Chapter 3

Question 1

what is the eq of strain?

Answer 1

change in length divided by the initial length

Question 2

what is shear stress?

Answer 2

τ = F/A.

Question 3

what is shear strain rate/rate of change of deformation?

Answer 3

ε = V/d

Question 4

viscosity eq’n

Answer 4

h=τ/ε

Question 5

Pure water at 20°C has a viscosity of 1.0 cP`

Answer 5

ye

Question 6

what is pseudoplastic viscosity?

Answer 6

the faster they are stirred, forced through a syringe, or squeezed, the less viscous and more fluid they become.

Question 7

what are dilatant materials?

Answer 7

materials/fluids that become more viscous with increased strain rate (stirring, etc)

Question 8

A er a substance has been permanently deformed (plastic deformation), there are trapped internal stresses. For example, in a crystalline substance such as a metal, the atoms in the crystal structure are displaced and the system is not in equi- librium. Similarly, in amorphous structures, some molecules are too close together and others too far apart when the sub- stance is permanently deformed.

Answer 8

ye

Question 9

what is creep?

Answer 9

Creep is de ned as the time- dependent plastic strain of a material under a static load or constant stress

Question 10

Also, if an observer looks at a red object for a reasonably long time, receptor fatigue causes a green hue to be seen when he or she then looks at a white background. For this reason, if a patient is observed against an intense-colored background, the dentist or clini- cian may select a tooth shade with a hue that is shi ed some- what toward the complementary color of the background color. For example, a blue background shi s color selection toward yellow, and an orange background shi s the color selection toward blue-green.

Answer 10

ye

Question 11

what is metamerism?

Answer 11

Objects that appear to be color-matched under one type of light may appear di erent under another type

Question 12

In general, thermal conductivities increase in the following order: polymers < ceramics < metals, although there are exceptions.

Answer 12

ye

Question 13

define thermal diffusivity?

Answer 13

a measure of the speed with which a temperature change will spread through an object when one surface is heated

h= κ/(cp ×ρ)

where h is thermal di usivity, κ is thermal conductivity, cp is heat capacity at constant pressure, and ρ is the temperature- dependent density in grams per cm3

Question 14

thermal expansion equation

Answer 14

α=∆L / (L×∆T)

Question 15

why is thermal expansion so important?

Answer 15

restorations, and preparing metal-ceramic crowns and bridges. A tooth restoration may expand or con- tract more than the tooth during a change in temperature; thus there may be marginal microleakage adjacent to the restoration, or the restoration may debond from the tooth; For example, an accurate wax pattern that ts a prepared tooth contracts signi cantly when it is removed from the tooth or a die in a warmer area and then stored in a cooler area. is dimensional change is transferred to a cast metal restoration that is made from the lost-wax process. Similarly, denture teeth that have been set in denture base wax in a relatively warm laboratory may shi appreciably in their simulated intraoral positions a er the denture base is moved to a cooler room before the processing of a denture.

Question 16

one can experience pulpal pain from an electrical current causes by dissimilar metals in the mouth

Answer 16

ye

Question 17

why might a bit of corrosion be helpful?

Answer 17

a limited amount of corrosion at the margins of dental amalgam restorations may be bene cial, since the corrosion products tend to seal the marginal gap and inhibit the ingress of oral uids and bacteria.

Question 18

define dilatant

Answer 18

Resistance to flow increases as the rate of deformation (shear strain rate) increases. The faster that fluids are stirred or forced through a syringe, the more viscous and more resistant to flow they become

Question 19

define pseudoplastic

Answer 19

Viscous character that is opposite from dilatant behavior in which the rate of flow decreases with increasing strain rate until it reaches a nearly constant value. Thus, the more rapidly that pseudoplatic fluids are stirred or forced through a syringe, the more easily they flow.

