Solved Question And Answer

SOLVED QUESTION AND ANSWER Suggest materials which are suitable for (a) polishing of glass insulators, (b) pivots for instrument, and (c) giving colures to discharge tube. Ans. Cerium Ce is suitable for glass polishing. Its specific gravity is 6.77 and melting point is 7040C. (b) Iridium Ir is suitable for pivots of instruments. Its specific gravity is 22.5 and melting point 2454oC. (c) krypton Kr is suitable for giving colures to discharge tube. Its melting point is -157.2oC. (2) Which materials are used (a) as power source in satellite, (b) in producing electricity for ‘pace maker’? Ans. (a) Neptunium NP is used as power source in satellite. Its melting points is 673oC. (b) Plutonium PU is used for producing electricity for ‘pace maker’. Its specific gravity is 19.5 and melting point is 640oC. (3) Name the major application of rhenium Re, samarium Sm, Selenium se tantalum Ta, and thorium Th elements. Ans. Major application and properties of these elements are as follows. (1)

Element Re Sm Se Ta Th (4) Ans.

(5) Ans.

(6 ) Ans.

(7) Ans.

Major use Electric furnace winding High temperature superconductors Xerox-type photocopiers Capacitors for electronics industries Incandescent gas mantles

Specificgravity 21.04oC 7.49 4.79 16.6 11.66

Melting point 3180oC 1072o C 217oC 2996oC 1750oC

Enumerate major uses of Yttrium in electrical and other fields. It is used as ‘red’ phosphor for TV tube’, for synthetic garnet in radar, as gemstones, for making high temperature ceramic superconductors in conjunction with barium and copper ect. Its specific gravity is 4.47 and melting point is 1509oC. Differentiate between ‘mylar’ and ‘kevlar’ as electrical materials, quoting their uses. Mylar is aluminized plastic film and Kevlar is polyimide fibre. Mylar is used as reflective insulation for the tankers containing liquid O2, liquid N2 and H2. Kevlar is used as reinforcing fibre to produce polymeric composites for use as solid dielectric. Mylar is generally used is sheet form while the Kevlar is used in fibre. Specific gravity of Kevlar is 1.45. What is meant by thermally actuated process ? Name some thermally actuated processes in materials. Material processes are generally influenced by thermal energy. Therefore, such processes are referred as thermally actuated processes. Some of theses process are (i) Diffusion (ii) Hot working (iii) Recrystallization (iv) Phase transformation (v) Polymerization (vi) Conductivity Semiconductor in semiconductors is also a thermally actuated process. What is diffusivity ? What is its application in electrical materials ? How is diffusivity influenced by temperature ? Diffusivity is a flow process that governs the movement of atoms and molecules in solids. Doping of intrinsic semiconductor (pure elements like Si, Ge) by dopants (e.g.

1

(8) Ans.

(9) Ans.

(10) Ans.

(11) Ans.

(12) Ans.

(13) Ans.

