Physical Properties Flashcards
Latent heat of vaporization
The amount of energy needed for transformation of a boiling liquid into vapour.
Latent heat of fusion
The amount of energy released when a liquid freezes.
Melting temperature
The amount of energy required for the transformation of solids into a liquid state.
Sublimation
The process in which some solids transform directly into a gas phase.
Different types of bonds
- Ionic bond (Gypsum, phosphate cements)
- Covalent bond (Dental resins)
- Metallic bond (Pure gold)
- Hydrogen bond (Water)
Glass transition temperature
The temperature at which there is an abrupt increase in the thermal expansion coefficient, indicating increased molecular mobility.
It is characteristic of a particular glassy structure.
Cohesion
Force of attraction between like molecules
Adhesion
Force of attraction between unlike molecules.
Adhesive
Material or film added to produce adhesion.
Adherend
The material to which an adhesive is applied.
What property of a material affects adhesion most?
Surface energy of the material
Relation of surface energy of restorative materials to tooth
Materials have greater surface energy than tooth
How does difference in surface energy of materials and that of tooth affect behaviour of restorations?
There is a greater tendency for the surface and margins of the restoration to accumulate debris with resultant high incidence of marginal caries.
Hue
The specific COLOUR produced by a specific wavelength of light. Describes the dominant colour of the object, eg. Red, green or blue.
Value, aka
Brilliance
Value
The lightness or darkness of an object.
High value =
light shade.
Saturation, aka
Chroma
Chroma
The amount of colour per unit area of an object (degree of saturation of a certain hue).
Translucency
The property of an object that permits the passage of light through it but does not give any distinguishable image.
Metamerism
The tendency of objects that appear to be colour matched under one type of light to appear different under another light source.
Fluorescence
The energy that a tooth absorbs is converted into light with longer wavelengths, and becomes a light source.
What properties of teeth does fluorescence affect?
- Brightness
2. Vital appearance.
Benzold-brucke effect
Change in colour due to change in brightness.
Rheology
The study of matter flow characteristics. (Viscosity)
Viscosity
Resistance to motion
Newtonian fluid
Ideal fluid with constant viscosity that demonstrates a shear stress that is proportional to the strain rate. (Straight line graph) |/_
Pseudoplastic
A material whose viscosity decreases with increasing shear rate.
Dilatant
Liquids that become more rigid with increasing shear rate - as the rate of deformation increases, these liquids become more viscous.
Thixotropic
A liquid that becomes less viscous and more fluid under pressure.
Examples of thixotropic materials.
- Dental prophylaxis pastes
- Plaster
- Resin cements
- Some impression materials.
Water contact angle of hydrophobic materials
95 degrees
Polysulphide, addition and condensation silicones
Water contact angle of hydrophilic materials
30-35 degrees
Alginate, agar, polyether
Stress (definition)
Stress is the force per unit area
Stress (explanation)
It is the internal reaction to external force
Tensile stress
It tends to stretch or elongate a body.
Compressive stress
Tends to compress a body
Shear stress
Resists the sliding or twisting of one portion of a body over another.
Strain
The change in length per unit length of body when a stress is applied.
Poisson’s ratio
When an object is subjected to a tensile or compressive stress, there is simultaneous axial and lateral strain.
Within the elastic range, the ratio of lateral to the axial strain is called poisson’s ratio.
Ideal Poisson’s ratio
For an ideal isotropic material of constant volume - 0.5.
Elastic limit
The maximum stress that a material can withstand without permanent deformation.
Proportional limit
It is the greatest stress that may be produced in a material such that the stress is directly proportional to the strain.
What are the conditions required to be met for a material to satisfy Hooke’s law?
The elastic stress should be proportional to the elastic strain
Young’s Modulus
It describes the relative stiffness or rigidity of a material.
How do you calculate Modulus of elasticity?
It is the ratio of stresses to strain within the proportional limit.
Elastic modulus
= tensile stress/tensile strain
= compressive stress/compressive strain
Unit of modulus of elasticity
Giga Pascal
Shear modulus
38% of elastic modulus
Yield strength
The strength at which a material begins to function in a plastic manner.
Yield strength (definition)
The stress at which a material exhibits a limiting deviation from proportionality of stress to strain.
Resilience
The amount of energy absorbed by a structure when it is stressed, not to exceed its proportional limit.
Lateral strain as expressed in dental terms
Axial strain
Toughness
The amount of energy required to fracture a material. It is a measure of a material’s resistance to fracture.
Factors that affect toughness of a material.
- Strength
2. Ductility
Brittleness
The inability of a material to sustain plastic deformation before a fracture occurs.
Ductility
The ability of a material to withstand permanent deformation under a tensile load without rupture.
Malleability
The ability of a material to withstand permanent deformation under a compressive load without rupture.
Flexibility
The strain that occurs when the material is stressed to its proportional limit.
