6.1 Planning and evaluating experiments (A2)
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A2 content for Unit 6 (WPH16): planning, implementation and evaluation skills. Uncertainties and log graphs are on 6.2, the A2 core practicals on 6.3, the Unit 3 basics on 3.1.
What this page covers
8 points
- What an A2 planning or evaluation question asks for
- Specifying apparatus: range, resolution and the relevant dimension (for example a pendulum length measured to the centre of the bob)
- When to use a data logger, and how to choose its sampling rate
- Calibration and zero checks, including background count and fixed-point calibration
- Techniques that improve a measurement: set squares, fiducial marks, data loggers, video
- Control variables, repeats, and the number and range of readings; how the data will be used
- Safety with radioactive sources, high voltages, hot liquids and charged capacitors
- Recording results, anomalous readings, and the words precision, accuracy, resolution and sensitivity
In one minute
6 points
- A good A2 plan specifies every instrument with its range and resolution and says exactly which dimension is measured and where from, such as "to the centre of the bob" or "from the front of the source holder to the window of the tube".
- Use a data logger when the event is too fast (or too slow) for hand timing; choose a sampling rate that gives many readings within each time constant or cycle.
- Check zeros and calibrate against known values; subtract the background count from every count rate.
- Plan at least six values over the widest sensible range, spaced evenly in the quantity you will plot; repeat where readings scatter, count for longer where they are random.
- Safety must name the hazard of this experiment: tongs and distance for sources, current-limited supplies for high voltages, stable mounting and care with hot liquids.
- Precise = little spread; accurate = close to the true value; sensitive = large output change per unit input change; resolution = smallest change shown.
Contents
14 sections
- What does an A2 planning question ask you to do?
- How do you choose apparatus, and when do you need a data logger?
- What is a "relevant dimension", and which techniques make it reliable?
- How do you calibrate and check instruments at A2?
- How do you plan for safety in A2 experiments?
- How many readings, over what range, and how will the data be used?
- How should results be recorded, and what about anomalies?
- What do precision, accuracy and sensitivity mean, and how do you improve an experiment?
- Exam tips for 6.1
- Common misconceptions
- How this topic connects
- Key equations
- Check your understanding
- Specification checklist
What does an A2 planning question ask you to do?
Unit 6 asks the same planning questions as Unit 3, but with experiments from Units 4 and 5 and sharper detail. The paper is written, but every question is about a real experiment, often one you have not done: write a plan, criticise one, process readings or evaluate a result.
| Part of the plan | What A2 detail looks like |
|---|---|
| Apparatus | instrument, range and resolution, and the exact dimension measured: "metre rule (1 mm) from the bottom of the clamp blocks to the top of the bob, plus half the bob diameter from a vernier caliper (0.1 mm)" |
| Calibration and zero | background count measured and subtracted; balance tared; Hall probe zeroed away from magnets; thermometer checked at fixed points |
| Technique | set squares, fiducial marks, many cycles timed, data-logger sampling rate, eye level with the scale |
| Variables | independent, dependent, and each control variable with how it is kept constant |
| Readings | how many values, over what range, how spaced; repeats or longer counts |
| Safety | the specific hazard (radioactive source, high voltage, hot liquid) and the precaution that deals with it |
| Use of the data | what is plotted against what, the expected shape, and what the gradient or intercept gives |
Plans gain credit for detail that could only belong to this experiment. Before writing, ask: what is hard to measure here, what could change by accident, and what could hurt someone? Answer those three and most of the plan writes itself.
How do you choose apparatus, and when do you need a data logger?
Choose each instrument so its range covers all the readings and its resolution is small compared with the smallest reading.
| Instrument | Typical resolution / setting | Use in A2 experiments |
|---|---|---|
| data logger with voltage, current, temperature, light-gate or pressure sensor | sampling rate chosen by you, e.g. 50 or 1000 samples per second | fast or slow changes: capacitor discharge, cooling, collisions, induced e.m.f. |
| oscilloscope | time-base (e.g. 1 ms per division) and Y-gain (e.g. 0.5 V per division) | rapidly changing p.d.s; periods of short cycles |
| Geiger–Müller tube and counter (or ratemeter) | counts in a set time | count rates from radioactive sources; background count |
| Hall probe and meter | e.g. 0.01 mT | magnetic flux density near magnets and coils |
| thermometer or temperature probe | 0.5 °C (liquid in glass) or 0.1 °C (digital) | gas laws, specific heat capacity, cooling |
| top-pan balance | 0.01 g | masses; small forces such as the force on a current-carrying wire |
| travelling microscope | 0.01 mm | very small distances that cannot be touched, e.g. a capillary bead or ring spacing |
- Use a data logger when the change is over in a second or two, when many readings are needed at precise times, when two quantities must be read at the same instant, or when a slow change needs readings over a long time.