P, As, Ga etc.) is accomplished by diffusion process. Effect of temperature is to enhance the rate of diffusion in materials. Can we produce a material having a bonding force of infinite value. No, this is not possible. We can produce a material having bonding forces equal to its theoretical strength only. Theoretical strength σth of a material is the maximum bonding force per unit cross-sectional area (σth = Fmax/A). According to calculations as per Coulomb’s theory, the theoretical strength may reach almost to the value of Young’s modulus E of the material. Thus for steel, σth = E = 200 GPa. What is Arrhenius law and what is its importance in the studies of materials technology ? Arrhenius law relates the rate of a process as a function of temperature. According to this law, the rate of process φ is related to the temperature T by φ = A e(-Eα/RT) …. (2.10) where A is a pre-exponential (or reaction) constant, Eα is activation energy, and R is gas constant. Arrhenius equation is important in the studies of material process such as creep, corrosion, diffusion, conduction etc.; and in understanding the behaviour of materials at higher temperatures. What are ‘electron affinity’ ‘electronegativity’ and ‘ionization energy’ in respect of atomic bonding ? Electron affinity is the work done to attract an extra electron from infinity to the outermost orbit of a neutral atom. Electronegativity is the tendency of an atom to attract electrons during bond formation with other atoms. Ionization energy is the energy required to remove a loosely bound electron from any orbit of an atom. The concept of electron affinity (expressed in volt/atom) influences upon the atomic behaviour in bonding. The electronegativity demarcates between metals and nonmetals, and ionization energy has an effect on the properties of materials. What is void ? How do they influence the crystal structure ? The vacant space between atoms is called void. Depending upon the packing configuration of atoms in a solid, the voids can be of two types. These are * Tetrahedral voids * Octahedral voids A tetrahedral void is formed in a four atoms system, whereas an octahedral void forms in six or more atoms system. Voids affect the atomic packing, density, conductivity and alloying properties of solids. What is the maximum size of a foreign atom that can be fitted into voids of different geometries ? Why such fitment is required ? Maximum size of a foreign atom that fits into tetrahedral void is 0.225r, where r is the radius of closed packed parent atom. This size of foreign atom in octahedral void is 0.414r. Consideration of such fitments is essential for alloy making. What is meant by allotropy of metals ? Discuss allotropic formes of iron. Some substances exist in more than one crystalline form. Different forms of crystal structure occur at different temperature (or pressure, or both). This behaviour of metals is known as allotropy. Several metals exhibit such behaviour e.g. Sn, Mn, Co, Fe, Ti and Cr. Iron exists in three allotropic forms These are • BCC form upto 9100 C 2

(14) Ans.

(15) Ans.

(16) Ans.

(17) Ans.

(18) Ans.

• FCC form between 9100C – 1410 0C • BCC form again between 14100C-15390C What is metallography ? How is the magnification of a microscope specified ? Metallography deals with the studies of macro and micro structures of metals. It incorporates determining sub-structure and crystal structure also. Besides X-rays and other techniques for crystal studies, microscopes are also used for structure studies. For that a magnification of 1500 × to more than 100 000 × is generally required. Magnification of a microscope is specified by the product of magnifying power of its eye piece and objective lens. For example, a microscope mounted with an eye piece of 20 × and objective lens of 100 × will have its magnification = 20.100 = 2000 . What is slip and a slip system ? What is its importance in understanding the behaviour of metals ? A polycrystalline material deforms by slip which is an irreversible shear deformation. Planes on which slip takes place under the influence of shear stress, are called slip planes. The directions in which they occur are known as slip direction. Combination of a slip plane and a slip direction lying on closely packed planes and in closely packed directions, are termed as slip system. There are 3 slip systems in HCP, 12 in BCC and FCC crystals. Different types of slip systems influence the properties of materials as described in the next question. Electrical materials like copper, aluminum, silver etc. are soft and ductile but molybdenum, vanadium, tungsten etc. are hard and brittle-why ? Softness or hardness of a material is influenced by the force (or the stress) required to deform the planes of atoms. The ductility and brittleness are governed by the amount by the amount of slip in them. If slip is more the metal will be more ductile. Since the slip occurs easily on FCC crystals due to easy slip directions in them, therefore Cu, Al Ag etc. are soft and ductile. Contrary to this, the slip does not occur easily in BCC crystals, hence Mo, Va, W etc. are hard and brittle. How are grain sizes specified in materials as per ASTM specifications ? Grain sizes are designed in accordance with American Society for Testing of Materials (ASTM) from ASTM no. 1 to ASTM no. 12. A grain is called (i) coarse grain for ASTM no. 3 and less, (ii) medium grain for ASTM no. 4 to 6, (iii) fine grain for ASTM no. 7 to 9, and (iv) ultrafine grain for ASTM no. 10 to 12. The average diameter of equivalent spherical grain of ASTM no. 1 is 287 micron, of ASTM no. 6 is 50.7 micron, and that of ASTM no. 12 is 6.4 micron. Explain the relaxation process and relaxation time. Relaxation is a time dependent phenomenon that occurs due to collision of electron with the obstacles present within the crystal (material). Jumping of atoms by diffusion, atomic vibration, flow of grain boundaries, oscillation and damping are some relaxation processes. Time interval of applies forces/fields and the time taken form completion of these relaxation processes may be comparable in some cases and quite different in other cases. A relaxation process may be too slow or to fast as compared to force/field

3

(19) Ans.

