flowing and is calibrated to read directly in amperes. Such an instrument is termed as thermo ammeter. The thermo ammeters are of importance in measuring alternating currents of high frequency, for which the ordinary types of measuring instruments are quite ineffective.

When a number of thermocouples are connected in series or in parallel, it is called a thermopile. The series of circuit provide high sensitivity than one thermocouple. Some of the circuit diagram is given in Figure.
Thermocouple Circuit
The connection leads are commonly used to contact the measuring junction with the reference junction when the connections are greater than 6 metres. The extension lead should have better mechanical characteristics, lowest resistance and lower cost than that of the thermocouple wire. Material combination of thermocouples can be used upto a temperature of 1150°C.

Properties and Uses of High Resistivity Materials

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Properties and Uses of Mercury:


It is silvery white metal. It is in the liquid state at room temperature and dissolves most metals and forms product abeld amalgums. it is a good conductor of heat and electricity. It is heavy in the liquid state. It gets oxidised if heated beyond 300°C and in the presence of oxygen or air.

Some important properties are listed below :

(a) It is the only metal in liquid state at room temperature.
(b) It is poisonous.
(c) Its expansion and contraction is uniform over a wide range of temperature changes.
(d) Its gets oxidised in the presence of oxygen if heated beyond 350°C.
(e) Its boiling point is 357°C.

It is used in mercury vapour lamps with a high luminous efficiency of about 40 lumens per watt and as arc rectifiers to convert AC to DC. It is used for making and breaking contact in buchholz relay and in thermometers.

Properties and Uses of Tungsten:


Some of the important properties of Tungsten are given below

(a) It is grey in colour and is one of the standard resistance material.
(b) It has the highest melting point of 3300°C amongst all the metals. Therefore, its refractory qualities are very much favourable.
(c) It is very hard metal and does not become brittle at high temperatures. (d) It can be drawn into xery thin wires for making filaments.
(e) Its resistivity is about twice that of aluminium.
(f) It has very high tensile strength in its thinnest form.
(g) It oxidises very quickly in the presence of oxygen even at temperatures of a few hundred degrees centigrade.
(h) It does not exhibit magnetic properties when pure but can easily alloy with steel called Tungsten Steel which is a magnetic material of top quality
(i) Its Atomic weight is 184 and resistivity 5.46 micro ohm / cm2.
(j) In the atmosphere of inert gases or in vacuum it can easily work upto 2000°C.

It is used in incandescent lamps as filaments due to its high melting points. As heater coil in electron tubes.



Properties and Uses of Molybdenum :


It has a high melting point of 2620°C and its boiling point is 3700°C. Its thermal co-efficient of expansion is 5.3 X 10-6 / degree and the resistivity ρ= 0.048 0 mm2/mm. Its normal temperature resistance co-efficient α = 0.047 / degree. It is mostly used as a target in X-ray tube and structure member in high vacuum electron tubes because of its ability to form a tight seal with glass.

Properties and Uses of Tantalum :


It is a material having resistivity (ρ) of 1.24 μΩ - cm and its temperature co-efficient of resistance (α) is 0.0036 / degree. Its melting point is 2900°C and its specific gravity is 16.6. Its efficiency for lamp filament is about 1.6 watts per candle power. Due to its low efficiency it is seldom used.

Properties and Uses of Manganin :


It is an alloy of 86% copper, 12% manganese and 2% nickel. Nickel serves to limit the thermo e.m.f to a low value of about 1 micro V/°C. The physical properties are given below :
Specific gravity - 8.4
Resistivity at 20°C - 48 x 10-8 ohm-m
Temperature co-efficient of resistance - 1 x 10-8/°C
Working temperature - 60 to 70°C
Melting point - 102°C

It can easily be drawn into thin wires. It has high electrical resistance but posses a low value of temperature co-efficient of resistance. It is used in making wire-wound precision resistances for measuring instruments, shunts for electrical measuring instruments, resistance boxes, standard resistance coils and coils for precision electrical measuring instruments. Table 3.1 gives the resistivity, temp. co-efficient and thermo e.m.f. of some compositions of Mn, Ni, Cu.

Composition and constant of Manganin

Composition %
Resistivity
Temp. co-efficient of resistivity per °C
Thermo e.m.f. with respect to copper micro V/ °C
Mn
Ni
Cu



67
1
.5
2.03
12 x 10-5
- 0.5
67
-
33
18.8
Almost zero
- 1.0
67
28
5
22
-29 x 10-5
0.12
67
30
10
20.5
-10 x 10-5
Almost zero

In the second group of materials, it may have large thermo e.m.f. and temperature co-efficient of resistance but must have high working temperature and low cost as these materials are required in large quantities. The principle alloy in this group is Constantan (Eureka).

Properties and Uses of Nichrome :


It is an alloy of 80% of nickel and 20% Chromium or 60% nickel 15% Chromium and 25% iron. Some of the properties of Nichrome is given below :

(a) It is silver white in colour.
(b) It is ductile and can be drawn into thin wires.
(c) Its maximum permissible temperature is 1100°C.
(d) It has a high value of resistivity (100 x 10-8 ohm-m) at 20°C.
(e) Its temperature co-efficient of resistance is 0.0001.

It is used in making heating elements for electric heaters, electric ovens, electric iron, room heaters, electrical furnaces etc.

