Insulated copper wire.
Explanation
An electromagnet is constructed by winding a coil of wire around a ferromagnetic core, such as soft iron. When electric current flows through the coil, it generates a magnetic field similar to that of a bar magnet. The effectiveness of the electromagnet depends on the current following the specific helical path of the coil.Analysis of Options:
- Bare copper wire. Bare copper wire (Incorrect): If bare wire is used, the adjacent turns of the coil will come into direct electrical contact. This creates a short circuit, causing the current to flow straight across the bundle of wire rather than spiraling through the coil. Consequently, the required magnetic field will not be generated.
- Insulated copper wire. Insulated copper wire (Correct): The wire must be insulated to prevent electrical contact between adjacent turns. This forces the current to travel the full length of the helical winding, creating a strong, concentrated magnetic field inside the core. Copper is preferred due to its low electrical resistance, which minimizes heat loss and allows higher currents to flow.
- High-resistance nichrome wire. High-resistance nichrome wire (Incorrect): Nichrome has high electrical resistance, which causes significant heat dissipation ($I^2R$ loss) when current flows. Electromagnets require high current to generate a strong magnetic field ($B \propto I$), making high-resistance wires inefficient and prone to overheating.
- Low-melting point fuse wire. Low-melting point fuse wire (Incorrect): Fuse wire is designed to melt when current exceeds a certain limit. It lacks the mechanical strength and thermal stability required for the construction of an electromagnet coil.
Key Takeaway: The wire in an electromagnet must be insulated to prevent short-circuiting between turns and must have low resistance (e.g., copper) to maximize current flow and magnetic field strength while minimizing heat generation.