(20) Ans.

(21) Ans.

(22) Ans.

(application) time interval. Relaxation processes are specified by relaxation time tr, which is expressed as p = 1 − e −t / tr ) where p is the fraction of relaxation process and t is any arbitrary time. When t= tr then p = 0.63 i.e. the relaxation time is the time taken to complete 63% of a process. For different processes, tr varies from 10-13 second to 107 second. What is electron scattering ? How does it influence the behaviour of electron and the conducting crystal ? Collision of electrons with obstacles within a conducting material, is an inherent phenomenon, Depending upon various factors, they scatter over certain angle θ, after collision. This is called electron scattering. The phenomenon of electron scattering causes loss in its velocity after collision. It also causes transfer of heat energy form electron to crystal lattice. Probability of scattering remains a constant in case the scattering is spherically symmetric. What are the sorueces of presence of obstacles in conducting materials ? Why do we consider an average value of collision time in electrical designing ? The obstacles in materials remain present in different forms. They may be present in natural form such as cementite in steel and iron, oxides in copper and aluminum; or may be present in manmade (unnatural form) such as grain boundaries. These osbstacles are located randomly in different orientations within the (conducting) materials. That is why the collision of electrons with them occurs at different intervals of time. Therefore, the collision time is determined as an average value, and is defined as the average time elapsed between two successive collisions. Discuss the differences between Joule effect and Peltier effect. The above two effects are different in the following respects. S. No Joule effect Peltier effect 1. It is irreversible effect. It is reversible effect. 2.

Heat is always evolved

Heat is evolved at one junction and absorbed at the other.

3.

Heat is produced all along the Heat is produced at one junction conductor. only.

4.

Heat generated is proportional to Heat generated is proportional to the square of current. the current

Establish a relation between temperature coefficient of resistance α2 at t2 temperature and α1 at t1 temperature. With the help of Eq. 6.16, the relation among resistances R1, R2 and R3 at temperature t1, t2 and t3 respectively may be written as R2 = R1 [1 + α1 (t2 – t1)], R3 = R1 [1 + α1 (t3 – t2)], and R3 = R2 [1 + α2 (t3 – t2)] A simple manipulation of above equations will yield a relation given as

4

α2 =

1 1

α1 (23) Ans.

(25) Ans.

(26) Ans.

+ (t 2 − t1 )

What is Wiedemana-Franz law and Lorentz number ? According to Wiedemana-Franz law, the electrical conductivity of solids can be related to their thermal conductivity. It is because all solids conduct heat and electricity. Whereas the thermal conductivity in ionic, covalent and molecular solids is primarily through lattice vibration; the transport of thermal energy in metals and alloys in mainly by free electrons. ne 2 t and thermal conductivity As the electrical conductivity σ= m nπ 2 K 2 Tt where K is Boltzmann constant, therefore their ratio may be Kt = 3m expressed as

π 2K 2T σ 3e 2 K t π2K 2 = ( say L) …. (6.35) = or σT 3e 2 The ration Kt/σT = L for all metals is a universal constant and is known as Lorentz number. Its value is 2.45 × 10-8 WΩ/(Kelvin)2. What is polarization vector ? How is it related to polarization ? Total polarization P expresses the dipole moment per unit volume of the dielectric material. If there are N number of molecules in this volume, the dipole moment/molecule can be expressed as r P = NP (7.26) r in which the quantity P is termed as polarization vector. What is electric susceptibility ? How is related to dielectric constant ? r Since the polarization vector P is proportional to total electric field intensity E and is r in the same direction as E, hence P can be expressed as r r P = ε 0χe E (7.27) χe in this expression is known as electric susceptibility. It is a characteristic of dielectric materials. It is related to dielectric constant as χe= (εr – 1) (7.28) Discuss polarization of a polyatomic gas and show its dependence on dielectric constant. Kt

(24) Ans.

