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A copper rod of length 25 cm and cross-section 3 mm² is joined end to end with an identical-dimension aluminium rod. Find the resistance of the series combination between ends A and B. (Resistivity of Cu = 1.7 * 10⁻⁸ Ohm.m, resistivity of Al = 2.6 * 10⁻⁸ Ohm.m)

1. 2.170 mOhm
2. 1.420 mOhm
3. 0.0858 mOhm
4. 0.858 mOhm

Correct answer: 0.858 mOhm

Solution

Each individual rod is about 1.42 mOhm (Cu) and 2.17 mOhm (Al); the figure shows them connected in parallel between A and B, giving about 0.858 mOhm.

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