Electrical Materials
What the exam tests
This domain covers conductors, insulators, magnetic materials, contact materials and heat-resistant materials used in domestic appliances. Questions ask why a material is chosen for a cable, element, soleplate, insulation or contact.
Core rules
- Conductors have low resistivity. Silver is the best conductor, copper is the standard for cables because it is ductile and joins well, and aluminium is lighter and cheaper but needs a larger cross-section and careful oxide removal.
- Insulators resist current: PVC for general cable, rubber for flexible cords, porcelain and ceramic for heat, mica for element insulation, bakelite for handles and bases.
- Magnetic materials such as soft iron and silicon steel carry flux in motors and relays. Contact materials need low contact resistance and resistance to arcing; silver alloys are common.
- Heating elements use nichrome because of high resistivity and oxidation resistance at high temperature. Insulation classes limit temperature; Class B is 130 °C, Class F 155 °C and Class H 180 °C.
Worked example
A 1000 W heater element must run red-hot in air. Copper is rejected: its resistivity is too low, so a long thin wire would be needed, and it oxidises. Nichrome is chosen for the element, mica or ceramic for electrical insulation, and a metal sheath for mechanical protection.
Traps
Do not say copper is the best conductor; silver is. Do not substitute aluminium for copper without increasing size and using suitable terminals. Do not use PVC where a hot element touches it. Do not confuse temperature class with ambient temperature.