… (6.34)

=

In polyatomic gas, the contribution to total polarization is made by electric polarization, orientation polarization and ionic polarization. If the gas contains N molecules/m3 and various polarizability are pe, p0 and pi respectively, than

5

⎛ p2 ⎞ P = N⎜⎜ 0 + p i ⎟⎟E ⎝ 3kT ⎠

(7.29a)

From macroscopic theory we know that P = ε0 (εr -1), therefore

(27) Ans.

(28) Ans.

(29) Ans.

(30) Ans.

(31) Ans.

⎛ ⎞ p2 (7.29b) ε 0 (ε r − 1) = N⎜⎜ p e + 0 + p i ⎟⎟ 3kT ⎝ ⎠ This equation shows that εr is independent of pe and pi, but is temperature dependent due to orientation polarizability. Is the behaviour of dielectrics same under d.c. field and a.c. field ? If not, how does it differ ? The behaviour of dielectric materials under alternating field is different from the behaviour under static (d.c.) field. It is mainly in respect of dielectric constant whose response is frequency dependent. Consequently, the dielectric constant under alternating conditions is a complex quantity whose real part is a measure of dielectric constant and the imaginary part depicts dielectric losses of material. The polarizability p and dielectric constant εr under static field, take on to complex values p* and ε r* under an applied electric field E0 cosωt. Here ω is angular frequency of applied field. Thus p* = p’ – jp” and ε *r = ε 'r − jε "r (7.30) in which the first terms on right side of equations represent real [Re] parts and second terms represent imaginary [Im] parts. Above quantities are different for different mechanisms of polarizations processes. What is meant by ‘optical absorption’ and infrared absorption ? The dielectric losses in optical frequency region associated with electrons are termed as optical absorption. This is the deciding factor for colour of materials (≈ 1010 Hz for red colour to ≈ 1020 Hz for violet colour). The infrared absorption refers to dielectric losses on account of ionic vibrations whose frequencies are in infrared region (1012 to 1014 Hz). How can the ferroelectric materials be classified on the basis of their chemical composition and structure ? Based on chemical composition and structure, the ferroelectric materials may be classified into following groups. 1. Tartrate group such as Rochelle salt (KH3PO8), NH4Rb. 2. Oxygen octahedron group such as BaTiO3. 3. Phosphates and arsenates group of alkali metals such as KH2PO4. What are anti-ferroelectric materials ? Similar to antiferromagnetic materials (see chapter 8), these are ionic crystals having lines of ions spontaneously polarized in anti-parallel direction with respect to alignment of neighboring ions. Examples of such materials are WO3, NaNbO3, PbHfO3, Ag2H3IO9, etc. Their curie temperatures are in the range of 227 K to 1010 K. What are polar and non-polar dielectrics ? How do they differ ? A dielectric that does not posses any permanent dipole is called non-polar dielectric, whereas a dielectric having permanent dipole moments is known as polar dielectric.

6

The non-polar atoms or molecules have induced dipole moments in the presence of an applied field. These dielectrics are compared below for their characteristics. Description Permanent dipole moments

Polar dielectric Non-polar dielectric Present, even in the Absent absence of an electric field

Alignment of dipoles

They align in the No alignement as direction of applied dipoles do not exist electric field

Induced dipole moments

Yes, always present

Yes, in the presence of an applied electric field

Absorption and emission

Occurs in infrared range

Does not occur

Polarization

Temperature dependent

Tempreature independent

Centre of Examples

(32) Ans.

(33) Ans.

the

symmetry No, they don not have Yes, they have HCI, H2O, CO etc, O2, H2, N2 etc.

