AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Required practical 5: Determination of resistivity of a wire using a…
Full specification wording
Resistivity: Required practical 5: Determination of resistivity of a wire using a micrometer, ammeter and voltmeter.
In this required practical you measure the resistivity of a metal wire.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In this required practical you measure the resistivity of a metal wire. Set up the wire in series with an ammeter and a cell, and connect a voltmeter in parallel across a measured length of the wire. For several lengths, record the current I and potential difference V, then calculate resistance using R = V/I. Measure the wire diameter at several positions and orientations with a micrometer, average the readings, and find the cross-sectional area using A = πr², remembering that r is half the diameter. Plot a graph of R against L; the gradient equals ρ/A, so resistivity is gradient × A. This method assumes constant temperature, so keep the current small and switch off between readings. Assessed skills include correct circuit assembly, accurate measurement, uncertainty handling and clear recording.
What earns the marks
- Connect the ammeter in series with the wire and the voltmeter in parallel across the measured length of wire.
- Measure the diameter with a micrometer at several positions and orientations, then average the readings to reduce random error.
- Calculate cross-sectional area using A = πr², converting the diameter from mm to m before halving it to find r.
- Plot a graph of resistance R against length L and use the gradient, since gradient = ρ/A, to determine ρ.
- Keep the current small and switch off between readings so that the wire's temperature, and therefore its resistivity, stays approximately constant.
Common mistakes and how to put them right
Connecting the voltmeter in series or the ammeter in parallel.
Put it right by placing the ammeter in the main series path and the voltmeter across the wire section.
Using the diameter as the radius in A = πr².
Put it right by halving the diameter first.
Leaving the circuit switched on throughout, which heats the wire and changes its resistance.
Put it right by using a low current and switching off between readings.
Exam tips
- Record raw micrometer readings with a sensible number of decimal places and show how you averaged them.
- State the equation you use before substituting values, and keep units consistent throughout.
- Comment on the main sources of uncertainty, such as wire diameter and temperature change, when the question asks for an evaluation.
Try the lesson
The start of the Resistivity: Required practical 5: Determination of resistivity of a wire using a micrometer, ammeter and voltmeter. lesson, made from checked teaching notes for this specification point.
Have a go first
In the determination of the resistivity of a wire, how should the diameter of the wire be measured to ensure an accurate result?
Listen
In this required practical you measure the resistivity of a metal wire. Set up the wire in series with an ammeter and a cell, and connect a voltmeter in parallel across a measured length of the wire. For several lengths, record the current I and potential difference V, then calculate resistance using R = V/I. Measure the wire diameter at several positions and orientations with a micrometer, average the readings, and find the cross-sectional area using A = πr², remembering that r is half the diameter. Plot a graph of R against L; the gradient equals ρ/A, so resistivity is gradient × A. This method assumes constant temperature, so keep the current small and switch off between readings. Assessed skills include correct circuit assembly, accurate measurement, uncertainty handling and clear recording. A strong answer earns marks for these points. Connect the ammeter in series with the wire and the voltmeter in parallel across the measured length of wire. Measure the diameter with a micrometer at several positions and orientations, then average the readings to reduce random error. Calculate cross-sectional area using A = πr², converting the diameter from mm to m before halving it to find r. Plot a graph of resistance R against length L and use the gradient, since gradient = ρ/A, to determine ρ. A common slip: Connecting the voltmeter in series or the ammeter in parallel. Instead, placing the ammeter in the main series path and the voltmeter across the wire section. Exam tip: Record raw micrometer readings with a sensible number of decimal places and show how you averaged them.
Key words
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Circuit construction and meter placement
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Measurement uncertainty and control of variables
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Graphical analysis using gradients
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Experimental determination of resistivity
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See every definition
- Circuit construction and meter placement
- The ammeter is connected in series with the wire and the voltmeter in parallel across the measured length of wire.
- Measurement uncertainty and control of variables
- The diameter is measured at several positions and orientations and averaged to reduce random error, and the current is kept small and switched off between readings so temperature stays approximately constant.
- Graphical analysis using gradients
- A graph of resistance R against length L is plotted, and since the gradient equals ρ/A, resistivity is found from gradient × A.
- Experimental determination of resistivity
- The resistivity of a metal wire is measured using a micrometer, ammeter and voltmeter, with resistance calculated using R = V/I and area using A = πr².
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