Types of Solids

1
Types of Solids

• There are three main types of solid
• Crystalline solids
• Amorphous solids
• Polymers

2
Crystalline Solids

• The atoms in a crystalline solid are arranged in
  a regular, repetitive manner forming a 3-D
  lattice.

3
Amorphous Solids

• Solids which have their atoms arranged in a
  completely irregular structure are called
  amorphous (without shape) solids.
• e.g. Glass

4
Polymers

• Polymers consist of huge molecules, each composed
  of a long flexible polymer chain of vast numbers
  (gt104) basic atom groups called monomers.

5
Mechanical Properties of Solids (1)

• Strength
• The force required to break a given material is a
  measure of its strength.
• The breaking force depends on
• the shape of the solid,
• the size of the solid,
• the type of the material.

6
Mechanical Properties of Solids (2)

• Breaking stress
• Breaking stress is the breaking force per unit
  area of a material, the force being at right
  angles to the area.

Unit Pa or N m-2
7
Mechanical Properties of Solids (3)

• Stiffness
• Stiffness is a measure of the difficulty of
  changing the shape of an object.
• The stiffness is measured by the Youngs modulus
  of the material.
• Brittleness
• Brittle materials are very stiff but will snap if
  too much force is applied.

8
Mechanical Properties of Solids (4)

• Ductility
• Materials which show a large amount of plastic
  deformation under stress are said to be ductile.
• Plastic behaviour
• Plastic behaviour occurs when a material is
  deformed beyond its elastic limit.
• In plastic deformation, bonds between atoms are
  broken one at a time.

9
Mechanical Properties of Solids (5)

• Hardness
• Hardness is a measure of the difficulty of
  scratching a material.
• Creep
• Creep is the continuous deformation that occurs
  from prolonged static stress.
• Creep occurs when a material, acted on by
  constant forces, changes its shape even though
  the forces on it remain constant.

10
Deformation

• The tension (force) verses the deformation

11
Elasticity

• The elasticity of a body is its ability to return
  to its original form after the distorting forces
  have been removed.

12
Stress

• Types of stress
• Tensile stress
• Compressive stress
• Shear stress
• Stress (Tensile)
• Tensile stress is defined as the tension per unit
  area applied normal to that area.

13
Strain

• Strain is a measure of the extent of deformation
  of an object.
• Strain is defined as the extension of an object
  per unit length.

14
Hookes Law

• When stress is applied to a material, strain is
  produced in the material.
• Strain ? stress provided the limit of
  proportionality is not exceeded.

where E is called the Young modulus.
15
Young Modulus (E)
http//www.matter.org.uk/schools/Content/YoungModu
lus/experiment_1.html

• Young modulus is defined by the equation

Unit Pa or N m-2

• Young modulus is a measure of the ability
• of the material to resist distortion.

16
Stress-strain Curve for a metal
The Stress verses Strain graph has the same shape
and regions as the force verses deformation
graph.
The slope of the linear part gives the value of
the Young modulus.
17
Energy Stored in a stretched wire

• Consider a wire which is stretched without
  exceeding the proportional limit.

Energy stored area under the graph ½ Fo?lo
Fo
Energy stored per unit volume ½ stress ? strain
?lo
18
A Model for a solid

• A model is something which allows you to describe
  and explain some phenomenon.
• Mathematical model
• Hookes law is a mathematical model. It is a
  successful model because of its universality and
  simplicity.
• Analogical model
• The behaviour of solids can be described by a
  model which assumes that matter is made of atoms
  with the properties of sticky tennis balls.

19
Intermolecular Forces

• The intermolecular forces arises from two main
  causes
• The potential energy of the molecules, which is
  due to the electromagnetic interactions with
  surrounding molecules.
• The thermal energy of the molecules, which is the
  KE of the molecules and it depends on the
  temperature.

20
Potential energy and Force
(P gt q)
Molecular binding energy
21
Properties of Solids from Molecular Theory (1)

• Equilibrium spacing of molecules (r ro)
• The potential energy is minimum.
• The repulsive force and the attractive force
  balance.
• The molecules oscillate about their equilibrium
  position.
• Elasticity
• Near the equilibrium position, ?r ? F.
• Intermolecular force constant, k - dF/dr.
• Youngs modulus k/ro

22
Properties of Solids from Molecular Theory (2)

• Breaking strain
• Beyond a separation, r OZ, the restoring force
  decreases with increasing separation.
• OZ is the separation between molecules at the
  breaking point of the solid.
• Breaking strain MZ/ro.
• Vaporization
• When the energy equals CM (latent heat), the
  molecules have little interaction and form a gas.

23
Properties of Solids from Molecular Theory (3)

• Thermal Expansion
• At a higher temperature, the mean position of the
  oscillation shifts to right due to the asymmetry
  of the curve.
• This corresponds to a greater separation than ro.
  Thus the solid expands.

24
Bonding (1)

• Ionic bonds
• Electrons are transferred between atoms
• The atoms become ions-one with a positive charge,
  the other with a negative charge.
• Covalent bonds
• The positive nucleus of both atoms has equal
  attraction for the electrons being shared.
• It is the attraction between nucleus and the
  shared electrons that holds the atoms together.

25
Bonding (2)

• Metallic bond
• Electrons are distributed equally through a
  metallic crystal.
• The positive nuclei are surrounded by a sea of
  electrons that are all attracted by the nuclei
  at the same time.
• Van der Waals bond
• More electrons may appear on one side of the
  nucleus to form an electric dipole.
• The electric dipoles have weak forces between
  them, called Van der Waalss forces.