Explain Lorentz expression for internal field. In case of solids and liquids, the atoms or molecules are closely packed, but in gases it is not so. Therefore in solids and liquids the internal filed Ei differs from the applied field E, in contrast to gases where Ei equals to E. In three-dimensional case, the expression for internal field is expressed as γP Ei = E + (7.31) ε0 where P is dipole moment per unit volume and γ is proportionality constant for internal field. Although the value of γ = 1.2/π but in the particular case when the atoms in a solid are surrounded cubically by other atoms, the Lorentz internal filed comes into picture. In this case the value of γ = 1/3, therefore P (7.32) (E i ) Lorentz = E + 3 ε0 This equation is known as Lorentz expression. What is Clausius-Mosotti equation ? Some materials like diamond and sulphur consist of a single type of atoms. These materials exhibit only electronic polarization since they neither contain ions not permanent dipoles. If pe is polarizability per atom and N is the number of atoms/m3, then polarization is given by P = N pe Ei Considering Lorentz’s expression for Ei and P = ε0 (ε1 – 1), the above equation may be transformed in terms of εr. Then it can be written as 7

N.p e

(34) Ans.

(35) Ans.

(36) Ans.

(37) Ans.

(38) Ans.

=

ε r −1 εr + 2

(7.34) 3 ε0 This equation is known as Clausius-Mosotti relation. What is the significance of Bohr magneton from the view point of quantum theory of atoms ? Since the orbital angular momentum of an electron is conveniently expressed in the unit of h/2π, therefore the Bohr magneton is also expressed as an order of h/2π, as e h 1 Bohr magneton = . 2m 2π It is the atomic unit of magnetic moment and is of the order of orbital magnetic moment of an electron in an atom. How can the susceptibility χ and permeability μr of a diamagnetic and a paramagnetic material be determined ? These can be determined by measuring the force exerted upon the material in an inhomogeneous filed, by using a ‘Gouy balance’. Since (μr -1) = χ, hence determination of any one parameter will yield the value of other parameter. What are paramagnetic salts and where are they used ? Paramagnetic salts are the rare-earth based compounds and are used to obtain very low temperature (<-2720C) by adiabatic demagnetization. They are used in solid state MASER. Cu, Mn and Al each as seperte metal does not exhibit ferromagnetism but their alloy Cu + Mn +Al (commonly known as Heusler alloy) exhibits ferromagnetism – Why ? A metal or an alloy exhibits ferromagnetism below its ferromagnetic curie temperature Tfc. Tfc of a metal or alloy can be altered (moved up or down) by addition of other elements. The Tfc of Fe is (7720C), of Ni is (3570C) and of Co is (11220C). But an addition of Cu of melting point 10830, Mn of melting point 12450C and Al of melting point 6600C shifts the Tfc of Heusler alloy, hence it exhibits ferromagnetism. Prove that the values of magnetic moments (in Bhor magneton) in iron, cobalt, and nickel are 4,3 and 2 respectively. The atomic number of iron, cobalt and nickel respectively are 26, 27 and 28. The electronic configuration of iron is 1s2 2s2 2p6 3s6 3d6 4s2. The configuration of cobalt and nickel are the same except for the arrangement in 3d-suboribit. Instead of 3d6 in Fe, it is 3d7 for cobalt and 3d8 for nickel. Thus

Element

Number of electrons in 3dsuborbit

Arrangement of spin moment

Fe Co Ni

6 7 8

↑↑↑↑↑↓ ↑↑↑↑↑↓↓ ↑↑↑↑↑↓↓↓

Value of magnetic moment in Bhor magneton = (upward moment – downward moment) 5–1=4 5–2=3 5–3=2

