Q4. The extension-load graph for a wire 2.5 m long and 1.0 mm in diameter has a slope of 0.28 mm per kg. Find Young’s modulus and suggest the metal, using NCERT Table 8.1. Take g = 9.8 m s.
πr = π(0.50 × 10), about 7.85 × 10 m. Y = gL/(πr × slope) = 9.8 × 2.5/(7.85 × 10 × 0.28 × 10), about 1.1 × 10 N m, which matches copper.
Q5. Why should each reading be taken a little while after a load is added, and why should the loads be added gently?
The wire takes a short time to settle to its new length, and a sudden load makes it oscillate and can briefly stretch it more than the steady load would. Gentle loading and a short wait give the true steady extension.
Q6. A student adds loads up to a large maximum. While unloading, every reading is noticeably larger than the loading reading for the same load, and with all the loads removed the wire does not return to its starting reading. What has happened, and what should be done?
If zero drift and clamp slippage have been excluded, the likely cause is loading beyond the elastic limit and leaving a permanent set. Do not average away this systematic mismatch; check the apparatus and repeat with a fresh wire and a lower maximum load.