- Sampling rate: choose it so there are many readings, at least 10 to 20, within each time constant, period or event. Too slow a rate misses the peak of a short pulse.
- A data logger removes reaction time, but its sensor still needs a suitable range and resolution, and zeroing or calibration.
A student plans to measure how the p.d. across a 220 µF capacitor, charged to 6.00 V, changes as it discharges through a 2.2 kΩ resistor. She intends to read a voltmeter every few seconds while timing with a stopwatch. (a) Calculate the time constant. (b) Explain why her method is unsuitable. (c) Suggest suitable apparatus and settings.
(a) \(RC=2.2\times10^3\times220\times10^{-6}=\) 0.484 s
(b) A reaction-time error of about 0.2 s is \(0.2\div0.484\times100=41\%\) of the time constant, and the discharge is nearly complete in about \(5RC=2.42\ \text{s}\), so there is no time to read the meter more than once or twice.
(c) A data logger with a voltage sensor across the capacitor, sampling at 50 readings per second (every 0.020 s): about 24 readings in each time constant and about 121 over 5RC.
Alternatively, use a 100 kΩ resistor: \(RC=22\ \text{s}\) and the 0.2 s error is under 1%. Changing the apparatus to suit the instrument is a good planning answer too.
What is a "relevant dimension", and which techniques make it reliable?
Many equations contain a length measured between particular points; saying which points is part of specifying the apparatus.
| Quantity in the equation | What you actually measure |
|---|---|
| length \(l\) of a simple pendulum | from the point of suspension to the centre of mass of the bob: string length + half the bob diameter |
| radius \(r\) of a whirled object | from the axis of rotation to the centre of the object |
| length of a solenoid (turns per metre) | the length of the winding, not of the tube it is wound on |
| distance from a radioactive source to a detector | a measured distance plus an unknown offset (the source sits inside its holder), handled with a graph, as in Worked example 4 |
| separation of capacitor plates | the gap between the inner faces, set with spacers of known thickness and checked at several points |
Techniques and extra apparatus worth naming
- Set square: stand it on the bench against a rule to make the rule vertical, or line up the bottom of a bob or pointer with the scale.
- Fiducial mark: a pin, or a line drawn with a marker pen, at the centre of an oscillation, where the object moves fastest and its passing is sharpest. Count passes in the same direction and start the stopwatch on "zero".
- Many cycles: time 10 or 20 oscillations so the reaction-time error is a small fraction of the total.
- Small amplitudes for pendulums and springs (under about 10°) so the motion is simple harmonic and the period does not depend on amplitude.
- Data logger, light gates or video against a scale remove reaction time.
- Clamp blocks at the top of a pendulum give a fixed, sharply defined point of suspension.
A student measures the string of a pendulum from the bottom of the clamp blocks to the top of the bob as 0.842 m, and the bob diameter with a vernier caliper as 24.6 mm. Three timings of 20 oscillations give 37.0 s, 37.2 s and 37.1 s. (a) Determine \(g\). (b) A second student uses 0.842 m as the length. Compare the error this causes with the uncertainty from the timing.
(a) Relevant length: \(l=0.842+0.0123=0.8543\ \text{m}\). Mean of 20 oscillations = 37.1 s, so \(T=37.1\div20=1.855\ \text{s}\). Rearrange \(T=2\pi\sqrt{l/g}\): \(g=4\pi^2l/T^2=4\pi^2\times0.8543\div1.855^2=\) 9.80 m s⁻²
(b) Leaving out the radius makes \(l\) \(0.0123\div0.8543\times100=1.4\%\) too small, giving \(g=9.66\ \text{m s}^{-2}\): every reading would be low by the same proportion, a systematic error. The timing half-range, 0.1 s on 37.1 s, is 0.27% in \(T\), so about 0.5% in \(g\) (\(T\) is squared). The missing radius is bigger than the timing uncertainty. (Combining percentage uncertainties for powers and products is on 6.2.)
How do you calibrate and check instruments at A2?
- Zero checks: tare a balance with the apparatus in place; zero a Hall probe away from magnets and currents; check a sensor reads zero with no input; close the jaws of a caliper.
- Background count: with the source locked away, count for a long time (several minutes) and work out the background count rate. Subtract it from every count rate before analysing.
- Calibration compares an instrument with known values. Two fixed points give a straight calibration line: melting ice (0 °C) and steam above boiling water (100 °C at standard pressure) for a thermometer; a known field inside a long solenoid for a Hall probe; known masses for a force sensor.
- A zero error moves every reading by the same amount and cancels when you take a difference. A scale (gradient) error changes readings in proportion and does not cancel in a difference; it must be corrected using the calibration line.
A thermometer reads 2.0 °C in melting ice and 96.0 °C in steam at standard pressure. During an experiment it reads 20.0 °C at the start and 60.0 °C at the end. (a) Determine the true final temperature. (b) Determine the true temperature rise and the percentage error if the readings were used uncorrected.
(a) Gradient of the calibration line: \((96.0-2.0)\div(100-0)=0.94\) (reading per °C). Invert: true = (reading − 2.0) ÷ 0.94, so \((60.0-2.0)\div0.94=\) 61.7 °C
(b) True start: \((20.0-2.0)\div0.94=19.1\ ^\circ\text{C}\), so the rise is \(58.0\div0.94-18.0\div0.94=\) 42.6 °C. The uncorrected rise is 40.0 °C: \((42.6-40.0)\div42.6\times100=\) 6% too small. The offset of 2.0 cancels in the difference, but the scale error does not.
How do you plan for safety in A2 experiments?
A safety answer names the hazard in this experiment and the precaution that deals with it.
| Hazard | Where it appears | Precautions |
|---|---|---|
| ionising radiation from a sealed source | count rates, inverse-square law, absorption | handle the source only with long tongs, holding it at arm's length and pointing it away from people; keep the time out of its lead-lined box short; work at the largest practical distance; return it to the box and store it securely straight after use |
| high voltage | electron beam tubes, deflection plates (kV supplies) | use a current-limited supply; make and change connections only with the supply switched off; use fully shrouded leads; never touch terminals while it is on |
| glass vacuum tube | electron beam and deflection tubes | handle gently, do not knock; the tube could implode |
| hot liquids and surfaces | water baths for gas laws, specific heat capacity, cooling curves, thermistor calibration | stand the beaker on a heat-proof mat in a stable clamp; do not carry beakers of hot water; let water cool before emptying; keep electrical leads away from the water |
| stored charge | large capacitors at more than a few volts | discharge through a resistor before handling; observe polarity of electrolytic capacitors |
| moving or whirling masses | circular motion, oscillating trolleys, swinging pendulums | safety goggles; clear space; secure the load to the string; use small masses |
Writing "wear gloves" or "wear a lab coat" for a gamma source. Neither stops gamma rays. The precautions that work are distance (tongs), short exposure time and shielding (the lead-lined box).
How many readings, over what range, and how will the data be used?
- Number: at least six (better seven or eight) values, so the shape is clear and an odd point stands out.
- Range: as wide as the apparatus and safety allow. For a decay or discharge, cover at least two or three time constants or half-lives.
- Spacing: aim for points spread evenly along the axis you will plot. If you plot \(1/d^2\), readings evenly spaced in \(d\) crowd together at the small-\(1/d^2\) end of the axis, so take more readings at small \(d\), where the quantity changes fastest.
- Repeats: repeat where readings scatter (hand timing, rebound heights); for random processes such as radioactive decay, count for longer so each count contains several hundred events.
- Control variables: name each one, say why it matters and how it is fixed: "same source and same GM tube, at the same voltage setting, throughout".
- Use of the data: rearrange into a straight-line form; say what is plotted, the expected shape, and what the gradient and intercept give.
| Relationship (A2 context) | Plot | Result from |
|---|---|---|
| \(T=2\pi\sqrt{l/g}\) | \(T^2\) against \(l\) | gradient = \(4\pi^2/g\) |
| \(F=BIl\) (wire in a field, on a balance) | reading change \(\Delta m\) against \(I\) | gradient = \(Bl/g\) |
| \(p\propto(\theta+273)\) at constant volume | \(p\) against \(\theta\)/°C | intercept on the \(\theta\) axis ≈ −273 °C |
| \(C=k/(d+d_0)^2\) (gamma count rate) | \(C^{-1/2}\) against \(d\) | gradient = \(1/\sqrt k\); intercept on the \(d\) axis = \(-d_0\) |
| \(V=V_0e^{-t/RC}\) | \(\ln V\) against \(t\) (see 6.2) | gradient = \(-1/RC\) |
A sealed gamma source sits a few millimetres inside its holder, so the true source-to-detector distance is \(d+d_0\), where \(d\) is the distance measured from the front of the holder to the window of a GM tube and \(d_0\) is unknown. Theory predicts a corrected count rate \(C=k/(d+d_0)^2\). (a) Plan an experiment to test this relationship and find \(d_0\). (b) Use the graph below to determine \(d_0\).
