Chapter 5 Flashcards
How did Rayleigh carry out his oil drop experiment?
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
Equation for volume of sphere
4/3πr³
Equation for volume of oil patch
πR²h
What is the largest uncertainty in the oil drop experiment?
Comes from the difficulty in measuring the diameter of an oil drop more precisely than about +- 0.5mm
What are the 2 types of microscopes to view atoms?
Atomic Force Microscope (AFM)
Scanning Tunnelling Microscope (SEM)
Scanning tunnelling microscopes show [ ] structures
large scale
How does an AFM work?
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.
Metals are [ ]
Crystalline
What does crystalline mean?
The individual particles are arranged in a regular pattern over distances many times the spacing between the particles.
What are dislocations?
Mismatches in the regular rows of atoms - missing atoms in the otherwise orderly arrangement.
What makes some pure metals ductile? How does this apply to energy?
The movement of dislocations.
This greatly reduces the energy needed to deform the metal.
Why are ceramic metals brittle?
They have dislocations within their structure however they are not mobile
Are metal alloys usually less or more ductile than pure metals? Why?
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.
Look at pg 97 diagram and copy
Look at pg 97 diagram and copy
Solids form when liquids [ ]
Cool
The internal structure of a solid can be [ ] or [ ]
Crystalline or amorphous
Rapid cooling is more likely to result in [ ]
Amorphous state, resembling the disordered arrangement in a liquid
Slow controlled cooling results in a [ ]
a single pure Crystal
If a material is not crystalline or amorphous, it is?
Polycrystalline
Describe a Polycrystalline material
Consists a number of grains all orientated differently relative to one other but with an ordered, regular structure within each individual grain
How are Polycrystalline solids formed?
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
The strength of a material is affected by tiny [ ] and [ ] in the structure of the material
cracks and flaws
Describe the stresses around cracks and how that propagates
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)
Describe toughness in terms of cracks
Toughness is a measure of the energy needed to extent cracks through a material
What happens when stress is applied to a crack in a material?
The metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack
What does microscopic mean?
Not visible to the unaided eye
Ceramics undergo [ ] fracture
Brittle
What are the three types of bond?
Ionic
Covalent
Metallic
Which bonds are directional?
Bonds in ceramics and ionic compounds
What does directional bonds mean?
This means that the atoms are locked in place and cannot slip, making the material hard and brittle
Ceramics have [ ] structures
rigid
Describe ceramics, using bonds
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
Describe metals, using bonds
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
Describe elasticity in metals, using strain
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
Describe elasticity in polymers, using strain
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
What are polymers?
They are long-chained molecules. Long chains of monomers
Draw a diagram of a polymer expanding.
Page 101
How do you produce a stiffer material from polymers
Adding cross-linkages, where polymer chains are tied together at regular intervals along the chains, produces a stiffer material
What is vulcanisation?
Where natural rubber is heated with sulfur
What is the cubic (or isometric) crystal system?
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.
What are the 3 varieties of crystals?
Primitive cubic
Body-centred cubic
Face-centred cubic
What is the coordination number?
Coordination number is the number of nearest neighbours of a central atom in the structure.
Describe the primitive cubic system (cP)
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.
Describe The body-centred cubic system (cI)
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.
Describe The face-centred cubic system (cF) .
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
List the features of crystals
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.
List the features of polycrystals
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.
List the features of Amorphous or glassy solids
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!
How are crystals formed?
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.
What happens to a material at its solidification temperature?
Atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals.
Describe the size of crystals
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.
Describe cracks in brittle materials
microscopic flaws (crack) will tend to elongate rather induce plastic deformation when a stress is applied.
Describe cracks in plastic materials
cracks tend to emitdislocationsand become more blunt as the raft is stressed