8

Explain as to why the ionization energy of impurity carrier is higher in silicon than in germanium. Ans. The ionization energy is the amount of energy needed to detach the valance electron from the impurity atom. The total energy of an orbiting electron is sum of its kinetic energy and potential energy given as q2 1 (9.36) E total = E KE + E P = mv 2 − 2 4ππ0 ε r r Above equation shows that for other factors being constant, the ionization energy is inversely proportional to dielectric constant εr. Since εr = 16 for germanium and 12 for silicon, hence ionization energy is higher is silicon than in germanium. (40) Why is the impurity concentration in extrinsic semiconductor kept very small ? Ans. The electrical conductivity of pure semiconductors is primarily of electron carrier type. The hole conductivity is comparatively weaker. This makes the semiconductors extremely sensitive to the presence of any impurity. Hence impurity concentration is kept very small, about 1:106 to 1:109. (41) What are different laws of diffusion ? What are their importance to electronic materials and devices ? Ans. Classical laws of diffusion were propagated by Fick. Hence they are known as Fick’s laws. There are tow laws viz. Fick’s 1st law and Fick’s 2nd law. Fick’s 1st law describes the flow (of electrons and holes etc.) under steady state conditions, while the 2nd law states non-steady state flow. These laws are of are great importance to electronic materials and devices because the mechanism of semiconduction by electrons and holes, and the doping process etc. are governed by them. (42) What is LCD ? What are its uses ? Ans. LCD (liquid crystal display) is a liquid crystal cell that consists of a thin layer of a liquid crystal sandwitched between two glass sheets. The electrodes are deposited on the inside faces of these sheets. LCD consumes extremely low power. Consumption of power is much less than the power requirement of LED. The LCD is generally used in • watches • portable instruments • screens of B/W pocket TV receivers etc. (39)

(43) Ans.

(44)

What is cryotron ? What purpose does it serve and how does it work ? A cryotron is an assemble of a superconducting wire (core material) A surrounded by a coil of another superconducting material B. When temperature of a system in which the cryotron is used, is below the transition temperature of two materials, both A and B are superconducting; otherwise they attain a normal state (see Fig. 10.3). Based on this fact, a cryotron is used as an element in control devices such as flip-flop in a computer. Its working may be understood as follows. The current IA in the wire is controlled by current IB in the coil. It is because of the fact that magnetic field produced by the coil normally exceeds the critical field of the wire (core material) at operating temperature. The ntensity of controlling current IB required to make the core in normal state, depends upon d.c. current flowing through the core. It is so as the core current also produces a magnetic field. Suggest suitable materials for making a cryotron. 9

Ans.

(45) Ans.

(46) Ans.

(47) Ans.

(48) Ans.

(49) Ans.

(50) Ans.

Tantalum is a suitable material for making wire (or core), if the operating temperature is extremely low (≈ 4.2 K which is liquification temperature of helium). Since the coil has to be superconducting even if the control current flows, hence niobium or lead are suitable coil materials. What are supercooled coils ? Supercooled coils are such electrical systems which can produce a flux density of 10 tesla or more in superconducting state. This flux density is ‘many ten’ times more than the flux density produced by normal coils at room temperature. What are cryogenic temperature, cryogenic industries, and cryogenic engine ? 115.5 K (-157.50C) is known as cryogenic temperature. Those industries in which the operations are performed below this temperature are called cryogenic industries. Industries involved in manufacturing of liquid oxygen (at -1830C), liquid nitrogen (at1960C), liquid ammonia in fertilizer industry (at = - 1900C) etc. are such examples. Engines using liquid fuels at such low temperatures are called cryogenic engines. High frequency transistors are generally made of germanium and not of silicon, but silicon is used in electronic device operating over a wider temperature range –why ? The carrier mobility of silicon is lower than that of germanium. Therefore, it is inferior to germanium for high frequency services. But the energy gap in silicon is higher than that of germanium. So it offer a greater temperature range service for electronic devices. What are the textured magnetic materials? Textured magnetic materials (also called grain- oriented materials) are made by orienting the crystal /grains in a preferred direction. The orientation is mainly accomplished by rolling process. By texturing the grains, the materials exhi0bits improved magnetic properties in the direction of the aligned orientation than in nonaligned direction. They can be easily magnetized also with a smaller magnetic field. Cold rolled sheet used for making transformer core is an example of textured (or grain- oriented) magnetic material. It has a lower hysteresis loss due to texturing. Also by orienting the grains parallel to electric flux path, a high permeability is obtained which is desired property for transformer core. What are amorphous or oxide magnetic materials? Non –crystalline solids are called amorphous materials. They may be oxide, carbides, halides nitrides etc. Most ceramics are metallic oxides, hence amorphous. Thus, the ceramic based magnetic materials are called amorphusor oxide magnetic materials. They may be amorphous paramagnetic such as Fe2 O3, DAG (Dy3Al5 O12); amorphous ant ferromagnetic such as MnO2, FeO, CoO etc.; and amorphous ferrites such as PbO. 6Fe2 O3, BaO. 6Fe2 O3 ect. What are the effect of different alloying elements on magnetic properties of allowed magnets? Effects of different alloying elements are widely varying. Even different composition of the same element has different effects. These effects are briefly illustrated as follow. Alloying Effects on magnetic properties elements

