Read Wire Extension: Compare Compliance, Not Only Length
Wire extension is ΔL = F L/(A Y). Length, cross-section area and Young's modulus all matter, and the radius enters through area squared in a ratio.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Start with the condition
For wires under the same tension, divide their extensions directly: ΔL-steel/ΔL-brass = (L-steel/L-brass) times (A-brass/A-steel) times (Y-brass/Y-steel).
Build the governing relation
Convert a radius ratio β into an area ratio β squared before simplifying.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Using radius rather than cross-sectional area makes the extension ratio off by a power of two.
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