Question 20

define rheology

Answer 20

Study of the deformation and flow characteristics of matter

Question 21

define thixotropic

Answer 21

Property of gels and other fluids to become less viscous and flow when subjected to steady shear forces through being shaken, stirred, squeezed, patted, or vibrated. When the shear force is decreased to zero, the viscosity increases to the original value. Also known as shear thin- ning, in which the greater the applied shear force, the less the resistance to flow.

Question 22

define viscosity

Answer 22

Resistance of a fluid to flow

Question 23

define creep

Answer 23

Time-dependent plastic strain of a solid under a static load or constant stress.

Question 24

define sag

Answer 24

Irreversible (plastic) deformation of metal frameworks of fixed dental prostheses in the firing temperature range of ceramic veneers.

Question 25

define absorption

Answer 25

The extent to which light is absorbed by the material in an object.

Question 26

define chroma

Answer 26

Degree of saturation of a particular hue (dominant color)

Question 27

define colour

Answer 27

Sensation induced from light of varying wavelengths reaching the eye.

Question 28

define esthetics

Answer 28

Principles and techniques associated with development of the color and appearance required to produce a natural, pleasing effect in the dentition. Aesthetics is an alternative spelling with the same meaning.

Question 29

define hue

Answer 29

Dominant color of an object; for example, red, green, or blue.

Question 30

define metamerism

Answer 30

Phenomenon in which the color of an object under one type of light source appears
to change when illuminated by a different light source

Question 31

define opacity

Answer 31

The extent to which light does not pass through a material. No image and no light can be seen through an opaque object.

Question 32

define reflection

Answer 32

The amount of light that reflects from the surface of an object.

Question 33

define refract/refraction

Answer 33

The degree to which light is bent when it passes from one medium to another. This makes a spoon appear bent in a glass of drinking water when light passes from air through glass into water, reflects off of the spoon, and then passes back through water and glass into air. The index of refraction is a measure of this effect.

Question 34

define translucency

Answer 34

The quality of light passing through an object in a diffuse manner, only to reveal a distorted image that can be viewed through the material.

Question 35

define transmit/transmittance

Answer 35

The amount of light passing through an object.

Question 36

define transparency

Answer 36

The extent to which light passes through a material and to which an undistorted
image can be seen through it.

Question 37

define value

Answer 37

Relative lightness or darkness of a color. Also known as the gray scale.

Question 38

define coefficient of thermal expansion

Answer 38

linear coefficient of expansion)—Change in length per unit of the original length of a material when its temperature is raised by 1 K (1 °C).

Question 39

what is K?

Answer 39

The kelvin (K) temperature scale extends the degree Celsius scale such that zero degrees K is defined as absolute zero (0 K = −273.15 °C). Temperatures on this scale are called “kelvins,” not “degrees kelvin,” kelvin is not capitalized, and the symbol (capital K) stands alone with no degree symbol. 1 K = 1 °C, K = °C + 273.15.

Question 40

If the thermal gradient does not change (i.e., heat is supplied at one end and withdrawn at the other end at a steady rate), the thermal conductivity is a measure of the heat transferred. If, however, the temperature gradient does change with time, which is invarably the case in the mouth, thermal di usivity will determine the amount of heat transferred and, consequently, the change in temperature experienced.

Answer 40

ye

Question 41

how do we protect against corrosion?

Answer 41

can use Au because it is noble (inert); but gold is soft and if it is pitted so that the metal is exposed–>will corrode very quickly bc concentration cells and two dissimilar metals are in contact;

as well other coatings failed because they: (1) were too thin, (2) were incomplete, (3) did not adhere to the underlying metal, (4) were readily scratched, or (5) were attacked by oral uids.

can paint the gold with a non-conductive coating–>won’t be able to accept e-s, a scratch in the coating will not lead to rapid attack of the gold/noble metal

Iron, steel, and certain other metals that are subject to corrosion may also be electroplated with nickel followed by chromium for corrosion protection and esthetic reasons.

important to include that if noble metal is used in alloy, the rest of the alloy must be equally dispersed and that at east half the atoms should be noble metals

Question 42

Titanium and its alloys are widely used in dentistry because of their favorable combination of chemical, physical, and biological properties as well as their resistance to corrosion. eir consequent biocompatibility is among their major advantages.