10

(51) Ans.

(52) Ans.

(53) Ans.

(54) Ans.

Nickel

Improves initial and maximum relative permeability, reduces hysteresis loss and saturation flux density.

Silicon

Improves initial and maximum relative permeability.

Copper

Reduces the net magnetization.

Zink

Reduces the net magnetization, considerably.

Aluminum

Reduces the net magnetization, considerably.

Cobalt

Enhances coercive force and energy product, and imparts magnetization at high temperature.

Manganese

Increase coercivity and hardness.

Tungsten

Increase remanence and abrasion resistance.

What are powder magnetic materials and elongated single domain magnets? How are they useful for application requiring miniature si0ze magnets? These are hard magnetic materials composed of fine particles, bonded together by organic resins or non –magnetic metal binders such as lead. The fine particles in the form of powder are of 1to 10 μm sizes and are manufactured by powder metallurgy technique. These can be produced as Elongated single domain (ESD) magnets also by precipitating on a liquid mercury cathode. Powdered magnetic materials are used to make miniature magnets for fractional horsepower motors for use in hearing aids, earphones, car windshield wiper- motors, clocks etc. What are carbon pile resistors? These resistors are made up of a number of carbon plates, by piling one over the another. The plates are maintained under pressure. The carbon composition consists of finely ground carbon, resin binder and insulating filler. Such resistors are used in electronic circuits for setting biases, controlling gain, loading the circuits etc. in oscillators, precision voltage dividers and communication systems. How can the conductivity of carbon be increased ? To increase the conductivity of carbon, different kinds of additives are used. The additives are mainly copper or bronze powder that are mixed with the carbon moulding compound. Permissible working temperature of manganin is low (about 600C to 700C) but that of the nichrome is high (about 3500C to 6500C) – why ? It is because the resistance coefficient of temperature of manganin is high at higher temperature (about 0.00002/K) than that of nichrome, which is about 0.00001/K. Moreover, the electrical resistivity of manganin is 43 × 10-8 ohm m as compared to 108 × 10-8 ohm m for nichrome. Their other properties, which are also the deciding factor in this regard, are as follows. Tensile strength (MPa) Material Density (kg/m3) Manganin 8200 420

11

(55) Ans.

Nichrome 8410 1000 Nichrome is often designated as nichrome I, III, V; and Hastelloy by hastelloy A,B,C,D. What meaning do they convey ? These designations specify different commercial grades of nichrome and hastelloy. Each grade differs in composition, properties and characteristics; and hence is suitable for different specific applications. The details given below are self-illustrious in this regard. Alloy Composition (%) Application Nichrome I Ni 80%, Cr 20% Thermocouple, heating element of furnaces and ovens, and strain gauges Hastelloy A Ni 60%, Mo 30%, Fe 5% Parts of chemical plants

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