12th Class Physics Chapter 6 Physics of Solids Short Questions Answer

Crystaline sold:- A solid having an ordered arrangement of molecules is called a crystalline solid. This arrangement in a regular pattern is constant throughout the crystal. Thus the crystalline solids have ordered molecular structure. For example metals such as copper iron and zinc ionic compounds such as sodium chloride ceramics such as zirconia. Amorphous(or glassy solids). The word amorphous means without structure. A solid having irreqular arrangement of molecules is called an amorphous solid. It means that amorphous solid are more like liquids with the disordered molecular structure. For example an ordinary glass which is solid at ordinary temperature has no regular arrangement of molecules. Polymeric solid:- Partially or poorly crystalline solids are called as polymeric solids. For example plastic and synthesis rubber are the examples of polymeric solids. They consist of long chain of three dimensional molecules formed by the chemical reaction called polymerization.

a) The force applied on unit area to produce any change in the shape length or volume of a body is called stress.
If the force F is applied on a unit area A then the stress is mathematically given by
Stress = Ԑ F/A
Units:- SI unit stress Newton’s per square metre (N-m^-2) which is called pascal.
ii) Strain:- The fractional change in length per unit original length due to applied  length force is called strain.
Let ΔI be the change in length and l be the original length then strain is mathematically given by
Strain = Ԑ=ΔI/I
Unit:- since strain is ration of two lengths there force it has no units.
b) Modes of stress:-
i) Tensile stress:- When a stress changes the length of a body is called tensile stress.
Compressive stress:-  When a stress changer the volume of a body is called compressive stress.
Shear stress:- When a stress changes the shape of a body is called  shear stress.
ii) Modes of strain:- It strain is due to tensile stress it is called tensile strain. It can  be given as
Strain (Ԑ) =Chane in length(ΔI)  = ΔI
              original length(I)          I
It has no umit. Both are similar quantities.
Compressive strain:- If it is produced as a result of compressive stress it is called as compressive strain. Mathematically it can be written as

Volume strain = Change in volume  = Δv
                         Original length (I)      V
It has no unit because both are similar quantities.
Shear strain:-When the opposite faces of a rigid body are subjected to shear
Shear strain= Δa = TanΘ
                         a
Θ is measured in radians.

The ratio of stress to strain is called modulus of elasticity.
Mathematically it can be written as
Modulus of Elasticity = Stress
                                          strain
i)Units:- The SI units  of modulus of elasticity are Nm^-2 or pascal
ii) Unit of Modulus of Elasticity= Stress
                                                         strain
F/A = FL = N-m = N =   Nm^-2
ΔI/I    AΔI   m².    m
Hence both of them have the same units.
b) Three  kinds of modulus of elasticity:-
i) Young’s modulus:-
For linear deformation the ration of tensile stress to tensile strain is called Young’s modulus. It is denoted by Y’
Mathematically it can be written as
Y=F/A =      FI
ΔI/I        AΔI
ii) Bulk modulus:-
The ratio of applied stress  to volumetric strain is called Bulk modulus. It is denoted  by K’ Mathematically it can be written as
K= F/A     =    F.V
ΔV/V        A.ΔV
iii) Shear modulus:-
The ratio of shear stress to shear strain is called shear modulus. It is denoted by G’ .It can be mathematically written as
G= F/A     =    F
TanΘ        A.TanΘ

Strain Energy:-
The work done by the applied force to produce extension in a material is stored  as potential energy knowing as strain energy.
It can mathematically be written as
E= ½ I₁F₁
Where  F₁ is force and I₁ is the extension of wire.
It determination from Graph:-
Strain energy can be  determined by the help of area under the curve between force- extension graph.

a) There are three types of energy bands:- i) Valence Energy Bands:- The electrons revolving in outermost orbit or shell are called valence electrons. The energy band occupied by the valence electrons is called valuce energy band. It is the highest occupied energy band in an atom. This can be never empty. It is either completely filled or partially filled with electrons. ii) Conduction Energy Band:- The energy band above the valence band is called conduction energy band. The electrons corresponding to this energy band are called free electrons. The conduction band may be either empty or it may be partially filled. iii) Completely filled Energy Band:- The bands below the valence energy band are normally completely filled. They do not take part in the conduction process. Therefore we consider only two above mentional bands. b) Energy band theory can be applied to distinguish between i) Insulators ii) Conductors iii) Semi – conductors i) Insulators:- Insulators are those substances in which valence electrons are bound very tightly to their atoms and are not free. AN insulator has the following properties on basis of energy Bands. i) Conduction band is empty having no free electrons. ii) Valence band is completely filled. iii) There is large forbidden energy gap between valence and conduction energy bands. ii) Conductors:- Conductors are those solids which have very large number of free electrons for electric conduction. On the basis of energy bands we observe that i. Conduction band is partially filled. ii. Valence band is also partially filled. iii. A very narrow forbidden energy gap between the conduction and valence bands. iii) Semi – Conductors:- At room temperature, Ge and Si crystals are semi – conductor. valence band is completely filled. At ok Ge or Si is a prefect insulator.

