The big idea: A physical property is one you can measure or observe without the material changing in any way.
The test: Weigh it, heat it, pass a current through it — put it back and it is the same material. That test is the definition.
Each property drawn as the observation that measures it. Step through to the kettle at the end.
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Density
Mass per unit volume, in kg/m³ or g/cm³. Decides how heavy a product is for its size. Aluminium is about a third the density of steel — which is why a bicycle frame is one and not the other.
Thermal expansion
How much a material grows when heated and shrinks when cooled. Bridges need expansion joints; a metal lid on a glass jar loosens under hot water; two bonded materials with different rates will warp.
Thermal conductivity
How readily heat passes through. Copper conducts, so it is the pan base; polymer resists, so it is the handle. One product often needs both, deliberately.
Melting point
The temperature at which it turns liquid. Sets both what the product survives and how it is processed — a high melting point makes casting expensive, because the mould must survive it too.
Electrical conductivity
How easily current flows. Copper conducts, so it is the wire core.
Electrical resistivity
The inverse — how strongly the material opposes current. PVC resists, so it is the sheath. Say which of the two you mean; they are not interchangeable.
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A kettle is the cleanest example in the whole topic, because five of the six physical properties are chosen deliberately in one object.
| Part | Property chosen | Why |
|---|---|---|
| Body | Low density | Light to lift when full of water |
| Element | High melting point | Survives being heated red-hot in use |
| Element | High thermal conductivity | Heat reaches the water quickly |
| Handle | Low thermal conductivity | Stays cool enough to hold |
| Flex sheath | High electrical resistivity | Insulates the user from the core |
Opposites on purpose: The same kettle wants high thermal conductivity in the element and low in the handle. A property is never good or bad in itself — only right or wrong for that part.
Physical properties at work
Dense hardwood base
Hardwoods are usually denser than softwoods, so a hardwood base is heavy for its size and keeps a speaker or lamp stable.
Light balsa models
Balsa has a very low density, so models are light and quick to cut and carry.
Low density, less fuel
An aircraft made of low-density materials is lighter, so it burns less fuel on every flight.
Thermoplastics soften
A thermoplastic softens at a low melting point (around 200 °C), so it can be 3D printed, moulded and melted down to be recycled.
Hardness is not on this list
Older papers call hardness a physical property. In this course it is mechanical (how a material resists scratching under force).
How this is tested — explaining the physical properties and applying them to a product. It comes up two ways:
Paper 1 — multiple choice
- Identify which physical property a described requirement needs.
- Spot the property that is physical rather than chemical.
Paper 2 — analysing a product
- Explain the physical properties chosen for parts of a product.
- Explain why one product needs opposite values of one property.
The trap: Writing "it conducts heat well" without saying which part and why that matters. The mark is the design consequence, not the property name.
Explain the physical properties a designer would specify for the base and the handle of a frying pan.
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