To serve as an insulator and prevent electric shock
Explanation
Electrical wiring systems require a distinction between the path of current flow and the external environment. Materials are classified based on their ability to conduct electricity: conductors (low resistivity) allow current flow, while insulators (high resistivity) block it.
Option Analysis:
- To prevent atmospheric corrosion of the metal wire is incorrect: While a coating does protect the metal from oxidation and atmospheric corrosion, this is a secondary benefit. The primary functional requirement for the coating is electrical safety, not chemical preservation.
- To reduce the heating effect of the electric current is incorrect: The heating effect of electric current (Joule heating) is determined by the formula \( H = I^2Rt \). The coating does not reduce the heat generated by the current flowing through the wire. In some cases, thick insulation may actually retard heat dissipation.
- To serve as an insulator and prevent electric shock is correct: Plastic (such as PVC) and rubber are excellent electrical insulators. The primary scientific reason for coating wires with these materials is to confine the electric current within the wire and prevent it from passing into other conductive bodies or living beings upon contact, thereby preventing electric shocks and short circuits.
- To increase the conductivity of the wire is incorrect: The conductivity of the wire is a property of the metal core (e.g., copper or aluminum). An external insulating layer does not alter the intrinsic conductivity of the metal.
Key Takeaway:
Electrical wires are coated with insulating materials like plastic or rubber primarily to ensure safety by preventing electric shocks and short circuits, utilizing the high resistivity of these materials.