chapter 3 pt.2

Question 1

how are atoms arranged in

a crytalline material

Answer 1

they atoms / ions / molecules are arranged in a regular pattern that repeats itself

Question 2

how are atoms arranged in

a non-crystalline material

Answer 2

they are arranged in a random, haphazard way
–> such materials are called amorphous

Question 3

define

polymers

Answer 3

they are long chain molecules with some cross-linking between them
–> depending on the amount of tangling, a polymer
may be described as semi-crystalline or amorphous

Question 4

what is a

polymeric material

Answer 4

a material that can undergo great strain & deform to a very great degree
–> some examples include rubber, glass, silk, wool, DNA,
cellulose and proteins

Question 5

define

amorphous solid

Answer 5

a solid with no definite from where its molecules have a disordered arrangement, and any ordering is short range
–> particles have no regular pattern, except over very
short distances

Question 6

define

amorphous material

Answer 6

any non-crytalline solid that does not organise its atoms and molecules in a definite lattice pattern

Question 7

what do atoms do in crystals ?

Answer 7

due to the net attractive force between a crystal’s constituent electrons and atomic nuclei, its atoms arrange themselves in a lattice to form said crystal
—> which means that crystals are formed by the
bonding of atoms

Question 8

define

crystal

Answer 8

a material in which its atoms are arranged in a regular pattern

Question 9

state

types of crystal defects

Answer 9

1. vacancy defect
2. dislocation
3. impurity defect

Question 10

state three

crystals are grouped into types based on ….?

Answer 10

1. physical properties
2. types of bonds between their atoms
3. shape of their crystalline structures

Question 11

state six

what are the features of a crystal structure ?

Answer 11

1. atoms maintain a definite form
2. they can have long range ans shrot range order
3. can be insulators, conductors or semi-conductors
4. sometimes they are shiny or transparent
5. the pattern of ‘unit cell’ repeats again and again
6. there are many types of bonds such as metallic, covalence and ionic bonds

Question 12

describe

metallic crystalline solids

Answer 12

they are made up of many identical spherical atoms packed closely together in a regular, repeating array

Question 13

state three

crystal structure of metals

Answer 13

1. body centered cubic
2. cubic close packed (face centered cubic)
3. hexagonal close packed

Question 14

what is the effect of foreign atoms on the behavious of crystals ?

Answer 14

creates a crystal defect or an impurity

Question 15

define

impurity defect

Answer 15

it is crystal lattice distortion caused by an impurity occupying an interstitial site in the lattice, or replacing the parent atom in the regular sites

Question 16

state three

types of adding foreign atoms

Answer 16

1. perfect crystal —> atom similar to the original
2. substitutional atom —> takes the original atom’s place
3. interstitial defect —> is shoved in the lattice to prevent the sliding of atoms over each other

Question 17

state four

types of point defect

Answer 17

1. vacancy
2. interstitial atom
3. small substitutional atom
4. large substitutional atom

Question 18

state three

what happens when foreign atoms are introduced into crystals ?

Answer 18

1. crystal structure is deformed and so energy is stored in the crystal
2. the foreign atom is mostly in a location that is normally occupied in the crystal structure
3. the radius of the foreign atom is about the same as the radius of the atoms in the crystal

Question 19

state two (1 in solids, and 1 in liquids)

factors that make it difficult for dislocations to move

Answer 19

1. foreign atoms (in solid solutions)
2. precipitated particales (in liquids)

Question 20

define

dislocation

Answer 20

an incomplete plane of atoms inside a crystal

Question 21

state why

crystal structures normally have imperfections

Answer 21

they allow particles to change their relative positions
–> which means that solids will be either more easily
deformed or strengthened

Question 22

explain

how does deformation occur at the atomic level ?

Answer 22

• it occurs by the slipping of atoms on specific planes of a crystal
• this slipping occurs by the movement of linear lattice defect called dislocations

Question 23

explain the relationship between

shaping and slipping

Answer 23

to shape a metal, one layer must be made to slide over another

Question 24

what causes dislocations in crystals ?

Answer 24

• crystal surfaces can generate dislocations. the stepped nature of most crystal surfaces results in localized stresses higher than the average lattice stress
• these stresses cause dislocations, and dislocation movement - -facilitates slip, which is a form of plastic deformation

Question 25

- what are the two types of dislocations ? - how does the dislocation density affect the strength of the material?

Answer 25

1. edge and screw 2. high dislocation density results in a large number of dislocation interactions, so the material becomes strong and hard

Question 26

what are the effects of dislocations in crystals ?

Answer 26

- dislocation movement, known as glide or slip, enables atomic layers to slide past one another at low stress - while a complete dislocation restores the crystal structure on either side of the slip plane, atoms on one side are displaced by one atomic spacing - in contrast, a partial dislocation does not fully restore the crystalline order

Question 27

what is the role of dislocation on crystal properties?