(a) Background: with the source in its box, count for 300 s: here 108 counts → 0.36 s⁻¹, subtracted from every count rate.
Measurements and technique: clamp the source holder and the tube on a line along a bench rule (1 mm); vary \(d\) from 0.050 m to 0.200 m in seven steps, using a set square to line up the front of the holder and the tube window with the rule; count for 100 s at each distance so every count is several hundred or more.
Controls and safety: same source, tube and counter voltage; nothing moved between source and tube. Source handled with tongs, pointed away from people, returned to its lead-lined box as soon as the readings are complete.
Use of the data: rearrange to \(C^{-1/2}=(d+d_0)/\sqrt k\). Plot \(C^{-1/2}\) against \(d\): a straight line confirms the law; it crosses the \(d\) axis at \(-d_0\), and \(d_0\) = y-intercept ÷ gradient.
(b) At \(d=0\): 0.0275 s^½; at \(d=0.200\ \text{m}\): 0.483 s^½. Gradient \(=(0.483-0.0275)\div0.200=2.28\ \text{s}^{1/2}\,\text{m}^{-1}\). \(d_0=0.0275\div2.28=\) 0.012 m (about 12 mm)
The graph turns a systematic error you cannot measure directly into an intercept you can read. That is why a plan should say how the data will be used, not just which readings to take.
How should results be recorded, and what about anomalies?
- Every column heading is quantity / unit, for example d / m or C / s⁻¹. Units go in the heading, not after each value.
- Raw readings are recorded to the resolution of the instrument and to the same number of decimal places down a column: 0.050, 0.075, 0.100 m, never 0.05, 0.075, 0.1.
- A calculated column has the same number of significant figures as the least precise raw data used, or one more.
- The unit of a derived quantity is worked out, not guessed: the unit of \(T^2\) is s²; of a count rate, s⁻¹; of \(C^{-1/2}\), s^½.
- Put repeats and the mean in the same table, so an anomalous repeat can be seen.
A student records the first four rows of the experiment in Worked example 4 like this (background 0.36 s⁻¹):
| Distance | Counts in 100 s | Count rate / s | Corrected rate |
|---|---|---|---|
| 0.05 | 5036 | 50.36 | 50.0 |
| 0.075 | 2575 | 25.75 | 25.39 |
| 0.1 | 1568 | 15.68 | 15.32 |
| 0.125 | 1060 | 10.6 | 10.24 |
Identify the faults and write out a corrected table, adding a column for \(C^{-1/2}\).
Headings: "Distance" has no unit → d / m; "Count rate / s" has the wrong unit → / s⁻¹; "Corrected rate" has no unit → C / s⁻¹.
Decimal places: distances measured to 1 mm must all show three decimal places; 10.6 should be 10.60 and 50.0 should be 50.00, to match the rest of their columns.
| d / m | counts in 100 s | count rate / s⁻¹ | C / s⁻¹ | C^(−1/2) / s^½ |
|---|---|---|---|---|
| 0.050 | 5036 | 50.36 | 50.00 | 0.141 |
| 0.075 | 2575 | 25.75 | 25.39 | 0.198 |
| 0.100 | 1568 | 15.68 | 15.32 | 0.255 |
| 0.125 | 1060 | 10.60 | 10.24 | 0.313 |
| 0.150 | 768 | 7.68 | 7.32 | 0.370 |
| 0.175 | 586 | 5.86 | 5.50 | 0.426 |
| 0.200 | 464 | 4.64 | 4.28 | 0.483 |
Anomalous readings
An anomalous reading is one that does not fit the pattern of the others: a repeat far from the other repeats, or a point well off the best-fit line.
- Spot it while the experiment is running (plot as you go, compare repeats) so it can be measured again. If it cannot be repeated, leave it out of the mean and the best-fit line, but keep it in the table, marked, with the reason.
- Suggest a physical cause where you can: a balance not re-zeroed, a wire that had warmed up, a timing started late, a source moved.
- Do not call a point anomalous just because it is inconvenient. A curve in the data is a result, not an anomaly.
In the experiment in the figures, the length of wire in the field is 45.0 mm. The balance has a resolution of 0.01 g. (a) Identify the anomalous reading and describe what the student should do. (b) Use the graph to determine the magnetic flux density \(B\). (c) Explain why the smallest current gives the least reliable point.
