By the end of this topic, you should be able to...
explain density, thermal expansion, thermal conductivity, melting point, electrical resistivity and electrical conductivity.
Guiding Question
How do material properties and classifications aid material selection for a specified manufacturing process?
💡 Did You Know? The Mars Climate Orbiter was lost on 23 September 1999 — a $327 million spacecraft destroyed because one engineering team used imperial units and another used SI units in the navigation software. The underlying physical property at fault was impulse — a product of force and time — but the incident illustrates a broader truth: physical properties are only useful when measured, communicated, and applied with precision. Density, thermal conductivity, electrical resistivity, and optical transmittance are not approximate descriptors — they are quantitative design inputs that determine whether a product functions, survives its service environment, or fails. Physical properties are the numerical language in which material behaviour is specified.
What Are Physical Properties?
Physical properties aren't abstract numbers in datasheets—they're the reason products survive (or fail) in the real world. Density determines whether your drone flies or crashes under its own weight.
Physical properties describe how a material responds to physical stimuli — gravitational fields, thermal energy, electrical fields — without undergoing chemical change or permanent mechanical deformation. They are intrinsic properties of the material — governed by atomic mass, crystal structure, and atomic bonding — and cannot generally be changed by the designer in the way that mechanical properties (strength, hardness) can be modified through heat treatment or cold working.
As introduced in A3.1.3, the three groups of material properties are:
Property Group | Nature of Stimulus | Properties Covered |
Physical | Non-mechanical, non-chemical stimuli (thermal, electrical, optical, magnetic) | Density, thermal expansion, thermal conductivity, melting point, electrical resistivity, electrical conductivity |
Chemical | Chemical environment, biological systems | Corrosion resistance, biocompatibility, degradability |
Mechanical | Applied forces and deformations | Strength, stiffness, toughness, hardness, fatigue |
This sub-topic provides the precise quantitative treatment of the six core physical properties. Each property is defined, quantified, and related to specific design applications so that its evaluation in material selection is practical, not abstract.
Density
Density is defined as the mass per unit volume of a material. It is a fundamental physical property governed by the atomic mass of the constituent elements and the efficiency with which atoms are packed together in the material's crystal structure or molecular arrangement.
Product Design Application — school bag structure:
A school bag frame must be rigid enough to support the load of books and equipment (mechanical — stiffness) but as light as possible to avoid adding to the user's carry load (physical — density). Aluminium alloy tube sections provide structural rigidity at significantly lower mass than steel — approximately one-third the mass for identical cross-sectional geometry. High-end expedition packs (Deuter, Osprey) use CFRP sheet or aluminium alloy for framesheet and stay elements precisely because density is the dominant physical property selection criterion in wearable load-bearing systems.
Manufacturing application — density in casting:
Density affects manufacturing process behaviour in casting. Liquid aluminium flows less aggressively under gravity than liquid steel, requiring pressure die casting or careful runner design to ensure complete mould filling. The lower hydrostatic pressure of liquid aluminium in a mould requires compensating design of gating systems.
Thermal Expansion
Thermal expansion is the tendency of a material to change its dimensions (length, area, volume) in response to a change in temperature.
When a material is heated, increased thermal energy causes atoms to vibrate more energetically about their equilibrium positions, increasing the average inter-atomic spacing and thus causing macroscopic dimensional increase.
Thermal Conductivity
Thermal conductivity (k) is the rate at which thermal energy (heat) is transferred through a material by conduction, per unit area, per unit temperature gradient. It quantifies how readily a material allows heat to flow through it.
Product Application — Cookware Selection
Cookware Material | k (W/m·K) | Cooking Performance | Limitation |
Copper (base or fully clad) | 401 | Fastest, most uniform heat distribution; most responsive | Expensive; reacts with acidic food; heavy |
Aluminium (anodised, clad) | 205 | Fast, good uniformity; lightweight | Softer; requires anodising or cladding |
Cast iron | 46–50 | Slow to heat (high thermal mass) but retains heat very well | Very heavy; requires seasoning (rust-prone) |
Stainless steel (clad over Al or Cu) | 16 | Poor conductor alone — needs conductive core layer | Low kkk creates hot spots if used alone |
Ceramic (alumina, stoneware) | 1–3 | Very slow, even heat once at temperature; excellent retention | Thermal shock risk; fragile |
Melting Point
Melting point is the temperature at which a material transitions from solid phase to liquid phase at a given pressure (typically stated at standard atmospheric pressure, 101.325 kPa).