In pure semi – conductors there are two kinds of charges a free electron and a hole electrons and holes in semi – conductors are equal in number and they are moving at random. When a certain amount of voltage is applied to the semi – conductor an electric field is produced inside it and thus an electric force acts upon free electrons and holes. Due to this electrical force a directed flow of electrons and holes takes place. The electrons get drift velocity on one side whereas the holes have their drift velocity in the opposite direction. This is how the current flowing through the semiconductor is carried by both electrons and holes. The total current flowing through the semi – conductor is equal to the sum of current due to motion of free electrons and the current due to motion of holes.

The area of hysteresis loop is the measure of the energy required to magnetise and then to demagnetize a magnetic material. Thus work is to be done against then internal friction of the domains. During this process energy is dissipated in the form of heat and is called hysteresis loss. The loss of this energy depends upon the area of the hysteresis loop. The magnetic materials which have small area of hysteresis loop will dissipate less energy because the area enclosed by the hysteresis loop represents energy per unit volume of the material. Hence in order to reduce the core losses in a transformer we should use the material for the core with a narrow hysteresis loop.

A crystalline structure which is build up by the repetition of a unit cell is called crystal lattice. A crystalline solid consists of three dimensional pattern that repeats itself over and over again. This smallest three dimensional basic structure of a crystal lattice is called unit cell. For example a crystal lattice structure of NacI is cubical.

The inter atomic force present between the atoms of solid structure is called cohesive force.

Crystals have ionic bonds the orbital electrons are strongly bound to each other. Thus there are no free electrons in them. These fore they do not conduct electricity easily.

Such constants which can be experimentally verified and always remain constant are called elastic constants. For example the ratio of stress to strain is always constant provided the external force is not too great. This constant is called modulus of elasticity. There are other elastic constants which are known as Bulk modulus K, Young’s modulus Y’ and shear modulus G’.

Water and sugar consist of polar molecules. When sugar is mixed with water they come to close to each other forming close chains. Oil molecules are non- polar in nature. Therefore they cannot form closed chains with water molecules. Thus oil remains separate from water.

A temporary deformation is called an elastic deformation while a permanent deformation produced in some body is called as plastic.

Such substances which start breaking just after the elastic limit is reached are called brittle substances. Glass and high carbon steel are examples of brittle substances. Ductile substances:- Such substances which undergo permanent deformation are called ductile substances. Lead and copper are examples of ductile substances.

Conductors:- Those substances which have plenty of free electrons conduct electricity within a conductivity order of 10⁷ (Ωm)^-1.
Insulators:- Those substances in which valence electrons are tightly bound to their atoms are called insulators. Insulators can conduct electricity within  a  conductivity order of 10^-10 to 10^-20(Ωm)^-1.
Semi- Conductors:- Those substance which gave intermediate range of conductivities are known as semiconductors. They can conduct electricity within a conductivity order of c10^-6 to 10^-4 (Ωm)^-1.

The n- type substances:- A single crystal of silicon or germinimum formed after the addition of a pentavalent substance is called a n- type substance. They have excess of electrons in them which results conduction throough them. Arsenic phosphorous and antimony etc are the examples of n- type substances. The p-type substances:- A single crystal of silicon or germanium fromed after the addition of trivalent substance is called the p-type substance. They have excess of holes in them which results conduction through p-type substances. Induim boron and aluminium etc are the examples of p-type substances.

A combination of p- type and n – type substances is called a semi- conductor diode or p-n junction. In a semi – conductor diode conduction is done byt the help of two types of charge carriers i.e. a free electron and holes respectively.

Valence band:- A band occupied by valence electors is called as valence band. It is either completely filled or partially filled. It lies below the conduction band. Conduction band:- A band in an atom lying above the conduction band is called as conduction band. The property of the conduction band is that it is either completely empty or partially filled. Current in solids flows due to the presence of conducting band.

A narrow gap exists between  the valence and conduction bands. The electrons can easily jump from valence  band to conduction band by the supply of small energy through the  increase of temperature. This makes the semi- conductor diode conducting.

At a temperature pf zero Kelvin the semiconductor diode behaves as perfect insulator. It means it will not conduct any electricity at temperature of 0 kelvin.

