Uses of metals
We focus on three key metals in this syllabus: Aluminium (Al) and Copper (Cu). Building on the concept of density from earlier topics, we compare these metals qualitatively rather than using specific numerical values.
| Metal | Key Property | Common Use |
|---|---|---|
| Aluminium | Low density (lightweight) | Aircraft manufacture |
| Aluminium | Good electrical conductivity + Low density | Overhead electrical cables |
| Aluminium | Resistance to corrosion | Food containers (e.g., cans) |
| Copper | Good electrical conductivity + Ductility | Electrical wiring |
Why it matters: Aluminium has a much lower density than steel or iron. This makes it ideal for aircraft, where reducing weight is critical for fuel efficiency and lift.
Why it matters: Both copper and aluminium are good conductors. However, for overhead cables, aluminium is preferred over copper because it is lighter (lower density), making the cables easier to support between pylons.
Why it matters: Aluminium forms a protective oxide layer that prevents further corrosion. This makes it safe and durable for food containers, ensuring the metal does not react with the food or degrade quickly.
Why it matters: Copper is highly ductile, allowing it to be pulled into long, thin wires for electrical wiring in buildings and appliances.
Reasoning:
- The metal must be strong enough to hold its shape.
- The metal must be lightweight to allow the plane to fly efficiently.
- Aluminium is chosen because it has a low density compared to other structural metals like steel.
Key Takeaway: If the question asks why aluminium is used in aircraft, the primary property to cite is its low density.
Reasoning:
- The metal must conduct electricity well (low resistance).
- The metal must be flexible enough to be installed in walls.
- Copper is chosen because it has good electrical conductivity and is ductile (can be drawn into wires).
Key Takeaway: For copper, you must mention both conductivity and ductility if asked for two properties.
The Correction: While aluminium is a good conductor, copper is actually a better conductor. The reason aluminium is chosen for overhead cables specifically is the combination of good conductivity AND low density. If only conductivity were considered, copper would be the choice. The low density reduces the load on the support pylons.
The Correction: Copper does not corrode easily under normal conditions, but it is not typically used for food containers because it can react with acidic foods. Aluminium is the standard metal for food cans due to its specific resistance to corrosion and light weight. Do not assume copper is used for food storage.
Strategy: When evaluating options, look for the direct link between property and use. For example, if the use is 'aircraft', the property must include 'low density'. If the option lists 'high density' or 'poor conductivity' alongside aircraft, it is incorrect.
Why this works: Examiners test your ability to apply knowledge logically. They accept answers that correctly pair the specific physical property with its functional advantage.
Strategy: Do not quote specific density values (e.g., '2.7 g/cm³') unless they are provided in the question. Instead, use comparative qualitative terms like 'lower density than steel' or 'lightweight'.
Why this works: The syllabus requires you to understand the concept of low density relative to other metals, not memorize constants. Quoting unprovided numbers can sometimes lead to errors if you misremember them, whereas qualitative comparisons are always correct in this context.
- Good electrical conductor (of electricity). 2. Low density.