At the melting point, the thermal energy supplied to the material is sufficient to overcome the inter-atomic or inter-molecular bonding forces that maintain the ordered solid crystal structure (or, for amorphous materials, the network structure).
Why Melting Point Matters in Design
A material used in a heated environment must have a melting point (or in the case of polymers, a glass transition temperature) well above the maximum service temperature. Industry practice uses a safety margin — typically the maximum service temperature is limited to approximately 60–80% of the absolute melting temperature
Electrical Resistivity
Electrical resistivity is an intrinsic material property that quantifies how strongly a material opposes the flow of electric current through it. Unlike electrical resistance (which depends on component geometry), electrical resistivity is a material constant — independent of the size or shape of the sample.
Materials selected for resistance heating (electric stovetop elements, industrial furnaces, hair dryers, toasters) require deliberately high resistivity combined with:
High melting point (to survive operating temperature)
Oxidation resistance (to function in air at elevated temperature)
Physical formability (must be drawnable into wire)
Electrical Conductivity
Electrical conductivity (σe\sigma_eσe) is the reciprocal of electrical resistivity — it quantifies how readily a material allows electric current to flow through it.
Key Vocabulary
These definitions must be precise. In an explain question, the examiner is looking for accurate use of technical language with units.
Term | Precise Definition |
Density (ρ\rhoρ) | Mass per unit volume, measured in kg/m³. Determined by atomic mass and atomic packing arrangement |
Thermal expansion | The increase in dimensions of a material when temperature rises, quantified by the coefficient of linear thermal expansion α\alphaα (K⁻¹) |
Coefficient of thermal expansion (CTE) | The fractional change in length per unit change in temperature, symbol α\alphaα, units K⁻¹ or °C⁻¹ |
Thermal conductivity (kkk) | The rate of heat transfer through a material per unit area per unit temperature gradient. Units: W/m·K. Higher kkk = better heat conductor |
Melting point | The temperature at which a material transitions from solid to liquid at standard pressure. Directly related to the strength of atomic bonds within the material |
Glass transition temperature (Tg) | The temperature range over which an amorphous polymer transitions from a rigid glassy state to a flexible rubbery state — the polymer equivalent of a melting point |
Electrical resistivity (ρe\rho_eρe) | The intrinsic opposition of a material to electric current flow, independent of geometry. Units: Ω·m. Lower resistivity = better conductor |
Electrical conductivity (σ\sigmaσ) | The reciprocal of electrical resistivity — the ability of a material to carry electric current. Units: S/m. Higher conductivity = better conductor |
Conductor | A material with very high electrical conductivity (low resistivity) — typically a metal with freely moving electrons |
Insulator | A material with very low electrical conductivity (very high resistivity) — electrons are tightly bound and cannot move freely |
Semiconductor | A material with intermediate electrical conductivity that can be precisely controlled by temperature, light, or doping |
Bimetallic strip | A component made from two metals with different coefficients of thermal expansion bonded together — bends predictably when temperature changes, used in thermostats |
Specific strength | Tensile strength divided by density — allows comparison of structural efficiency of materials of different densities (Ashby, 2011) |
Practice Questions
The learning objective for this topic uses the command term EXPLAIN — "give a detailed account including reasons or causes." A description alone is never enough. For every property you name, you must state what it is, why it occurs at the atomic or structural level, and what the consequence is for design. These questions mirror that expectation precisely.
Question 1 (3 marks)
Explain why aluminium alloy is preferred over steel for the construction of a commercial aircraft fuselage, with reference to density and one other physical property.
Examiner's hint: Define density. Explain why lower density is advantageous in aviation — mass directly drives fuel consumption and payload capacity. Select a second relevant property (thermal expansion: aircraft fuselage experiences large temperature swings between ground and cruise altitude; or thermal conductivity: cabin insulation requirements). Explain the property definition, the mechanism, and the design consequence. Do not simply list — EXPLAIN with reasons.
Question 2 (4 marks)
Explain why expansion joints are incorporated into the design of long steel bridges. Include the relevant physical property, its definition, and the consequence if expansion joints were omitted.
