A copper rod of 88 cm and an aluminium rod of unknown length have their increase in length independent of increase in temperature. The length of aluminium rod is : (α_Cu = 1.7×10^-5 K^-1 and α_Al = 2.2×10^-5 K^-1)

1. 6.8 cm

2. 113.9 cm

3. 88 cm

4. 68 cm

Doubtly Asked 30 Oct 2026 NEET UG 2019 Practice test neet Save WhatsApp Report

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Answer: 4. 68 cm

The difference in length stays the same at all temperatures only if both rods expand by the same amount: \(\Delta l_{Cu}=\Delta l_{Al}\).

\(l_{Cu}\alpha_{Cu}\Delta T=l_{Al}\alpha_{Al}\Delta T\Rightarrow l_{Al}=88\times\dfrac{1.7}{2.2}=68\) cm.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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