Launch practical
About 50 minutes
- Set up the bench with the 7 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 44 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Ammeter, 10 A range: it reads to 0.01 A. Record every digit it shows.
What it shows
V = I R
Resistance is how hard it is for a current to flow: resistance = potential difference ÷ current. Every centimetre of the same wire adds the same resistance, so a longer wire has more. In series, resistances add; in parallel, the total is less than the smallest.
The graph to expect. The wire’s own resistance is directly proportional to its length, so the points lie on a straight line. The measured line is not exactly proportional: it can cross the resistance axis a little above zero, from the clips’ contact resistance and the fixed clip not being exactly on 0 cm.
On the AQA 2027 Physics Equations Sheet. Rearrange to R = V ÷ I for each length of wire.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Low-voltage power pack (0–12 V DC) | Drives a current round the circuit. | — |
| Digital ammeter (multimeter on the 10 A range) | Measures the current through the wire, in series. | 0.01 A |
| Digital voltmeter (multimeter on the 20 V range) | Measures the potential difference across the wire, in parallel. | 0.01 V |
| Resistance wire taped to a metre rule | The wire under test; the rule measures its length. | 1 mm |
| Two crocodile clips | One fixed at 0 cm, one moved along the wire to change the length. | — |
| Five 4 mm leads | Connect the circuit. | — |
| Resistors A (6.8 Ω) and B (10 Ω) on holders | For the series and parallel part. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical differently. Your course setting picks one; you can change it before you start.
Why: The circuits, the bench and the exam questions follow your board’s version.
Put the fixed clip on the 0 cm mark
Look straight down at the fixed clip. Slide it until the middle of its jaws sits exactly on the 0 cm mark of the rule.
Why: The length you write down is read from the rule. If the clip is past the zero, every length of wire is longer than you record.
Connect the supply + to the ammeter (lead 1)
Start at the red + socket of the power pack. Plug the other end of this lead into the ammeter’s 10 A socket.
Why: Starting at the positive terminal and following the loop round makes the circuit easy to check against the diagram.
Connect the ammeter to the clip at 0 cm (lead 2)
From the ammeter’s COM socket to the crocodile clip at the zero end of the wire.
Why: The ammeter is in series with the wire, so the same current flows through both.
Connect the sliding clip to supply − (lead 3)
From the sliding crocodile clip back to the black − socket of the power pack. This lead is your switch.
Why: This completes the main loop. Unplugging it between readings switches the current off.
Connect voltmeter V to the clip at 0 cm (lead 4)
From the voltmeter’s V socket to the clip at 0 cm (the clip the ammeter is joined to).
Why: The voltmeter goes across the wire, in parallel, so it measures the potential difference across just the length between the clips.
Connect voltmeter COM to the sliding clip (lead 5)
From the voltmeter’s COM socket to the sliding clip.
Why: Now the voltmeter is connected across the length of wire between the two clips.
Set the power pack
Choose the potential difference for the power pack. It stays switched off until your first reading.
Do: 2 V, a low pd, so the wire hardly warms.
Why: A bigger pd gives bigger currents, which the ammeter measures more precisely, but a short length of thin wire then carries a large current and gets hot.
Take the readings
At each length of wire, 20, 30 … 100 cm: move the crocodile clip to the length, switch on, read the voltmeter and the ammeter as soon as they settle, then switch off (each length twice if you repeat). Work out resistance = potential difference ÷ current for each length and plot resistance against length. Then resistors A and B alone, in series and in parallel.
Repeats: AQA’s handbook takes one pair of readings at each length. Repeating each length (unclip, clip on again, read again) and using the mean resistance is good practice and is credited in mark schemes: it reduces the effect of random errors such as the clip’s contact and the meters’ last digit. It does not remove a systematic error such as the clip being past the zero.
Make safe and clear upSafety
Switch off the power pack and turn it down to 0 V. Let the wire cool before you touch it or the clips. Unplug the leads, unclip the crocodile clips and coil the leads. Turn both meters to OFF and return the kit.
While you take readings
Switch off between readings
Unplug lead 3 (your switch) as soon as you have read both meters, and plug it in again only for the next reading.
Do: Unplug lead 3 after each pair of readings, and plug it in only for the next.
Why: Current heats the wire. Keeping it on only while you read keeps the wire close to room temperature, so its temperature stays the same (a control variable) and it does not burn you.
Repeat each length
At each length, read both meters, unclip, clip on again at the same mark and read them again. Use the mean resistance.
Do: Read both meters twice at each length, clipping on afresh the second time.
