Electrical Materials
Electrical Materials
Electrical Materials tests why copper, aluminium, PVC, porcelain, bakelite, mica, nichrome and silicon steel are chosen for particular parts. Questions often ask property-to-application matching and simple resistance calculations.
Core rules and concepts
- Conductors: copper resistivity is about 1.68×10⁻⁸ Ω·m; aluminium about 2.82×10⁻⁸ Ω·m. Copper conducts better and is used for contacts and small appliance wiring; aluminium is lighter and cheaper for overhead lines but forms an insulating oxide and needs special joints.
- Insulators: PVC for general domestic wiring (about 70°C), XLPE for higher temperature, rubber for flexible cords, porcelain for high-voltage insulators, bakelite for switch bodies and handles, mica for heating-appliance insulation.
- Magnetic materials: soft iron and silicon steel for cores because they are easily magnetised and demagnetised; ferrites for high-frequency cores.
- Heating and protection: nichrome (about 80% Ni, 20% Cr) for elements because of high resistivity, oxidation resistance and high melting point; tin-lead alloy for fuse wire; 60/40 tin-lead solder for joints.
- Selection depends on resistivity, temperature coefficient, dielectric strength, thermal conductivity, mechanical strength, cost and workability.
Worked example
Why is nichrome, not copper, used for a heater element? A 1000 W, 230 V element needs R = V²/P = 52.9 Ω. Copper would need a very long, thick conductor to reach 52.9 Ω and would oxidise and soften when hot. Nichrome reaches the same resistance with a short, manageable wire and survives red heat. Reasoning: the element must convert electrical energy to heat, not transport it efficiently.
Traps that cost marks
- Saying silver is used in domestic wiring because it has the best conductivity; it is too costly.
- Ignoring the insulating oxide layer on aluminium joints.
- Using PVC insulation where the operating temperature is high.
- Confusing mica, an insulator, with a conductor.