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Physics · Electricity · Resistance
Resistance vs length of wire
A length of constantan wire is stretched along a metre rule between two clips. A jockey contact slides along the wire, allowing different lengths to be tested. Measure R for each length and plot R against L — the gradient gives the resistance per metre.
length under test (drag jockey)
L = 50 cm · R = 2.50 Ω
Results table
| L (cm) | R (Ω) | R/L (Ω/m) |
|---|---|---|
| 20 | — | — |
| 40 | — | — |
| 60 | — | — |
| 80 | — | — |
| 100 | — | — |
Plot · R vs L
Straight line through origin → R ∝ L. Gradient ≈ 5.0 Ω/m.
Exam-style questions
1. For a uniform wire, how does resistance depend on length L?
2. Why must the wire be kept COOL (low current)?
3. If you double the wire's CROSS-SECTIONAL AREA but keep L the same, what happens to R?
4. From the slope of R-vs-L you can calculate ρ. Formula: