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8463 · RP3; 8.2.3Required practical activitiesCore

RP3: Wire Resistance and Resistor Combinations

Connect the ammeter in series with the investigated wire and the voltmeter across the selected wire length. Measure both potential difference and current at each setting, then calculate R = V/I using amperes. Changing the contact position changes the length included between the voltage connections. Keep wire material and cross-sectional area constant when investigating length.

35 min Video by Malmesbury Education Required practical activities
Resistance of a Wire - GCSE Science Required PracticalCurated by Kepler, not produced by us. Watch it before the notes below.
Open on YouTube

Before you start

  • Use your teacher’s tier checklist: this overview can include Higher-only material, and is not a tier-specific revision list.
  • AQA separate GCSE Physics 8463. Use the specified required-practical reference and the centre’s approved apparatus and supervised procedure.
  • Read an electrical circuit and calculate resistance from potential difference and current.
  • Use the centre’s approved supervised method and equipment limits. These notes prepare you to explain and evaluate practical work; they do not replace carrying it out and recording the evidence.

Watch, pause, try

Video by Malmesbury Education. The embedded demonstration focuses on wire length. The written guide and additional resistor-combination video cover the separate series/parallel part.

Viewing focus: Measure resistance for a controlled wire-length comparison

After watching, attempt these questions before opening the answers. You can also use the written lesson below.

After watching 1: Why could a longer measurement session change resistance even if the contact position stayed fixed?

Show answer and reasoning

Current can heat the wire, changing its temperature and resistance. Control current and measurement/cooling timing rather than treating length as the only possible cause.

After watching 2: What evidence would distinguish a genuine length trend from an added contact resistance?

Show answer and reasoning

Several R-versus-length points can reveal a gradient and a nonzero intercept. Check contacts and the length reference; a single ratio does not separate these effects.

What you'll be able to do

  • Measure resistance for a controlled wire-length comparison
  • Construct and check series and parallel resistor circuits
  • Distinguish a material trend from heating and contact effects
1

Measure the selected length without changing other factors

Connect the ammeter in series with the investigated wire and the voltmeter across the selected wire length. Measure both potential difference and current at each setting, then calculate R = V/I using amperes. Changing the contact position changes the length included between the voltage connections. Keep wire material and cross-sectional area constant when investigating length.

Use a suitable low-current arrangement, make readings promptly and switch off between measurements as directed. Wire heating can change resistance and confound a length comparison. Keep contacts firm and use the same definition of the measured length. A wire that has been stretched or damaged may no longer have uniform cross-sectional area.

2

Interpret resistance against length

For a uniform wire of fixed cross-sectional area and temperature, resistance is proportional to length. Plot calculated resistance against measured length and inspect whether a straight-line relationship is supported. An unexplained intercept can indicate lead/contact resistance or a length-zero problem. Do not force the fitted line through zero merely because the ideal model predicts proportionality.

The gradient’s unit depends on whether length is plotted in metres or centimetres. A slope in Ω m⁻¹ is not itself resistivity: the wire’s cross-sectional area would also be required for that extension calculation. The GCSE investigation can establish a length relationship without claiming that every conductor’s resistance is independent of temperature.

3

Compare arrangements as well as individual resistors

Use a circuit diagram to build and check two resistors in series, then a suitable parallel arrangement. In series the same current passes through both and the total potential difference is the sum of the component differences. In parallel the branches have the same potential difference and the supply current is the sum of branch currents. Measure the relevant quantities rather than judging only brightness.

For two ideal resistors, series resistance is the sum. Adding a parallel path reduces the equivalent resistance below either positive branch resistance. Specify the supply and resistor ratings and avoid a short circuit or connecting an ammeter directly across the supply. A filament lamp heats and changes resistance, so do not substitute an assumption of fixed lamp resistance for actual operating measurements.

AQA RP3 requires both the wire-length investigation at constant temperature and combinations of resistors in series and parallel. Complete both parts; an I–V curve for a single resistor does not cover the combinations requirement.

4

Worked applications

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.

Example 1A 0.40 m wire has 1.20 V across it and carries 0.300 A. Find resistance. What does the uniform-temperature model predict for 0.80 m of the same wire?
1R = 1.20/0.300 = 4.00 Ω.
2Doubling length at unchanged area, material and temperature doubles resistance.
Answer
4.00 Ω measured; 8.00 Ω predicted under the stated controls.
Example 2Two ideal 6.0 Ω resistors are connected first in series and then in parallel. Find equivalent resistance in each case.
1Series: 6.0 + 6.0 = 12 Ω.
2In parallel, two equal paths share current, giving half one branch resistance.
Answer
12 Ω in series; 3.0 Ω in parallel.
Example 3An R-against-length line rises from 2.4 Ω at 0.20 m to 6.4 Ω at 0.60 m. Find its gradient and discuss the intercept.
1Gradient = (6.4 − 2.4)/(0.60 − 0.20) = 10 Ω m⁻¹.
2Extrapolated intercept = 2.4 − 10 × 0.20 = 0.4 Ω.
3Investigate contact/lead resistance or a length-zero error before assuming ideal proportionality.
Answer
Gradient 10 Ω m⁻¹; a 0.4 Ω intercept needs an experimental explanation.

Common mistakes to avoid

Leaving a thin wire heating while calling temperature controlled.
Use appropriate current and timed reading/cooling arrangements.
Calling the R-versus-length gradient resistivity.
State the units and the additional area factor needed for that extension.

Key takeaways

  • You can measure resistance for a controlled wire-length comparison.
  • You can construct and check series and parallel resistor circuits.
  • You can distinguish a material trend from heating and contact effects.

Apply it in an exam

Question 1: An R-against-length line rises from 2.4 Ω at 0.20 m to 6.4 Ω at 0.60 m. Find its gradient and discuss the intercept.

Show answer and reasoning

Gradient 10 Ω m⁻¹; a 0.4 Ω intercept needs an experimental explanation.

  • Gradient = (6.4 − 2.4)/(0.60 − 0.20) = 10 Ω m⁻¹.
  • Extrapolated intercept = 2.4 − 10 × 0.20 = 0.4 Ω.
  • Investigate contact/lead resistance or a length-zero error before assuming ideal proportionality.

Explore further

Return to this tomorrow

Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.

  • Where is the voltmeter connected?
  • What happens when a second positive-resistance branch is added in parallel?

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.

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