# KPK Board 12th class Physics Ch 16 Physics of Solids short questions answers

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Evaluate the importance of strength and stiffness in a design context.

Materials have specific specific uses in designing, depending upon their characteristics and properties such as strength and stiffness. Strength means that how much great force the material can support without breaking while stiffness gives the resistance of the material to be distorted in shape, size or in both. We make the use of such kinds of materials in daily life such as iron, aluminum etc, for designing window, cupboards, T-iron, guarder etc. of different shape, size and dimensions. These solids are used in engineering industry, where they are stretched, squeezed or twisted to design different objects for our daily uses.

Discuss the super conductivity of a conductor with the help of a curve.

The materials which conduct electric current without any resistance are called super conductors. OR The materials whose resistivity is zero below a certain temperature are known as superconductors, and the phenomenon is called super-conductivity. A class of materials such as metals, alloys, and ceramics show a remarkable change in resistivity with temperature. The curve drawn between temperature and resistivity shows that the decrease in temperature causes the decrease in resistivity. At certain temperature these materials lose their resistance suddenly which is known as critical temperature, denoted by Tc. Thus at Tc the resistivity of the material becomes zero (p =0) as shown in the curve.

Distinguish between crystalline, amorphous and polymer solids.
1. Crystalline Solids:
2. The solids whose atoms or ions or molecules are arranged in a regular repeated manner in three dimensions are called crystalline solids.
3. Crystals possess both short range and long range order.
• The arrangement of atoms or molecules of crystalline structure can be studied by using X-ray techniques.
1. The cohesive force between the atoms, molecules or ions, keeps them in particular order for long distance in spite of their fast vibrations.
2. Every crystalline solid has a definite melting point at which the disorder among the atoms or molecules takes place.
3. Metals such as copper, iron, zinc, sodium chloride (NaCl), sucrose diamond are the examples of crystalline solids.
4. Amorphous Solids:
5. The solids which have no regular arrangement of their atoms, molecules are called amorphous solids.
6. The word amorphous means without form or structure. Thus amorphous solids are quite different from crystalline solids.
• They have no definite melting point that is why there known as glassy solids.
1. glass is the example of amorphous solids.
2. Pol ymeric Solids:
3. The solids of organic compounds are called polymers.
4. They are organic compounds which consists of chemical combination of carbon with oxygen, nitrogen and other metallic or nonmetallic elements.
• The molecular structures of polymers are more ordered than amorphous but les than crystalline solids.
1. They consist of fine grains, having a size of 10° to 10 Aº separated by defined boundaries.
Define unit cell, basis and space lattice.
1. Units cell: The smallest three dimensional basic structure in a crystal is called a unit cell. The unit cell is smallest group of atoms or molecules in a crystal which is the representative of the crystal structure . Unit cell has the following characteristics,
2. unit cell is the smallest repeating unit in a crystal
3. The opposite faces of a unit cell are parallel.
4. The edge of unit cell connects equivalent points.
5. Unit cell is just like a box containing one or more atoms.
6. Basis: A set of atoms which is located near each site of a Bravais lattice is called basis. the type, number & arrangement of atoms inside the unit cell of a crystal. The basis identical in composition, arrangement & orientation, such as that the crystal appears exactly the same at one point as it appears at all other equivaim pus sit apears at all other equivalent points. In the regular & periodic arrangement of basis is the basic features of the crystal.
7. Space Lattice: The array of points showing the arrangement of atoms, ions or molecules at different sites in three dimensional spaces is called space lattice. A crystal is three dimensional designs in which identical points form a three dimensional network of cells. A three dimensional space lattice can be divided into unit cells described by three vectors a, b, c. Therefore space lattice contains a large number of unit cells repeated in all directions. Thus space lattice is the skeleton on which crystal structure is built by placing atoms on the lattice points.
Differentiate between paramagnetic, diamagnetic and Three dim space ferromagnetic materials with suitable examples.
1. Paramagnetic Materials: Those substances which are composed of toms in which the orbit and spin axis of electrons are so oriented that their magnetic field support each other are known as paramagnetic materials.
2. In paramagnetic materials each atom behaves like a tiny magnet.
3. When a paramagnetic substance is placed in a magnetic field, it is weakly magnetized in the direction of the field applied.
• Therefore paramagnetic substance are weakly attracted by a magnet.
1. The relative permeability (U) of paramagnetic materials, is slightly greater than I.
2. Examples: Aluminium, antimony, pollodium, manganese and other metallic salts are the examples of para magnetic materials.
3. Diamagnetic Materials:
4. Those substances which are composed of such atoms in which the orbital an axis of electrons are so oriented that their magnetic fields cancel each othe called diamagnetic materials.
5. In diamagnetic materials each atom is magnetically neutral.
• When a diamagnetic substance is placed in a magnetic field, it is well magnetized in opposite direction of the field applied.
1. Therefore a diamagnetic substance is weakly repelled by a strong magnetic field,
2. Their relativity permeability (H) is less than 1.
3. Examples: Bismuth, lead, gold, copper, sodium, chloride (NaCl).
4. Ferromagnetic Materials:
1. Those substances in which the atoms co-operate with each other in such a way so as to exhibit a strong magnetic effect are called ferromagnetic materials.
2. In ferromagnetic substances there are small regions called domains. Each domain is large enough to contain 10l2 to 10 atom.
• In each domain the magnetic fields of all the spinning electrons are parallel to each other.
1. Therefore each domain behaves like a small magnet with its own north and south poles.
2. They are strongly attracted by a magnet.
3. Examples: Iron, cobalt, nickel etc. are the examples of ferromagnetic substances.
Distinguish between soft and hard substances by drawing its curves.
1. Soft Substances: A material which has the tendency to break easily or suddenly without any extension is called soft or brittle. For example egg shells are brittle and can be easily broken without any extension. Similarly cast iron, concrete glass, ceramics are the examples of soft or brittle Stress materials. Such materials break just after elastic limit as shown in the curve drawn between stress and strain.
2. Hard Substances: A substance which can be easily stretched, bent, rolled into useful shape is called hard or ductile. Hard materials have a low resistance to impact, well known hard materials include diamond and steels. Such materials never break after elastic limit and the graph between stress and strain extends to yield point. If the stress is further increased the body is permanently deformed as shown in the curve.
Explain Hook's Law and Modulus of elasticity.