Answer 42

ye

Question 43

how to corrode a noble metal?

Answer 43

normally do not corrode bc their EMF is so positive; however overpotential is required–>ye

Question 44

Palladium has been found to be e ective in reducing the susceptibility to sul de tarnishing for alloys containing silver.

Answer 44

ye

Question 45

what is the clinical significance of glavanic currents?

Answer 45

cause corrosion
glavanic shock = pain and discomfort (to eliminate, put a varnish on the surface–>insulation from electrolight sol’n and no EC cell is created)

Question 46

what is magnetism?

Answer 46

occurs due to circulating electrical currents; e-s orbit within the atoms–>induces a net magnetic field in magnetic substances ONLY (field cancels out in other subs)

A magnetic field induces changes in the sur- rounding medium in proportion to its ux density, the unit of which is the Tesla (T), and the ux density is proportional to the size of the magnet.

Magnets very susc. to corrosion–must electroplate

In dentistry, magnetic materials are used that generate a constant eld that is permanent at mouth temperatures

Question 47

define thermal conductivity

Answer 47

(coefficient of thermal conductivity)—Property that describes the thermal energy transport in watts per second through a specimen 1 cm thick with a cross-sectional area of 1 cm2 when the temperature differential between the surfaces of the specimen perpendicular to the heat flow is 1 K (1 °C).

Question 48

define thermal diffusivity

Answer 48

Measure of the speed with which a temperature change will proceed through an object when one surface is heated.

Question 49

define concentration cell

Answer 49

Electrochemical corrosion cell in which the potential difference is associated with the difference in concentration of a dissolved species, such as oxygen, in solution along dif- ferent areas of a metal surface. Pitting corrosion and crevice corrosion are types of concentration cell corrosion.

Question 50

define crevice corrosion

Answer 50

Accelerated corrosion in narrow spaces caused by localized electrochemical processes and chemistry changes, such as acidification and depletion in oxygen content. Crevice corrosion commonly occurs when microleakage takes place between a restoration and the tooth.

Question 51

define corrosion

Answer 51

Chemical or electrochemical process in which a solid, usually a metal, is attacked by an environmental agent, resulting in partial or complete dissolution.

Question 52

define electromotive series

Answer 52

Arrangement of metals by their equilibrium values of electrode oxidation potential. Used to judge the tendency of metals and alloys to undergo electrochemical (galvanic) corrosion.

Question 53

define galvanic corrosion

Answer 53

Accelerated attack occurring on a less noble metal when electrochemically dissimilar metals are in electrical contact within a liquid corrosive environment.

Question 54

define galvanic shock

Answer 54

Pain sensation caused by the electrical current generated when two dissimilar metals are brought into contact in the oral environment.

Question 55

define pitting corrosion

Answer 55

Highly localized corrosion occurring at metal surface defects such as pits, scratches, and cracks in which the region at the bottom is oxygen-deprived and becomes the anode while the surface around it becomes the cathode. Thus metal at the base preferentially ionizes and goes into solution, causing the defects to enlarge.

Question 56

define stress corrosion

Answer 56

Degradation caused by the combined effects of mechanical stress and a corrosive environment, usually exhibited as cracking.

Question 57

define tarnish

Answer 57

Process by which a metal surface is dulled or discolored when a reaction with a sulfide, oxide, chloride, or other chemical causes surface discoloration through formation of a thin oxidized film.

Question 58

define magnet

Answer 58

Metallic material in which the component atoms are so ordered that it can attract iron- containing objects or align itself in an external magnetic field.

Question 59

define a tesla

Answer 59

Unit of flux density (T) of the magnetic field produced by a magnet.