Properties and Uses of Constantan (Eureka):


It is an alloy of 60 to 65% of copper and 35 to 40% Nickel. Soft constantan wire has resistivity of 45 to 48 x 10-8 ohm-m and hard constantan wire 46 to 55 x 10-8 ohm-m. Temperature co-efficient is nearly zero. Thermo e.m.f. is 39 micro V per degree centigrade with respect to copper and therefore it is suitable for measuring temperature upto 700°C. Its working temperature is about 500°C. When bare constantan wire is heated in air for 3 seconds at a temperature of about 900°C, it acquires a thin film of electrical insulating oxide which withstands turn to turn voltage upto 1 volt. In many instance the constantan is replaced by cheaper alloys Niclin, containing less nickel than constantan due to addition of zinc, is one of such alloys. Its resistivity is 40 micro ohm-cm and maximum working temperature is 300°C. Another alloy is nickel silver alloy with still greater zinc. Its resistivity is 30 to 32 micro ohm-cm and maximum working temperature is between 200 to 300°C. Some of the properties of Constantan is given below :

(a) It can be drawn into thin wires.
(b) Its maximum permissible temperature is about 500°C.
(c) Its melting point is 1300°C.
(d) Its specific gravity is 8.9.
(e) Its temperature co-efficient of resistance is 0.00002 to 0.00005.
(f) It is rust proof and does not corrode in the presence of air or moisture.

It is used in making resistance elements for items like loading rheostat and starters for electric motors, resistance boxes and thermo couples. It is also used for resistance elements in the field regulators used for regulating the generated voltage of a generator.

Properties and Uses of Lead :


It is a soft bluish grey metal. Its specific gravity is 11.36 and melting point is 326°C. It is malleable and ductile. It is mostly used as a fusing material and also for soldering material when alloyed with tin. Solder is an alloy of two or more metals of low melting point which is used to join two or more pieces of metals. The most common solder is an alloy of tin and lead. The most popular composition is 50% lead and 40% tin or 40% lead and 60% tin. The lead solder serves to join copper, bronze, brass, tinned iron, zinc etc. Its melting point is about 185°C and its electricity conductivity is about 10% of copper. There are mainly two types of solders viz., soft solder and hard solder.

Properties and uses of Soft Lead

Soft solders are composed of lead and tin in various proportions.  The most important application  is in the field of electronic devices, coating of iron or steel sheets for roofing and filling of hollow castings etc. The tensile strength is 5.7 kgf/sq. mm and melting point is upto 400°C. The joint made of this type of solder should not be subjected to mechanical stresses due to its poor mechanical strength. Its important properties for various percentage of tin content are given in Table.

Properties of Soft Lead


                              Percentage of tin
0
10
20
33
50
62
Specific gravity
11.4
11.0
10.2
9.6
8.9
8.4
Melting point °C
327
60
180
180
180
180
Tensile strength
1.3
2.8
3.5
4.2
5
3
Elongation %
60
30
30
30
40
30
Co-efficient of linear expansion (kgf / sq. mm)
29
28
26
25
23
21
Electrical resistivity (micro ohm-m)
21
19
18
17
15
14

Properties and uses of Hard Solders


A hard solder is an alloy of copper and zinc. It has a high melting point (790 to 860°C). It is used for joining brass, copper, iron and steel. For brass work the proportion is 3 : 2 zinc silder solders, aluminium solders also belong to this type. The important properties of hard solder are given in Table.

Properties of Hard Solders


Solder
Density kg / sq.mm
Tensile strength kg/sq.mm
Melting point °C
Metals joined
Copper-zinc,  54% copper
8.3
22
860
Copper and its alloys
Silver solder, 70% silver
9.8
30 - 35
730
Copper, brass
Silver solder, 25% silver
8.9
28
765
Copper and its alloys
Silver Cadmium solder
9.7
-
790
Copper and its alloys



Properties and Uses of Fusing Material :


A fuse is a protective device which consists of a thin wire or strip which melts when a particular value of current flowing through it exceeds. Most commonly used materials for fuse wire are lead, tinned copper, zinc, tin, silver, lead-tin alloy, silver alloy, copper alloys etc. Silver is considered to be the best material for fuse wire as it has high conductivity, free from oxidation, low specific heat and non deteriorating properties. The chemical composition and melting point of fusible alloys are given in Table. Chemical Composition and Melting Point of Fusible Alloys

Chemical Composition
Melting Point °C
Bi
Pb
Sn
Cd
Hg
20
20
-
-
60
20
50
27
13
10
-
72
52
40
-
8
-
92
53
32
15
-
-
96
54
26
-
20
-
103
29
43
28
-
-
132
50
50
-
-
-
160
33
-
67
-
-
166
20
-
80
-
-
200

Properties and Uses of Platinum : 


It has a high melting point of 1770°C. Its resistivity is 9.27 x 10-8 ohm-m. It is silver white in appearance and non-corrosive. It can be shaped easily and it is malleable and ductile. It is not affected by many chemicals. Some of the properties of Platinum are given below :

(a) It is even more costly than gold.
(b) It is rust proof and chemically inert.
(c) It can be drawn into thin wires and strips.
(d) Its resistance temperature co-efficient is 0.00307 / °C
(e) It possess thermal stability and does not get oxidised even at high temperatures.
(f) It combines with many metals to form useful alloys.

It is used as heating elements in laboratory ovens and furnaces It is used as electrical contact material and as material for grids in special purpose vacuum tubes. Platinum thermo-couples are used for measurement of temperature upto 1600 degrees centigrade.

Properties and Uses of Tin :


Tin is a white lustrous metal, which is malleable and can be hammered into thin foils. It is soft and weak and is highly resistant to corrosion. It possesses little strength. It is useful constituent for non-ferrous alloys such as bronze, solders etc. It is widely used in the manufacture of solders.

Properties and Uses of Zinc : 


Zinc is a bluish white metal which is moderately ductile and malleable. It is brittle at room temperature and has high creep rate. It is practically in corrodible under ordinary atmosphere. Zinc is mostly used as a protective material for iron and steel.