Examiner's hint: Define thermal expansion and CTE. Explain the atomic mechanism — increased temperature causes increased atomic vibration and increased interatomic separation. Calculate a sense of scale: a steel bridge 500 m long, α\alphaα = 12 × 10⁻⁶ K⁻¹, seasonal temperature range of ~40°C → ΔL = 12 × 10⁻⁶ × 500 × 40 = 0.24 m = 24 cm of movement. Without expansion joints: compressive buckling in summer, tensile cracking in winter.
Question 3 (4 marks)
Explain the difference between thermal conductivity and electrical conductivity as physical properties of materials. Use one named material example to illustrate each property in a design context.
Examiner's hint: Define each precisely with units. Explain the shared mechanism in metals — free electrons carry both heat and charge — which is why good electrical conductors are usually good thermal conductors (Wiedemann-Franz Law). Electrical: copper wiring in PCBs — high σ\sigmaσ, low energy loss. Thermal: expanded polystyrene in a cool box — low kkk, slow heat transfer into contents. Show you understand both properties at mechanism level, not just application level.
Question 4 (3 marks)
Explain how melting point influences the selection of a material for a solder joint in electronics manufacturing.
Examiner's hint: Define melting point as the temperature of solid-liquid transition, linked to atomic bond strength. Explain that solder must melt at a low enough temperature to flow and form a joint using a simple soldering iron (~200–250°C tip temperature), yet must remain solid during product operation (electronic components typically operate below 85°C). Sn-Pb solder melts at ~183°C, lead-free Sn-Ag-Cu (SAC) solder at ~217°C — both selected specifically because their melting points fall in the correct window. Also explain that the components being soldered (silicon chips, PCB substrate) must NOT reach their own melting/softening point during the process.
Question 5 (4 marks)
Explain why electrical resistivity increases with temperature in metals but decreases with temperature in semiconductors. Refer to the atomic-level behaviour of electrons in your answer.
Examiner's hint: In metals — the lattice vibrates more at higher temperatures, increasing the frequency of electron-lattice collisions, impeding electron flow, increasing resistivity. In semiconductors — at low temperatures, electrons are bound to atoms and cannot conduct. As temperature rises, more electrons gain sufficient thermal energy to break free into the conduction band, increasing the number of charge carriers, decreasing resistivity. This is the fundamental physics behind thermistors (NTC type) used in temperature sensors.
Question 6 (6 marks)
A designer is developing a handle for a professional chef's knife. The handle is riveted to the steel blade and will be used in a commercial kitchen environment involving heat, moisture, cleaning chemicals, and intensive daily use. Explain how three physical properties should influence the material selection for the handle. For each property, state the required value (high or low) and justify your reasoning.
Examiner's hint: This is a full explain response. Select three from: thermal conductivity (low — handle must not heat up when blade contacts hot surfaces), density (low to moderate — balance comfort and user fatigue during long service), melting point (adequately high — handle must not soften in dishwasher ~70°C or near hot surfaces), electrical conductivity (low — safety in wet environment near electrical equipment). For each: define the property, state the requirement, and explain WHY that requirement exists in this specific design context.
Sources
Ashby, M.F. (2011). Materials Selection in Mechanical Design. 4th edn. Butterworth-Heinemann, Oxford.
Callister, W.D. and Rethwisch, D.G. (2018). Materials Science and Engineering: An Introduction. 10th edn. Wiley, New York.
International Baccalaureate Organization (2024). Design Technology Guide: Diploma Programme. First Assessment 2027. IBO, Geneva.
Ulrich, K.T. and Eppinger, S.D. (2015). Product Design and Development. 6th edn. McGraw-Hill Education, New York.
Linking Questions
Why is a good understanding of material properties important when designing structural systems? (A3.2)
When do the physical properties of materials restrict the ability to use certain prototyping techniques? (A2.2)
How do the properties of a material influence the choice of manufacturing techniques for a product? (A4.1)
How can the characteristics of a material limit the effectiveness of modelling and prototyping as designs are developed? (B2.2)
How important is an understanding of the mechanical properties of a material when considering structural and mechanical systems, and their applications? (A3.2, A3.3, B3.2, B3.3)
Which classifications of properties are important when developing electronic systems and their applications? (A3.4, B3.4)
How could the continued development of biodegradable materials influence designers’ ability to address aspects of design for sustainability and design for a circular economy? (C2.1, C2.2)
Why is a thorough understanding of materials key for effective product analysis and evaluation of products? (C3.1)
How do design decisions related to the properties of materials and components impact a product’s life-cycle analysis? (C3.2)