Resistivity of a conductor changes with temperature. If the resistivity of a nichrome wire increases when it is heated, what happens to the current through it if it is connected to the same voltage source before and after heating? Justify your answer using relevant relationships.
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
When a nichrome wire is heated, its resistivity increases. Since resistance $R = \rho \dfrac{l}{A}$, an increase in resistivity causes an increase in resistance.
By Ohm's law: $I = \dfrac{V}{R}$
Since the voltage V remains the same but resistance R increases, the current through the wire decreases. Current is inversely proportional to resistance — if resistance increases, current decreases proportionally.
Source: Chapter 11 (Electricity), Sections 11.4 and 11.5
Explanation
- Key relationships to quote: $R = \rho \frac{l}{A}$ (links resistivity to resistance) and $I = V/R$ (Ohm's law linking current to resistance and voltage).
- Examiners want you to clearly state: resistivity ↑ → resistance ↑ → current ↓ (at same V), with the formulas justifying each step.
- Don't just say "current decreases" — you must justify why using both equations to earn full 3 marks.