Why: The clip’s contact and the meters’ last digit change a little every time. A mean of repeats reduces the effect of these random errors and shows up an anomaly.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| A large current in a short length of thin wire | The wire gets hot enough to burn skin (thin wire can glow red) | Use a low pd, switch off between readings and do not touch the wire while current flows |
| The two crocodile clips touching (a short circuit) | A very large current heats the leads and the power pack: burns | Keep the clips apart and switch off before moving a clip |
| A damaged mains lead on the power pack | Electric shock | Check the mains lead and plug before use and do not use damaged kit |
Variables
- Independent: you change
- Length of wire between the clips in cm
- Dependent: you measure
- Resistance of the wire in Ω
- Control: you keep the same
- Material of the wire. Use the same wire throughout.
- Diameter (thickness) of the wire. Use the same wire throughout.
- Temperature of the wire. Low pd, and switch off between readings.
Results from a specimen run
| Length in cm | Potential difference in V | Current in A | Repeat potential difference in V | Repeat current in A | Resistance in Ω |
|---|---|---|---|---|---|
| 20.0 | 1.18 | 1.31 | 1.19 | 1.30 | 0.91 |
| 30.0 | 1.39 | 1.01 | 1.37 | 1.04 | 1.35 |
| 40.0 | 1.48 | 1.50 | 0.85 | 1.74 | |
| 50.0 | 1.58 | 0.73 | 1.59 | 0.74 | 2.16 |
| 60.0 | 1.64 | 0.64 | 1.64 | 0.64 | 2.56 |
| 70.0 | 1.69 | 0.57 | 1.70 | 0.56 | 3.00 |
| 80.0 | 1.74 | 0.51 | 1.74 | 0.51 | 3.41 |
| 90.0 | 1.76 | 0.46 | 1.77 | 0.46 | 3.84 |
| 100.0 | 1.80 | 0.42 | 1.79 | 0.43 | 4.22 |
| Resistors | Potential difference in V | Current in A | Resistance in Ω |
|---|---|---|---|
| Resistor A | 1.91 | 0.26 | 7.35 |
| Resistor B | 1.96 | 0.19 | 10.32 |
| A and B in series | 2.01 | 0.11 | 18.27 |
| A and B in parallel | 1.79 | 0.43 | 4.16 |
Anomaly spotted: Current at 40 cm, retaken.
Processing the data
- Resistance of 20 cm of wire
- 0.908 Ω
- Shape of the graph
- A straight line: the resistance goes up by the same amount for every extra centimetre. It crosses the resistance axis a little above zero (the clips’ contact resistance), so the wire’s own resistance is directly proportional to its length.
- Gradient (resistance per cm)
- 0.0415 Ω/cm
- y-intercept
- 0.0877 Ω
- Resistance per metre
- 4.15 Ω/m
- Inside a longer wire
- Mechanism explained on the first choice
- A and B in series
- 18.3 Ω
- The rule for series
- R total = R A + R B
- A and B in parallel
- 4.16 Ω
- Resistors in parallel
- The total resistance of two resistors in parallel is less than the smaller of the two.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Constantan wire, 28 SWG (data book) | 4.15 Ω/m | 4.41 Ω/m |
| A and B in series (6.8 Ω + 10 Ω) | 18.3 Ω | 16.8 Ω |
| A and B in parallel (6.8 × 10 ÷ 16.8) | 4.16 Ω | 4.05 Ω |
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Leave it where it is
The clip stays where you saw it, past the zero, so every length of wire is longer than you record by the same amount (a zero error). Every resistance is a little too big: the line moves up but its gradient does not change.
The fix: Put the fixed clip on the 0 cm mark. The length you write down is read from the rule. If the clip is past the zero, every length of wire is longer than you record.
Method: 6 V, for bigger currents
Currents of up to 3–4 A at 20 cm: the wire gets hot enough to burn you, and a thin nichrome wire glows. Its resistance rises as it heats, so the short lengths read too high. The ammeter readings are more precise.
The fix: Set the power pack. A bigger pd gives bigger currents, which the ammeter measures more precisely, but a short length of thin wire then carries a large current and gets hot.
Method: Leave the current on from the first reading to the last
The wire stays hot all the time. A constantan wire’s resistance hardly changes, but a nichrome wire’s rises by a few per cent at the short lengths, so those points sit above the line. The hot wire is also a burn risk.
The fix: Switch off between readings. Current heats the wire. Keeping it on only while you read keeps the wire close to room temperature, so its temperature stays the same (a control variable) and it does not burn you.
Potential difference at 20 cm: Typed 1.31 V
The fix: That is the current from the ammeter. The potential difference is on the voltmeter, the meter connected across the two clips: read its display.
Current at 20 cm: Typed 1.18 A
The fix: That is the potential difference from the voltmeter. The current is on the ammeter, the meter in the loop with the wire: read its display.
