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Tuesday, 17 February 2015

F.Sc ICS Notes: Physics XII: Chapter 17 Physics of Solids Exercise Short Questions:

FSc ICS Notes: Physics XII: Chapter 17 Physics of Solids Exercise Short Questions:

Question 17.1 Distinguish between crystalline, amorphous and polymeric solids.
Answer 17.1 Crystalline Solids: Crystalline solids are those in which their atoms/molecules are arranged in particular fashion.They have definite space.Compound like sodium chloride and ceramics are crystalline solids. Atoms/molecules in crystalline structure vibrate continually about their means position with certain amplitude. The amplitude of vibration increases with rise of temperature of solid. The cohesive forces between atoms,molecules or ions in crystalline solids maintain the strict long-range order in-spite of atomic vibrations. For every solid, there is particular temperature at which the vibration of the atoms/molecules of solid become so large that the structure if the solid suddenly breaks and the solid melts. This temperature is called melting point. Every crystalline solid has a definite melting point.

Amorphous Solids: The solid which has no particular arrangement of their atoms/molecules is called amorphous or glassy solid. As there is no regular arrangement of atoms, therefore amorphous solids are more like liquid with the disorder structure frozen in e.g., Glass which is solid at ordinary temperature has no regular arrangement of molecules. On heating, it softens into paste like state before it becomes very viscous liquid at almost at 800 C. Thus amorphous solids are called glassy solids. These types of solids have no definite melting point.

Polymeric Solids: Polymeric solids are more or less solids materials with a structure between order and disorder. They can be classified as partially or poorly crystalline solids. e.g. Natural rubber which is in pure state composed of hydrocarbon. Plastic and synthetic rubber are termed as polymers. because they are formed  by p[polymerization reaction in which relatively simple molecules are chemically combined into massive long chain molecules have three dimensional structure. These materials have low specific gravity compared with even the lightest of metals. Polymers consist completely or in part of chemical combination of carbon with oxygen, hydrogen, and other metallic or non-metallic elements. polythene, polystyrene and nylon are examples of synthetic polymers.

Question 17.2 Define stress and strain. What are their SI units? Differentiate between tensile, compressive and shear modes of stress and strain.
Answer 17.2 Stress: It is defined as the force applied on unit area to produce any change in shape, volume, and  length of a body. Mathematically, if F is the force applied on area A, then stress can be expressed as:
                                                      Stress = Force/ Area = F / A
Unit of stress is Newton-meter or pascal. 

Strain: It is defined as the fractional change in length, volume or shape on the application of stress, it has no unit.

Tensile Stress: When a stress changes length it is called tensile stress.
Tensile Strain: It is defined as the fractional change in length on applying stress.

Compressive stress: It is an overall force per unit area (pressure) applied.
Compressive strain: The strain produced as a result of compressive stress.

Shear stress: It is a stress tending to produce an angular deformation or it changes the shape then it is called shear stress.
Shear strain: It occurs when an angular deformation occurs, and is equal to the angular displacement produced.

Question 17.3 Define modulus of elasticity. Show that the units of modulus of elasticity and stress are the same. Also discuss its three kinds.
Answer 17.3 Modulus of elasticity: The ratio of the stress on a body to the strain produced is called modulus of elasticity. It has three types.

Young’s modulus: It is defined as the ratio of tensile stress to tensile strain. Mathematically it is:
                            Young's Modulus = Tensile Stress / Tensile Strain

Bulk modulus: The ratio of volume stress to volume strain is called Bulk Modulus. Mathematically it is:                             Bulk Modulus = Comprehensive Stress / Volume Strain

Shear modulus: The ratio of the shear stress on a body to the shear strain produced in the body. Mathematically it is:
                            Shear Modulus = Shear Stress / Shear Strain

Units of E = units of stress / units of strain = units of stress / dimensionless = units of stress.
So units of modulus of elasticity and units of stress are same, as units of strain have no units.

Question 17.4 Draw a stress-strain curve for a ductile material, and then define the terms: Elastic limit, Yield point and Ultimate tensile stress.
Answer 17.4 
Elastic Limit: It is defined as the greatest stress that a material can endure without any permanent change in shape or dimension.

Yield Point: The point at which the material begins to ‘flow’, i.e. the strain increases with time up to breaking point without further increase in the stress.

Ultimate Tensile Stress: It is denied as the maximum stress that material can withstand and can be regarded as nominal strength of the material.

Question 17.5 What is meant by strain energy? How can it be determined from force-extension graph?
Answer 17.5 According to Hook's Law, the force-extension graph with in elastic limit is straight line. The energy stored in the solid when it is extended. When the deforming force changed form  0 to F then extension is produced. The average force is given by:
                                             (0 + F) / 2 = F / 2
The work done is extending the solid through x is given by
                                              W  = (F /2) * x
The work done in extending is stored as strain energy in solid.

