The point at which the molecules of a loaded wire begin to slide across each other resulting in a rapid increase in extension is

Analyze the question: The question is about a loaded wire. It asks for the point at which layers of molecules begin to slide over each other, causing a sudden, rapid increase in extension with little or no extra load.

Explain the concept: On a stress-strain (or load-extension) graph for a ductile material like a wire, the key points in order are:

1. Proportionality limit: Hooke's law holds here. Extension is directly proportional to load.
2. Elastic limit: The wire still returns to its original length when the load is removed.
3. Yield point: The molecular layers slip over one another. The wire extends rapidly without a significant increase in load, and the deformation becomes plastic (permanent).
4. Ultimate tensile strength and breaking point: The wire necks, then breaks.

Solution:

• Proportionality point (B): this is only where Hooke's law stops being obeyed. There is no sliding of molecules here.
• Elastic point (D): the wire still recovers fully, so there is no sliding.
• Deformation point (C): this is not a standard named point on the graph.
• Yield point (A): this is where molecules slide across each other and the extension rises rapidly. This matches the description exactly.

Answer: The correct option is A. yield point.

Exam tip: Remember "yield = give way." When the material yields, it gives way and flows. Whenever you see "sudden large extension with little extra force," think yield point.