Answer 27

- they can either weaken or strengthen a crystal - their motion produces a plastic deformation of crystals at stresses well below the theoretical sheer strength of a perfect crystal

Question 28

- do large interstitial atom defects in metals make them stronger ? - how to improve this ?

Answer 28

1. pure metals with large crystals are neither very stiff nor very strong because dislocations can spread through them easily, which allows slipping to take place 2. in order to improve its mechanical properties, the spread of dislocations must be reduced

Question 29

# making metals stronger by adding smaller grains

Answer 29

1. the smaller the grains the more dificult for dislocations to spread 2. they are produced if a molten metal is cooled and solidified rapidly ( grain size is also affected by later heating or mechanical treatment)

Question 30

# making metals stronger by forming alloys

Answer 30

- they are mixtures of metals and non-metals - they make the metal stronger, stiffer and tougher

Question 31

# what is work hardening

Answer 31

it occurs when a metal is hammered, stretched or bent, which in turn produces more dislocations that become so jumbled that they block the spread of others

Question 32

# defference between metals and alloys

Answer 32

alloys are genrally less ductile than pure metals

Question 33

# relationship between cracks and stress

Answer 33

under tensile stress, cracks propagate through materials

Question 34

# behaviours of - fine glass fibres - thin rod of glass

Answer 34

1. extremely thin, it only has a few layers so it cracks very easily 2. it has more layers than fine glass fibres, so it doesn't crack as easily

Question 35

why do brittle materials fracture ?

Answer 35

because of the propagation of cracks through the material

Question 36

how do cracks that lead to failure form ?

Answer 36

- as the crack grows, there is less material available to withstand the applied stress or strain - failure occurs when the material that has not been affected by the crack cannot withstand the applied stress --> this failure is often classified as ductile or brittle

Question 37

how does necking happen and how does it affect wires ?

Answer 37

in wires, when a sufficiently large force is applied, localised narrowing occurs at weak points & the wire eventually breaks at one of these points

Question 38

# difference between ductile cracks and brittle cracks

Answer 38

1. ductile fracture : fractures of material with large plastic deformation before it fractures 2. brittle fracture : fracture of materials without plastic deformation (or very small plastic deformation) before it fractures

Question 39

# compare ductle materials and brittle materials

Answer 39

ductile materials are easily stretched into wires and clearly show deformation, while brittle materials break rather than stretch (and often fracutre cleanly so the material could be put easily back together due to it not deforming before the fracture)

Question 40

# relatioship netween ductility and temperature

Answer 40

1. an increase in temperature will increase ductility 2. a decrease in temperature will cause a decrease in ductility and a change from ductile to brittle behavior

Question 41

describe, in terms of atomic structure, what happens when the stress is increased and when the stress is removed (before the elastic limit) in a wire

Answer 41

1. when stress increases, the bonds stretch so the atomic seperation increases --> stored as strain (EPE) 2. when the stress is removed, elastic behavior occurs and the wire returns to its original length because the atomic seperation returns to the original value when the load is removed --> energy is completely restored

Question 42

# define viscosity

Answer 42

- it is a frictional force that opposes relative motion between layers of fluid or between a solid and a fluid - the sliding between layers is a form of friction called viscosity

Question 43

# define laminar flow

Answer 43

occurs when viscous forces due to friction are large compared with the forces required to accelerate the relevant masses of fluid

Question 44

# define turbulent flow

Answer 44

occurs when viscous forces due to friction are small compared with the forces required to accelerate the relevant masses of fluid

Question 45

# state nine what is a characteristic of the laminar flow of a solution in a pipe ?

Answer 45

1. there is no turbulence where the solution is in contact with the pipe 2. there are frictional forces between the layers in a solution 3. the velocity of the solution is not the same at all point in the pipe 4. there are frictional forces where the solution is in contact with the pipe 5. the liquid flowing in the centre of the pipe flows faster than the liquid at the edge 6. speed is lowest at the edges and higher in the middle of the pipe 7. the velocity profile is parabolic 8. molecules next to the pipe stick to it and have zero velocity 9. molecules in the next layer slide over these and so on

Question 46

# state 4 the flow depends on : ?

Answer 46

1. the velocity 2. the pipe size 3. Reynold's number 4. the fluid's viscosity

Question 47

how does laminar flow turn into turbulent flow ?

Answer 47

laminar flow occurs in layers without mixing, so an obstruction in the vessel produces turbulence --> turbulent flow mixes the fluid

Question 48

# relationship between temperature and viscosity

Answer 48

1. in most liquids, viscosity decreases as temperature increases and vice versa 2. in most gases, viscosity increases as temperature increases and vice versa

Question 49

# relationship between drag force and : speed, effective area and viscosity of fluid

Answer 49

1. speed : drag force increases as speed increases and vice versa 2. effective area : the greater this area the greater the drag force and vice versa 3. viscosity of fluid : the greater the viscosity of a fluid, the greater the drag force and vice versa