Question 17.6 Describe the formation of energy bands in solids. Explain difference amongst electrical behavior of conductors, insulators and semiconductors in terms of energy band theory.
Answer 17.6  Electrons of an isolated atom are bound to the nucleus and can have energy levels which are distinct. However when a large number of atoms say N are brought close to one another to form a solid each energy level of the isolated atom splits into N- sub-levels, called states, under the action of the forces exerted by other atoms in the solid. These permissible energy states are discrete but so closely spaced that they appear to form a continuous energy band. In between two permissible energy bands, there is range of energy states which cannot be occupied by electrons. These are called forbidden energy states and its range is said to be forbidden energy gap.
  1. Valence Energy Band
  2. Conduction Energy Band
  3. Filled Energy Band
Conductors: Conductors are those substances which have plenty of free electrons for electrical conduction it means that conductors are those materials in which valence and conduction bands largely overlap each other. There is no physical distinction between the two bands which ensures the availability of a large number of free electrons due to overlapping of conduction and valence energy bands.

Insulators: Insulators are those substances in which valence electrons are  bound very tightly to their atoms and are not free to moves, it means that an insulator has an empty conduction band a full valence band and a large energy gap between them.

Semi Conductors: Semi conductors at room temperature have the following properties: partially filled conduction band, partially filled valence band and very narrow forbidden gap between valence and conduction band.

Question 17.7 Distinguish between intrinsic and extrinsic semi-conductors. How would you obtain n-type and p-type material from pure silicon? Illustrate it by schematic diagram.
Answer 17.7  Intrinsic semiconductor: A pure semiconductor material. In it the concentrations of negative charge carriers (electrons) and positive charge carriers (holes) are the same.

Extrinsic semiconductor: Such material in which certain impurities are added. Its conductivity increases considerably depending strongly on the type and concentration of the impurity.
From pure silicon N-type substance is obtained by adding impurity like a pentavalent element phosphorous 15P . Four valence electrons of P form covalent bonds with valence electrons of Si atoms, but one electron is left unbounded. It is called free electron.
From pure silicon, P-type substance is obtained by adding an impurity of trivalent element like Boron. Three valence electrons of surrounding 14Si , but one bond is not completed because no electron is available. This vacancy of electron is called a hole.

Question 17.8 Discuss the mechanism of electrical conduction by holes and electrons in a pure semi- conductor element.
Answer 17.8  The pure semi-conductors have equal number of holes and free electrons. As voltage is applied across the semiconductor, an electric field is produced. Due to electric field electrons get drift velocity opposite and to field and holes in the direction of filed. In this way the current is conducted in the pure-semiconductor.

Question 17.9 Write a note on superconductors.
Answer 17.9 Superconductors: Materials whose resistivity becomes zero at a certain temperature are known as superconductor. Once the resistance of a material drops to zero, no energy is dissipated and the current once established, continues to exist indefinitely without the source of emf. The temperature at which resistivity becomes zero is called critical temperature.for exp mercury Hg T = 4.2K, Aluminum Al T =1.18K. The first superconductor was discovered in 1911 by Kmaerlingh Ornes. He observed that electrical resistance of mercury disappears at 4.2 K. Some other metals such as Al, Sn and Pb also become superconductor at very low temperatures. In 1986 a new class of ceramic materials was discovered that become superconductors at 125 K. Recently Yttrium barium copper oxide (Yba2 Cu3 O7 ) have been reported to become superconductor at 163 K. Superconductors have many applications., e.g.magnetic resonance imaging
(MRI), magnetic levitation trains and faster computer chips.

Question 17.10 What is meant by para, dia and ferromagnetic substances? Give examples for each.
Answer 17.10  Paramagnetic substances: If the spin and orbital axis of electrons in an atom are so oriented that their fields support each other and the atom behaves like a tiny magnet then substance with such atoms are called paramagnetic substances. e.g., Manganese, Aluminum, Platinum etc.

Diamagnetic substances: Those substances in which the magnetic field produced by orbital and spin motion of the electrons may cancel each others effect are called Diamagnetic substances. e.g., the atoms of water, Copper Cu, Bismuth Bi, Antimony Sb.

Ferromagnetic substances: There are certain substances e.g. iron Fe, Cobalt Co, Nickel Ni, Chromium dioxide and Alnico in which atoms, co-operate with each other in such a way as to show strong magnetic effects are known as Ferromagnetic substances.

Question 17.11 What is meant by hysteresis loss? How is it used in the construction of a transformer?
Answer 17.11 The area of hysteresis loop is measure of energy required to magnetize and demagnetized the substance. This energy is dissipated in form of heat which is called hysteresis loss. So material for which this loss is small is used to form core of transformer.

Written By: Asad Hussain.

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