A 2‑metre long copper wire has a resistance of 0.04 Ω at 20 °C. If its length is doubled, what will be the new resistance, assuming the resistivity stays the same?

medium 2 marks Class 7 · Science · Electric Current and its Effects

Answer

0.08 Ω

The correct answer is 0.08 Ω — Step 3: Compute $R_{n e w} = 0.08, O m e g a$ .

Step-by-step solution

Step 1: Write the relation $R p r o pt o L$ for a material of fixed cross‑section and resistivity.
Step 2: Since $L$ doubles from 2 m to 4 m, the resistance also doubles: $R_{n e w} = 2 \times 0.04, Ω$ .
Step 3: Compute $R_{n e w} = 0.08, O m e g a$ .

Common mistake: Forgetting that resistance doubles when length doubles.

Concept tested: Resistance proportional to length.