Chapter 5

Question 1

How did Rayleigh carry out his oil drop experiment?

Answer 1

1) measuring the diameter of the oil drop, then calculating the radius
2) Placing the oil drop on still water to observe it spreading
3) Measuring the diameter of the patch of oil after it spreads, and calculating the radius of that patch

Question 2

Equation for volume of sphere

Answer 2

4/3πr³

Question 3

Equation for volume of oil patch

Answer 3

πR²h

Question 4

What is the largest uncertainty in the oil drop experiment?

Answer 4

Comes from the difficulty in measuring the diameter of an oil drop more precisely than about +- 0.5mm

Question 5

What are the 2 types of microscopes to view atoms?

Answer 5

Atomic Force Microscope (AFM)

Scanning Tunnelling Microscope (SEM)

Question 6

Scanning tunnelling microscopes show [ ] structures

Answer 6

large scale

Question 7

How does an AFM work?

Answer 7

It moves a needle over a sample to detect the contours of the surface. It can detect changes on an atomic scale. A fine point is mounted on the arm and forces between the surface and the tip make the arm bend. A laser beam is reflected from the arm detects the bending. One way of using the AFM is to move the specimen to keep the force on the tip constant. The up and down movement corresponds to the surface profile.

Question 8

Metals are [ ]

Answer 8

Crystalline

Question 9

What does crystalline mean?

Answer 9

The individual particles are arranged in a regular pattern over distances many times the spacing between the particles.

Question 10

What are dislocations?

Answer 10

Mismatches in the regular rows of atoms - missing atoms in the otherwise orderly arrangement.

Question 11

What makes some pure metals ductile? How does this apply to energy?

Answer 11

The movement of dislocations.

This greatly reduces the energy needed to deform the metal.

Question 12

Why are ceramic metals brittle?

Answer 12

They have dislocations within their structure however they are not mobile

Question 13

Are metal alloys usually less or more ductile than pure metals? Why?

Answer 13

Less ductile
Metal alloys can be formed by the addition of other metallic elements that usually have different size atoms. These can pin down the dislocations in the metal structure, making slippages between the layers of atoms more difficult.

Question 14

Look at pg 97 diagram and copy

Answer 14

Look at pg 97 diagram and copy

Question 15

Solids form when liquids [ ]

Answer 15

Cool

Question 16

The internal structure of a solid can be [ ] or [ ]

Answer 16

Crystalline or amorphous

Question 17

Rapid cooling is more likely to result in [ ]

Answer 17

Amorphous state, resembling the disordered arrangement in a liquid

Question 18

Slow controlled cooling results in a [ ]

Answer 18

a single pure Crystal

Question 19

If a material is not crystalline or amorphous, it is?

Answer 19

Polycrystalline

Question 20

Describe a Polycrystalline material

Answer 20

Consists a number of grains all orientated differently relative to one other but with an ordered, regular structure within each individual grain

Question 21

How are Polycrystalline solids formed?

Answer 21

As a liquid cools, crystals star to form at different points within it. Each crystal grows out into the remaining liquid until it runs into its neighbours. The result is a patchwork of tin crystals or grains. The interface where these grains meet is known as the grain boundary

Question 22

The strength of a material is affected by tiny [ ] and [ ] in the structure of the material

Answer 22

cracks and flaws

Question 23

Describe the stresses around cracks and how that propagates

Answer 23

The stress concentration around such cracks can be 100s or even 1000s of times the applied stress. This can lead to cracks working though a specimen until it fractures (Look at pg98)

Question 24

Describe toughness in terms of cracks

Answer 24

Toughness is a measure of the energy needed to extent cracks through a material

Question 25

What happens when stress is applied to a crack in a material?

Answer 25

The metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack

Question 26

What does microscopic mean?

Answer 26

Not visible to the unaided eye

Question 27

Ceramics undergo [ ] fracture

Answer 27

Brittle

Question 28

What are the three types of bond?

Answer 28

Ionic Covalent Metallic

Question 29

Which bonds are directional?

Answer 29

Bonds in ceramics and ionic compounds

Question 30

What does directional bonds mean?

