EXERCISE SHORT QUESTIONS
CHAPTER # 17: PHYSICS OF SOLIDS
Q # 1. Distinguish between crystalline, amorphous and polymeric solids.
Ans.
Crystalline Solids: The solids in which the atoms, ions and molecules are arranged periodically are called crystalline solids. Metals such as copper, zinc, iron etc., Ionic compounds such as sodium chloride and Ceramics such as zirconia are the examples of crystalline solids.
Amorphous Solids: The word amorphous means shapeless. Thus in amorphous solids, there is no regular arrangement of molecules like that in crystalline solids. The ordinary glass is an example of amorphous solids.
Polymeric Solids: Polymeric solids are more or less solid materials with a structure between order and disorder. Natural rubber which is in pure state composed of Hydrocarbons. Polythene, Polystyrene and Nylon are examples of synthetic polymers.
Q # 2. Define stress and strain. What are their SI units? Differentiate between tensile, compressive and shear modes of stress and strain.
Ans.
Stress: The force applied on unit area to produce any change in the shape, volume or length of a body is called stress. Mathematically, it is described as:
The SI unit of stress is newton per square meter, which is given the name pascal (Pa).
Strain: It is defined as the fractional change in length, volume or shape of a body when stress is applied on it. 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: The stress which causes change in volume of the body is called compressive stress.
Compressive Strain: This is the strain produced as a result of compressive stress.
Shear Stress: The stress tending to produce an angular deformation or change in the shape is called shear stress.
Shear Strain: This is the strain caused by angular deformation. It is equal to the angular displacement produced.
Q # 3. Define modulus of elasticity. Show that the units of modulus of elasticity and stress are the same. Also discuss its three kinds.
Ans.
Modulus of Elasticity: The ratio of stress to strain is a constant for a given material, provided the external applied force is not too great, called modulus of elasticity. Mathematically, it is described as:
Since the strain is a dimensionless quantity, the units of modulus of elasticity are the same as that of stress, i.e., Nm\textsuperscript{-2} or Pa.
Young’s modulus: Young Modulus is the ratio of tensile stress to tensile strain.
Bulk modulus: Bulk modulus is the ratio of volume stress to volume strain.
Shear modulus: Shear modulus is the ratio of shear stress to shear strain of a body.
Q # 4. Draw a stress-strain curve for a ductile material, and then define the terms: Elastic limit, Yield point and Ultimate tensile stress.
Elastic Limit: It is defined as the maximum stress a material can endure without any permanent deformation.
Yield Point: The stress at which the material start to be permanently deformed is called Yield Point.
Ultimate Tensile Stress: It is defined as the maximum stress a material can withstand.
Q # 5. What is meant by strain energy? How can it be determined from the force-extension graph?
Ans. The amount of P.E stored in a material due to displacement of its molecule from its equilibrium position, under the action of stress, is called strain energy.
Consider a wire whose one end is attached to a fixed support, is stretched vertically by connecting a weight at its lower end. The work done for extension \emph{l}\textsubscript{1} by a certain force F\textsubscript{1} will be equal to the area under force –extension curve, which is equal to the area of triangle OAB. Therefore,
This work done is appeared as strain energy inside the wire. So
Q # 6. Describe the formation of energy bands in solids. Explain the difference among electrical behavior of conductors, insulators and semi-conductors in terms of energy band theory.
Ans.
Energy Band: When the numbers of atoms are brought together, as in a crystal, they interact with one another. As the result, each energy level splits up into several sub-levels. A group of such energy sub-levels are called an energy band.
Conductors: In conductors, 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.
Insulators: In insulators, valence electrons are tightly bound to their atoms and are not free to move. An insulator has an empty conduction band, a full valence band and a large energy gap in between them.
Semi-conductors: At room temperature, the semiconductors have partially filled conduction band, partially filled valence band and very narrow forbidden gap between valence and conduction band.
Q # 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.
Ans.
Intrinsic semi-conductors: A semiconductor in its extremely pure form is known as intrinsic semiconductors.
P-type: These materials are obtained by doping semi-conductor with atoms of a trivalent impurity such as Aluminium. It creates a vacancy of an electron called a hole.
N-type: The N-type materials are obtained by doping semi-conductor with atoms of a pentavalent impurity such as Phosphorous. It leaves a free electron.
Q # 8. Discuss the mechanism of electrical conduction by holes and electrons in a pure semi-conductor element.
Ans. In a pure (or intrinsic) semi-conductor, the number of holes and free electrons is equal and both contribute to the flow of current through it. When voltage is applied across the semi-conductor, an electric field is produced. Due to this electric field, electrons get a drift velocity opposite to the electric field and holes in the direction of the electric field. The electronic current and the hole current add up together to give the current through semiconducting material.
Q # 9. Write a note on superconductors.
Ans. The materials whose resistivity becomes zero below a certain temperature are called superconductors. And the temperature at which the resistivity of a material falls to zero is called critical temperature.
Any superconductor having a critical temperature above 77K (the boiling point of liquid nitrogen) is referred as high temperature superconductor. Superconductors can be used in Magnetic Resonance Imaging (MRI), Magnetic Levitation Trains, Powerful but small electric motors and in Fast computer chips.
Q # 10.What is meant by para, dia and ferromagnetic substances? Give examples for each.
Paramagnetic Substances: If the spin and orbital axis of electrons in an atom are oriented in such a way that their fields support each other and the atom behaves like a tiny magnet. Such substances are called Paramagnetic substances. e.g., Manganese, Aluminium, Platinium etc.
Diamagnetic Substances: The substances in which the magnetic field produced by orbital and spin motion of the electrons may cancel each other’s effects are called Diamagnetic substances. e.g., the atoms of water, Copper (Cu), Bismuth (Bi), Antimony (Sb).
Ferromagnetic Substances: Ferromagnetic substances are those substances in which atoms co-operate with each other in such a way as to show strong magnetic effects e.g., Iron (Fe), Cobalt (Co), Nickel (Ni), Chromium dioxide and Alnico.
Q # 11.What is meant by hysteresis loss? How is it used in the construction of a transformer?
Ans. The area of hysteresis loop is the measure of energy required to magnetize and demagnetize a substance. This energy is dissipated in form of heat, which is called hysteresis loss. The materials, for which hysteresis loss is small, are used to form the core of transformers.
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