The entry below is taken from the 2008 OCR specification which combines the topics to be taught in Chapters 4 and 5.
This section is about materials and how their properties are related to their uses and their structures. Microscopic images are used to give evidence of structure at different scales.
The physics may be put into perspective through contexts such as the study of medical replacement materials, biological materials and engineering materials. Human and cultural issues arise, for example, in considering the impact of materials on technology and society and through
the aesthetic appeal of materials.
It is not intended that candidates acquire a detailed knowledge of a wide range of materials, and the terminology associated with each. It is intended that they have a reading comprehension of terms needed to understand accounts of the structure, uses and properties of materials. Examples should include: a metal, a semiconductor, a ceramic, a long-chain polymer and a composite material.
Properties to be studied are restricted to simple mechanical and electrical properties.
Candidates should develop skills of measurement, instrumentation and identification of uncertainty. It is expected that this will be taught in part through measurement tasks carried out in teams and reported to others.
Recommended Prior Knowledge
● Some of this work is provided with a foundation in PA Module 1 Communication, in particular
through ideas of resistance and conductance in circuits. If candidates are not confident with
measuring resistance with an ohmmeter or with thinking of current as a flow of charged
particles, that work may need to be brought forward here if PA Module 2 Designer Materials is
taught before PA Module 1 Communication.
Assessable learning outcomes
1. Knowledge and understanding of phenomena, concepts and relationships by describing and
explaining cases of:
(i) simple mechanical behaviour: types of deformation and fracture;
(ii) simple electrical behaviour: the broad distinction between metals, semiconductors and
insulators (only in terms of the number of mobile charge carriers, not their mobility);
(iii) direct evidence of the size of particles and their spacing;
(iv) behaviour and structure of classes of materials: metals, ceramics, polymers, composites;
(v) one method of measuring:
Young modulus and fracture stress;
electrical conductivity or resistivity.
2. Scientific communication and comprehension of the language and representations of physics,
by making appropriate use of the terms:
(i) for mechanical properties and behaviour: stress, strain, Young modulus, fracture stress and
yield stress, stiff, elastic, plastic, ductile, hard, brittle, tough;
(ii) for electrical properties: resistivity, conductivity, charged carrier density;
ability to sketch and interpret:
(iii) stress–strain graphs up to fracture;
(iv) tables and diagrams comparing materials by properties;
(v) images showing structures of materials.
3. Quantitative and mathematical skills, knowledge and understanding by making calculations and estimates involving:
(i) R = ρl ; G = σA
A l
(ii) tensile/compressive stress, strain, Young modulus E = stress .
strain
A Revision Checklist for Chapter 4 can be found on the Advancing Physics
CD-ROM.
Section 4.1: Just the job - Getting a feel for materials
Learning outcomes
● A material chosen for a particular application must have the right combination of
properties.
● Classes of materials include metals, glasses, ceramics and polymers.
● Composite materials combine the properties of more than one material.
● Compressive forces tend to squash an object; tensile forces tend to stretch it.
● Brittle materials snap easily; tough materials do not.
Materials to choose from
We open with a discussion of the properties of various materials and how these properties are suited to particular jobs. The sheer range of materials available to us should become apparent as should the enormous volumes being processed globally.
Question 10S Short Answer 'Exploring the range of materials'
This set of images portrays a range of materials in action, each with a question about its properties and uses. It emphasises non-physics aspects – including environmental and aesthetic issues convenient to create a broad context, and interest.
Getting a feel for materials
Here we move in from the broad picture to concentrate on the physical properties of materials. The experiments that follow are intended to provide a gentle introduction. Some of the activities will be revision of GCSE work. The point is to emphasise that each material has its own profile of properties. The experiments will be run as a circus, or part circus / part demonstration. A more formal version of tensile testing is found in the next section where the Young modulus is calculated.
Activity 10E Experiment 'Tensile testing: Getting a feel for materials 1'
Activity 20E Experiment 'Compressive testing: Getting a feel for materials 2'
Activity 30E Experiment 'Hardness testing: Getting a feel for materials 3'
Activity 40E Experiment ‘Tear testing: getting a feel for materials 4’
Activity 50E Experiment 'Measuring density: Getting a feel for materials 5'
Some values of density:
Material / Density in kg/m3 / Density in g/cm3air / 1.3 / 0.0013
wood (balsa) / 200 / 0.20
wood (oak) / 720 / 0.72
ice / 920 / 0.92
high density polythene / 960 / 0.96
bone / 1100 / 1.1
breeze block / 1400 / 1.4
brick / 1700 / 1.7
concrete (ordinary mix) / 2200 / 2.0
glass-reinforced plastic / 1900 / 1.9
glass / 2500 / 2.5
aluminium / 2700 / 2.7
steel (high-tensile) / 7800 / 7.8
copper / 8900 / 8.9
lead / 11400 / 11.4
mercury / 13600 / 13.6
uranium / 18700 / 18.7
platinum / 21500 / 21.5
Classifying materials
This second part of section 4.1 develops the idea that materials can be classified. Through a discussion of bones, the student book distinguishes 'tension' from 'compression' and 'brittle' from 'tough' materials. Later in section 4.2, you will begin to treat material properties quantitatively.
At this point we will introduce the materials database and materials selection charts. The charts plot one property against another (e.g. Young modulus for stiffness against density): the quantitative ranges in behaviour very clearly cluster like materials into classes. More than this, the range of values within a class of materials raises the question of why – and leads to consideration of how properties relate to material structure (taken up in chapter 5).
Both database and charts may be useful for your materials research and presentation.