Intrinsic semi-conductor:- A pure form of semi-conductor material without any impurity is called an intrinsic semiconductor. In other words undoped semi- conductors like pure elementail silicon and germanium are called intrinsic semi-conductors. Extrinsic semiconductor:- The semi-conductor to which some impurity is added to obtain the desired conduction properties is called extrinsic semiconductor. An impurity doped into a pure germanium or silicon is usually done in the ratio of 1 atom of impurity atom to 106 atoms of pure Ge or Si. Doping is said to be the addition of certain impurity to the pure semiconductor material.

The conductivity of a semi-conductor diode can be raised by the increase of temperature of a semi- conductor diode.

The substances which offer no resistance to electric current at a critical temperature of the sub stance are called as superconductors. The first superconductor was discovered in 1911 by Kmaerlingh omes. Supper conductors are used in some advanced technologies such as in high speed supper- computers magnetic resonance imaging and powerful but small electric motors.

A small portion of a substance of the order of 1 mm containg about 10¹² to 10¹⁶  number of atoms is called domain. It is to be noted that each domain of a substance acts kike a separate magnet.

Diamagnetic substance:- A substance in which the magnetic field produced by the orbital and spin motion of electrons add upto zero is called a diamagnetic substance. For example the atoms of water copper bismuth and antimory are diamagnetic substances. ii) Paramagnetic substances:- A substance in which the orbital and spin motion of electrona support each other and produce magnetic field is called a paramagnetic substance. The atom behaves like a tiny magnet. For example iron. iii) Ferromagnetic substances:- A substance in which the atoms co-operate with each other in such a way so as to show a strong magnetic effect is called a ferromagnetic substance. Ferrous, Cobalt, Nickle, and chromium dioxide are the examples of ferromagnetic substances.

A temperature at which the domains of a ferromagnetic substance start losing their orderliness is called a Curie temperature. For example iron has a Curic temperature for 705ᵒC.

It is that value of reverse current which is required by a substance for its demagnetization. It is to be noted that the coercive current fore steel is larger than that of iron because steel is harder than iron.

The magnetism lags behind the magnetizing current. This phenomenon is called as hysteresis.

The energy wasted during the work done against the internal friction of the domains of a substance is usually dissipated in the form of heat which is called as hysteresis loss. Hysteresis loss is generally shown in the from of a curve called hysteresis loop. As the hard material like steel cannot e easily magnetized or demagnetized. Thus its area of hysteresis loop is quite large than that of soft material like iron.

LWt us consider an isolated atom. Electrons of an isoltated atom are bound to the nucleus. They have distinct energy levels. Wnergy states:- When a large number of atoms say N are brought clase to one another to form a solid each energy level of the isolated atom splits into N sub-levels under the action of the forces exerted by others atoms in the solid. These sub-level are called energy states and they are very close to each other so that we can say that these form continuous energy bands. Forbidden Energy states and Forbidden Energy Gap. There is a large number of energy states between two consecutive permissible energy bands which cannot be occupied by electrons. These are called forbidden energy states. The gap consisting of the forbidden energy states is called forbidden range or energy gap.

A semi- conductor found in nature is called an intrinsic semi-conductor . It can be converted into an extrinsic semi-conductor by the process of doping. The process of adding small amount of impurity atoms to the intrinsic semi-conductor is called doping. Addition of pentavallent impurity to Ge or Si converts it into n-type substance and of trivalent impurity converts it into p-type substance.

The coercivity of steel is more than that of iron as more current is needed to demagnetize it. Once the material is magnetized the magnetization cure never passes through the origin instead it forms the closed loop which is called hysteresis lop.

Hard magnetic materials such as steel can not be easily magnetized or demagnetis so they have large loop area as compared to soft magnet material like iron which can easily be magnetized. The energy lost per cycle for iron is less than for steel. That is because loop for stell encloses more area than hysteresis loop for soft iron.

The ratio of applied stress to volumetric strain is called bulk modulus. It is denoted by K’
Mathematically it can be given as
Bulk modulus = volumetric stress
Volumetric strain

Or     K= FV
AΔV
Where ΔV is the change in volume and V is the original volume.
Unit:- The SI unit of Bulk modulus is N m^-2.

The temperature at which the resistance or resistivity becomes zero is called critical temperature.
It is denoted by T_c at a certain temperature the value of resistivity falls to zero.
A substance which shows zero resistance at a critical temperature is called the super conductor.

For linear deformation the ratio of tensile stress to tensile strain is called Young’s modulus. It is denoted by Y
Young’s modulus = Tensile stree
Tensile strain
Or     y= F/A  = FL
ΔI/L    AΔI

A crystalline solid consists of three dimensional pattern which repeats itself over and over again. This smallest three dimensional basic structure is called unit cell.