Investigating Resistance in Wires – Length and Thickness Effects

A classic GCSE & A-level Physics practical that still earns its place on the syllabus

Resistance in a wire depends on how long it is, how thick it is, and what it’s made from. This investigation is a cornerstone practical at GCSE and A-level Physics because it links abstract equations directly to what students can measure, plot, and explain.

At Hemel Private Tuition, this experiment works particularly well because students can slow it down, repeat measurements, and really see where uncertainty creeps in.

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The Physics Behind It

The key relationship is:

$$R = \rho \frac{L}{A}$$

Where:

• R = resistance (Ω)

• ρ = resistivity (material constant)

• L = length of wire

• A = cross-sectional area

This immediately gives two testable predictions:

• 📏 Doubling the length doubles the resistance

• 🧵 Increasing thickness (area) reduces resistance

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Experiment 1: Resistance vs Length

Method (GCSE & A-level friendly)

• Use a long wire (e.g. constantan or nichrome) taped to a metre ruler

• Measure resistance for increasing lengths using:

  • Ammeter + voltmeter or

  • A digital multimeter

• Keep current low to reduce heating

Expected Result

• A straight-line graph of resistance vs length

• Passing (close to) the origin

📌 A perfect opportunity to discuss why it might not pass exactly through zero.

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Experiment 2: Resistance vs Thickness

Method

• Use wires of the same material and length but different diameters

• Measure diameter with a micrometer screw gauge

• Calculate cross-sectional area

• Measure resistance

Expected Result

• Thicker wire → lower resistance

• At A-level: plot R vs 1/A to get a straight line

📌 This is where maths, physics, and experimental technique really come together.

Experiment 3: Resistance vs Thickness

Method

• Using Conductive Putty
• Measure out equal masses of resistance putty and roll them into different thicknesses and lengths.
• Measure the resistance for different thicknesses at the same length, for different lengths at the same thickness, and identify two lengths with various thicknesses that have the same resistance.
  

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Common Pitfalls (and Why They Matter in Exams)

• 🔥 Heating of the wire → resistance increases mid-experiment

• 📐 Inaccurate diameter measurement → large percentage error

• 🔌 Crocodile clips not making clean contact

• 📉 Poor graph scaling and missing uncertainty bars

These are exactly the points examiners reward when students explain limitations and improvements.

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Why This Practical Still Matters

✔ Reinforces proportionality
✔ Links equations to real data
✔ Builds graph and analysis skills
✔ Develops practical confidence

It’s also a great discriminator between students who know the formula and those who understand the physics.

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Teaching Tip from the Lab

At Hemel Private Tuition, we often run this experiment twice:

• Once quickly to see the trend

• Once slowly, focusing on precision and uncertainty

Students are often surprised how much their results improve the second time.

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If you’d like a fully guided worksheet, exam-style questions, or to run this practical in person or online using our multi-camera lab, just get in touch.