Which of the following is an essential physical property of the wires uses for making fuses ?

Analysis

The question asks for the specific physical property that makes a material suitable for use as a fuse wire in an electrical circuit.

Concept Explanation

A fuse is a safety device designed to protect electrical circuits from damage caused by excessive current (overload or short circuits).

To perform this function effectively, the fuse wire must be designed to "blow" (melt and break the circuit) whenever the current exceeds a safe limit. If the wire did not melt at a relatively low temperature, the excess current would continue to flow, potentially causing the wires in the walls or the appliance itself to overheat and catch fire.

Step-by-Step Solution

1. Understand the Function: The primary purpose of a fuse is to break the circuit when current becomes too high.
2. Analyze the Mechanism: When current flows through a wire, it generates heat (Joule heating). The amount of heat produced is proportional to the square of the current ($H = I^2Rt$).
3. Evaluate the Requirements: • If the fuse wire had a high melting point, it would require an extremely high amount of heat (and thus a very high current) to melt. By the time it melted, the rest of the circuit might already be damaged. • If the fuse wire has a low melting point, it will melt quickly as soon as the current exceeds the rated limit, effectively cutting off the power supply and preventing damage.
4. Review Options: • A (Low density): Density does not affect the melting behavior of the wire. • B (Hypothermal conductivity): This is not a standard term used in this context; high thermal conductivity would actually be a disadvantage as it would dissipate heat away from the fuse. • C (Low electrical resistivity): While fuse wires usually have low resistance to avoid unnecessary heating during normal operation, the essential safety property is the melting point. • D (Low melting point): This is the critical characteristic that ensures the fuse acts as a safety switch.

Conclusion

The most essential property for a fuse wire is its ability to melt quickly when the current limit is reached. Therefore, it must have a low melting point.

Exam Tip: Remember the "Fuse Rule": Low Melting Point, High Current Sensitivity. If you are asked about the material used, it is typically an alloy of lead and tin because these metals have low melting points compared to copper.