This reading introduces the materials selection charts:
Reading 100T Text to read: 'Introduction to materials selection charts'
This key reading explains in more detail what materials selection charts are, and through specific examples demonstrates their power as an aid to materials selection in the design process and also as a route to thinking about controlling properties (through understanding materials structure – the theme taken up in chapter 5). From here you can also find all related resources, including interactive charts embedded in a web site supplied on the Advancing Physics AS CD-ROM.
Materials selection charts are a novel approach to presenting mechanical characteristics, and help us to understand:
· how each class of materials (metals, ceramics, polymers etc) has a characteristic range
of values for a given property
· how the properties of a material reflect the underlying microstructure, and can therefore
be usefully improved by scientists and engineers
· the engineering context of material properties in designing and making things, since this
invariably means making trade-offs between two or more properties (including the cost!)
Five materials selection charts are introduced below. To understand how to interpret and use the charts in depth, study Reading 100T 'Introduction to materials selection charts'. More interactive versions of the charts can be found in File 5L 'Interactive materials selection charts'.
1. Young modulus and density – classes of materials
Physical Insights
· Stiffness measures how much something stretches elastically when a load is applied.
Young modulus measures stiffness and is a material constant, i.e. it is the same
whatever the size of the test-piece.
· Young modulus and density both depend on the atomic packing within the material, and
Young modulus depends on the type of bonding between the atoms (electron bond,
covalent, ionic etc.)
· Note how the materials all lie roughly on a diagonal – Young modulus is strongly
correlated to density.
· The metal and polymer bubbles are small – this is because material composition and
processing do not have a significant effect on density or Young modulus.
· Woods have very different stiffnesses depending on whether they are loaded 'with' or
'across' the grain. This is because of the aligned stiff cellulose micro-fibres. Both paper
and MDF are made from wood pulp and so have similar densities, but have little
directional variation in Young modulus.
· Foams have the lowest densities because they have pores full of air.
· Note that the scales are logarithmic, because of the large ranges of values.
Applications of the chart
· Stiff lightweight materials are hard to find, for things like sports products and bicycles –
composites appear to offer a good compromise, but they are usually quite expensive,
and wood is still used for cheaper products (e.g. oars).
· Many applications require stiff materials, e.g. roof beams.
· Many applications require low density materials, e.g. packaging foams.
· Polymers don't seem like a good choice for stiff, lightweight products – but they can be
reinforced by incorporating stiffening ribs into the design (for instance, look inside a
mains plug).
· Ceramics are quite light and very stiff – but their poor tensile strength and toughness
means they are likely to fracture.
2. Young modulus and density – metals and polymers
Physical Insights
· Stiffness measures how much something stretches elastically when a load is applied.
Young modulus measures stiffness and is a material constant, i.e. it is the same
whatever the size of the test-piece.
· Young modulus and density both depend on the atomic packing within the material, and
Young modulus depends on the type of bonding between the atoms (electron bond,
covalent, ionic etc.)
· Note how the materials all lie roughly on a diagonal – Young modulus is strongly
correlated to density.
· The metal and polymer bubbles are small – this is because material composition and
processing do not have a significant effect on density or Young modulus.
· Woods have very different stiffnesses depending on whether they are loaded 'with' or
'across' the grain. This is because of the aligned stiff cellulose micro-fibres. Both paper
and MDF are made from wood pulp and so have similar densities, but have little
directional variation in Young modulus.
· Foams have the lowest densities because they have pores full of air.
· Note that the scales are logarithmic, because of the large ranges of values.
Applications of the chart
· Stiff lightweight materials are hard to find, for things like sports products and bicycles –
composites appear to offer a good compromise, but they are usually quite expensive,
and wood is still used for cheaper products (e.g. oars).
· Many applications require stiff materials, e.g. roof beams.
· Many applications require low density materials, e.g. packaging foams.
· Polymers don't seem like a good choice for stiff, lightweight products – but they can be
reinforced by incorporating stiffening ribs into the design (for instance, look inside a
plug).
· Ceramics are quite light and very stiff – but their poor tensile strength and toughness
means they are likely to fracture.
3. Strength and toughness – classes of materials
Physical Insights
· Strength measures the resistance of a material to failure, given by the applied stress (or
load per unit area).
· The chart shows yield strength in tension for all materials, except for ceramics for which
compressive strength is shown (their tensile strength being much lower).
· Toughness measures the energy required to propagate a crack through a material; it is
important for things which suffer impact.
· The material bubbles are large, particularly for metals – this is because material
composition and processing have a significant effect on strength and toughness.
· The tensile strengths of brittle materials are very sensitive to the presence of flaws.
· Metals are tough because they deform plastically instead of propagating cracks.
· Cast iron is often brittle because it contains graphite flakes which behave like little cracks
within the metal.
· Quenching carbon steel makes it very hard but brittle; tempered steel is tougher but less
strong than after quenching.
· Note that the scales are logarithmic, because of the large ranges of values.
Applications of the chart
· There are many cases where strength is no good without toughness. Saw blades,
hammer heads and engine components are commonly made of quenched and tempered
steel to get moderately high strength with good toughness .
· Steel is often used to absorb energy in car impacts because it is tough and strong.
· Ceramics cannot be used for components which suffer impact because of their low
toughness.
· Polymers are quite sensitive to cracks and defects as they do not absorb much energy
when they fracture; they are often considered to be tough however because of their
ductility – energy is absorbed straining the material to failure. This makes them good for
children's toys, when their low strength is also not important.
4. Strength and toughness – metals
Physical Insights
· Strength measures the resistance of a material to failure, given by the applied stress (or
load per unit area).
· The chart shows yield strength in tension for all materials, except for ceramics for which