Answer 30

This means that the atoms are locked in place and cannot slip, making the material hard and brittle

Question 31

Ceramics have [ ] structures

Answer 31

rigid

Question 32

Describe ceramics, using bonds

Answer 32

Atoms share electrons with neighbouring atoms to form covalent bonds. These bonds are directional - they lock atoms in place, like scaffolding The bonds are strong - stiff The atoms cannot slip - hard and brittle

Question 33

Describe metals, using bonds

Answer 33

Non directional bonds Atoms in metals are ionised The free electrons move between the ions The -ve charge of the electrons 'glues' the ions together, but the atoms can easily change places The bonds are strong - metals are stiff The ions can slip - metals are ductile and tough

Question 34

Describe elasticity in metals, using strain

Answer 34

Metals behave elastically for small strains, up to strains of 0.1% Up to this point the metal extends because the pacing between the +ve ions increases. When the tensile force is removed the metal returns to its original length

Question 35

Describe elasticity in polymers, using strain

Answer 35

They can extend elastically up to 1% strains. Polythene is very floppy because it is free to rotate about its bonds. The bonds are strong so they are difficult to break. this gives polythene the macroscopic properties of strength and flexibility

Question 36

What are polymers?

Answer 36

They are long-chained molecules. Long chains of monomers

Question 37

Draw a diagram of a polymer expanding.

Answer 37

Page 101

Question 38

How do you produce a stiffer material from polymers

Answer 38

Adding cross-linkages, where polymer chains are tied together at regular intervals along the chains, produces a stiffer material

Question 39

What is vulcanisation?

Answer 39

Where natural rubber is heated with sulfur

Question 40

What is the cubic (or isometric) crystal system?

Answer 40

It is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.

Question 41

What are the 3 varieties of crystals?

Answer 41

Primitive cubic Body-centred cubic Face-centred cubic

Question 42

What is the coordination number?

Answer 42

Coordination number is the number of nearest neighbours of a central atom in the structure.

Question 43

Describe the primitive cubic system (cP)

Answer 43

It consists of one lattice point on each corner of the cube. Each atom at a lattice point is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom ( 1⁄8 × 8). Coordination number of 6.

Question 44

Describe The body-centred cubic system (cI)

Answer 44

has one lattice point in the centre of the unit cell in addition to the eight corner points. It has a net total of 2 lattice points per unit cell ( 1⁄8 × 8 + 1). Coordination number of 8.

Question 45

Describe The face-centred cubic system (cF) .

Answer 45

Has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of 4 lattice points per unit cell ( 1⁄8 × 8 from the corners plus  1⁄2 × 6 from the faces). Each sphere in a cF lattice has coordination number 12

Question 46

List the features of crystals

Answer 46

Periodic repetition of a basic pattern of atoms, in three dimensions (in other words a unit cell) The structure is stable. In a regular crystal, the view from any one atom location is the same as the view from any equivalent location. They are formed by solidification; growth centres tend to compete, with two important results. Defects in the homogeneous structure occur (such as vacancies and dislocations), and large crystals are rare.

Question 47

List the features of polycrystals

Answer 47

The structure within the crystallites or grains is the same. These crystallites are joined so that the networks are not continuous but change direction at grain boundaries. The properties depend on both the crystal (atomic) structure and the way the grains join together The structure is not stable because of the grain boundaries, where an atom may migrate from one grain to its neighbour. This diffusion process depends on both temperature and time, and some grains may be extinguished as a result of it. There may be anisotropies in the way the grains themselves are shaped and joined. Metals are polycrystalline, and there are no gaps at the interfaces between the metal crystals.

Question 48

List the features of Amorphous or glassy solids

Answer 48

this state of matter as homogeneous in every way think of the transition from liquid to solid. In this case, the arrangement of atoms is effectively the same as in the corresponding liquid melt from which it solidified. The material fails to crystallise!

Question 49

How are crystals formed?

Answer 49

The crystallization of a large amount of material from a single point of nucleation results in a single crystal. Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline solid forms.

Question 50

What happens to a material at its solidification temperature?

Answer 50

Atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals.

Question 51

Describe the size of crystals

Answer 51

The final sizes of the individual crystals depend on the number of nucleation points. The crystals increase in size by the progressive addition of atoms and grow until they impinge upon adjacent growing crystal.

Question 52

Describe cracks in brittle materials

Answer 52

microscopic flaws (crack) will tend to elongate rather induce plastic deformation when a stress is applied.

Question 53

Describe cracks in plastic materials

Answer 53

cracks tend to emit dislocations and become more blunt as the raft is stressed