Ans. According to Hook’s Law stress is directly proportional to strain within the elastic stress limit.

stress Stress

stress  = constant x strain

stress/ strain = constant

But the constant of proportionality is known as the modulus of elasticity. Therefore

Modulus of elasticity = Stran/strain

The unit of modulus of elasticity is the same as that of stress because strain is unit less. Let a graph is drawn between stress and strain for a soft iron wire as shown. The graph from 0 to A is straight line which shows the validity of Hook’s Law. m e WHICH shows the validity of Hook’s Law The stress at point A is called proportional limit.

Is there any difference in the length of a 20m steel girder when standing vertically and horizontally?

Yes there is slightly difference in the length of a 20m steel girder when standing vertically is compare to standing horizontally.

1. Vertical Position: When a 20m long girder is standing vertically its length is slightly decreased. Because its whole weight w is acting through the centre of gravity on the lower end surface. This gravitational force pushes down ward which the ground pushes upward by equal force. So the girder is under compression. The force exerted perpendicular per unit area to decrease the length of an object is called compressive stress.

Compressive stress = F/A = W/A

This compressive stress produces compressive strain given by

Compressive strain = Δl/L

Thus when the girder is standing vertically, its length is slightly decreased.

1. Horizontal Position: When the girder is lying horizontally, the force per unit area share stress is negligibly parallel to the surface to change its shape is very small. Hence share stress is negligible weak and length of the girder is nearly the same.
Steel reinforcing is used in concrete beams to prevent cracking Explain where the steel reinforcing should be placed in a concrete beam?

A proper steel rods frame is prepared based on construction engineer knowledge. This beam is then filled with concrete during construction. In this wa the steel rods lies inside the beam as well as near the outer surfaces of the beam. In this way the reinforcing steel is used in concrete beam to prevent cracking.

a) What is meant by the elastic limit of a material?

1. b) In what way does a material behave if it obeys Hook’s law?
2. a) The maximum stress which a material can endure without any permanent! change in shape or dimensions, is called the elastic limit, denoted by Sx. The end point of the elastic limit is known as yield point, as shown by B in the graph drawn between stress and strain.
3. b) Hook’s law states that strain is directly proportional to the stress with in the elastic limit.

stress Stress

stress  = constant x strain

stress/ strain = constant

Thus a material which obeys Hook’s Law must behave as an elastic body. It has to maintain direct proportionality between stress and strain. On the stress-strain curve from 0 to A the material shows direct proportionality between stress and strain. Here stress increases linearly with strain. The proportional limit is denoted by p.

Cast-iron beams are used in bridge and buildings construction. The lower part of the beam is thicker than the upper part. Why is it better for the lower part of the beam to be thicker than the upper part? Give reason for your answer including reference to the tensile and compressive strength.

Cast-iron beams are used in bridges and buildings where the lower part of the beam is made thicker than the upper part, so that to increases the strength of beam. Making the lower part of the beam thicker this ultimately increases the lower surface of the beam. As a result the compressive stress decreases as given by:

Compressive stress = F/A

Thus the decreases in compressive stress causes an increase in compressive strength. Moreover the decrease in compressive stress also results the decrease in compressive strain which cause compressive strength.

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