Exam answer: Leaving out: potential difference = current × resistance, V = I R (or any correct rearrangement, in words or symbols)
The fix: Potential difference = current × resistance, V = I R. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
Exam answer: Leaving out: Hazard: a high current makes the wire overheat or get hot (which can burn the skin)
The fix: Name the hazard: a large current makes the wire hot, which can burn the skin.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1V = I R · Knowledge[1 mark]
Write down the equation that links current (I), potential difference (V) and resistance (R).
Worked answer and how the marks are given
A full-mark answer
potential difference = current × resistance
V = I RMark scheme
- potential difference = current × resistance, V = I R (or any correct rearrangement, in words or symbols)
Where marks are usually lost
- Missing: potential difference = current × resistance, V = I R (or any correct rearrangement, in words or symbols). Potential difference = current × resistance, V = I R. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
- Question 2Resistance from V and I · Applying knowledge[2 marks]
Your first readings at 50 cm were a potential difference of 1.58 V and a current of 0.73 A.
Calculate the resistance of 50 cm of the wire.
Resistance = Ω
Worked answer and how the marks are given
A full-mark answer
R = V ÷ I
R = 1.58 ÷ 0.73
R = 2.16 ΩMark scheme
- R = 1.58 ÷ 0.73 (method)
- R = 2.16 Ω
- Allow answers from 2.143 to 2.186 Ω (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as ohm, ohms, Ω, Ω.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.462 Ω (no marks). Upside down: resistance = potential difference ÷ current = 1.58 ÷ 0.73 = 2.16 Ω.
- Answer 1.15 Ω (no marks). V × I is the power, not the resistance. Resistance = V ÷ I = 1.58 ÷ 0.73 = 2.16 Ω.
- Missing: R = 1.58 ÷ 0.73. Rearrange V = I R: R = V ÷ I. Put the potential difference on top.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to investigate how the resistance of a wire depends on its length. The wire is taped to a metre ruler.
Describe a method the student could use. Your answer should include a hazard and how to reduce its risk.
Worked answer and how the marks are given
A full-mark answer
1. Connect the wire in series with an ammeter and a power supply, with a voltmeter in parallel across the wire.
2. Fix one crocodile clip at the 0 cm end of the wire, which is taped to a metre ruler.
3. Put the other crocodile clip at 20 cm and measure the length between the clips with the ruler.
4. Switch on and record the current from the ammeter and the potential difference from the voltmeter.
5. Calculate the resistance using R = V ÷ I.
6. Move the clip to change the length in 10 cm steps up to 100 cm and repeat.
7. Repeat the readings for each length and calculate a mean resistance.
8. Plot a graph of resistance against length.
Hazard: a high current can make the wire overheat and cause burns, so use a low current and switch off between readings.Mark scheme
Indicative content:
- Ammeter in series with the wire; voltmeter in parallel (across the wire)
- Read the length of wire between the two clips on the metre ruler
- Length varied by moving a crocodile clip
- Current measured with the ammeter
- Potential difference measured with the voltmeter
- Calculate the resistance for each length using V = I R (R = V ÷ I)
- Take each length more than once and average the readings (or the resistances)
- Plot a graph of resistance against length
- Hazard: high current makes the wire overheat (melt) and could cause burns; use low currents (switch off between readings)
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| The fixed clip not exactly on 0 cm, and the resistance where the clips touch the wire | Systematic | Adds the same extra resistance to every reading, so the line misses the origin (a positive intercept) but its gradient is not changed. | Line the fixed clip up with the 0 cm mark, looking straight down, and use the gradient of the graph rather than a single reading. |
| The wire heating up while current flows | Systematic | A nichrome wire’s resistance rises when it is hot, so the short lengths (biggest currents) read too high and the line bends up at the start. Constantan hardly changes. | Use a low pd and switch off between readings so the wire stays close to room temperature. |
| The ammeter’s resolution (0.01 A) when the current is small | Random | At the long lengths on 2 V the current is only 0.2–0.4 A, so the last digit is a 2–5 % uncertainty and those resistances scatter. | Use an ammeter with a better resolution (for example a 2 A range reading to 0.001 A). |
| The sliding clip’s contact and exactly where it bites the wire | Random | Each placement adds a slightly different contact resistance and length, so the points scatter a little either side of the line. | Repeat each length, clipping on afresh each time, and use the mean resistance. |
Repeatable. When you clip on again at the same length with the same wire and meters, you get almost the same resistance.
Reproducible. Another group, with a different power pack and meters, gets almost the same resistance per metre for the same wire.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 3 | This page | — |
| Edexcel | 10.17 | Similar, with differences | Part b is series and parallel circuits. This board does not ask for resistance of a wire against its length. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.