Electrical Materials
This domain tests the properties and selection of conductors, insulators, magnetic materials and contact materials. The exam checks whether you can match a material to its job: copper for conductivity, aluminium for lighter overhead lines, PVC for insulation, and silver or silver alloys for low-resistance contacts.
Core rules
- Copper has low resistivity, about 0.0172 Ω·mm²/m, and resists corrosion better than many alternatives. Aluminium is lighter and cheaper but has higher resistivity, about 0.0282 Ω·mm²/m.
- Resistance is R = ρL/A. Increasing length raises resistance; increasing cross-sectional area lowers it.
- Insulation is rated by voltage, temperature and application. PVC is common for domestic wiring; XLPE withstands higher temperatures.
- Magnetic materials such as soft iron are used in cores because they are easily magnetised and demagnetised. Hard magnetic materials retain magnetism for permanent magnets.
- Contact materials need low contact resistance and resistance to arcing. Silver, copper alloys and tungsten are used according to the duty.
Worked example
Find the resistance of 20 m of 2.5 mm² copper conductor. R = 0.0172 × 20 / 2.5 = 0.1376 Ω. The same length of aluminium gives R = 0.0282 × 20 / 2.5 = 0.2256 Ω. The aluminium conductor has higher resistance, so it would produce more voltage drop and heat for the same current.
Traps
Do not substitute diameter for area. Do not ignore temperature rise, which increases resistance of copper. Do not choose insulation by colour alone. Do not use a contact material only because it is cheap.