(a) \(I=2.50\ \text{A}\), \(\Delta m=0.95\ \text{g}\): the line predicts about 0.80 g. Re-zero the balance and repeat the reading at 2.50 A; if the repeat lies on the line, use it. If not, exclude 0.95 g from the line and note it in the table.
(b) Gradient from a large triangle: \((1.28-0.16)\ \text{g}\div(4.00-0.50)\ \text{A}=0.32\ \text{g A}^{-1}=3.2\times10^{-4}\ \text{kg A}^{-1}\). Since \(\Delta m\,g=BIl\), gradient \(=Bl/g\), so \(B=3.2\times10^{-4}\times9.81\div0.0450=\) 0.070 T
(c) ±0.005 g on 0.16 g is 3.1%, but on 1.27 g only 0.39%: small readings carry the largest percentage uncertainty.
What do precision, accuracy and sensitivity mean, and how do you improve an experiment?
- A set of results can be precise but inaccurate (a systematic error moves them all the same way), or accurate on average but imprecise (large random scatter).
- Random errors limit precision; systematic errors limit accuracy. Repeating improves the mean of a scattered set, but it cannot fix a systematic error.
- "More accurate" does not mean "more decimal places", and many significant figures do not make a value "precise".
A temperature sensor gives an output p.d. that falls as the temperature rises: 3.92 V at 10 °C, 3.40 V at 20 °C, 1.78 V at 50 °C and 1.36 V at 60 °C. Its output is read with a voltmeter of resolution 0.01 V. (a) Calculate the sensitivity between 10 °C and 20 °C and between 50 °C and 60 °C. (b) Determine the smallest temperature change the system can detect in each range. (c) A student says the sensor is "more accurate at low temperatures". Comment.
(a) Sensitivity = change in output ÷ change in input: \((3.92-3.40)\div10=\) 0.052 V K⁻¹ (52 mV K⁻¹); \((1.78-1.36)\div10=\) 0.042 V K⁻¹ (42 mV K⁻¹).
(b) Smallest detectable change = resolution ÷ sensitivity: \(0.01\div0.052=\) 0.19 K; \(0.01\div0.042=\) 0.24 K.
(c) The sensor is more sensitive at low temperatures, so it can detect smaller changes there; whether its readings are accurate depends on how well it has been calibrated against known temperatures, not on its sensitivity.
Evaluating and improving
- Find the largest source of uncertainty by comparing percentage uncertainties (a squared quantity counts twice; see 6.2), and look for systematic errors: a non-zero intercept where theory says zero, results all on one side of the accepted value, a hidden offset such as \(d_0\).
- Suggest a specific improvement tied to that source and say what it improves; comment in context: does the result agree with the accepted value within its uncertainty, and do the assumptions hold (small amplitude, point source, constant temperature)?
| Problem | Improvement | What it improves |
|---|---|---|
| reaction time in hand timing | light gate or data logger; or time many cycles past a fiducial mark | precision of times |
| rule not vertical, pointer not level with scale | set square against the rule; eye level with the pointer | accuracy of lengths (removes a systematic error) |
| heat lost to the surroundings | insulate (lagging, lid); start below room temperature and finish the same amount above it | accuracy of thermal results |
| random counts from a source | count for longer; repeat counts and average | precision of count rates |
| very small readings | increase the range (bigger current, longer length) within safe limits; use an instrument with finer resolution | percentage uncertainty |
| wire or resistor warming up | small currents; switch off between readings | keeps a control variable (temperature) constant |
An improvement earns credit when it is specific and justified: "use a data logger sampling at 50 Hz, because the p.d. halves in about 0.3 s, far too quickly to read by hand". "Use better equipment" or "be more careful" earn nothing.
Exam tips for 6.1
"Metre rule" is a name. "Metre rule, resolution 1 mm, from the bottom of the clamp blocks to the top of the bob, plus half the diameter from a vernier" is a specification.
Compare the time scale of the change (a time constant, a period, a pulse length) with a reaction time of about 0.2 s, and state a sampling rate that gives many readings within that time.
Any plan with a radioactive source should measure the background count rate over several minutes and subtract it from every count rate before plotting.
End every plan with the graph: what goes on each axis, the expected shape, and which feature (gradient, intercept on either axis) gives the quantity asked for.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| A precise result must be accurate. | A systematic error can move every reading by the same amount while their spread stays tiny. | Precision is about spread; accuracy is about closeness to the true value. Check both. |
| More significant figures make a value more precise. | Digits beyond the resolution of the instrument carry no information. | Record to the resolution; precision comes from the spread of repeats. |
| Sensitivity and resolution are the same thing. | Sensitivity is output change per unit input; resolution is the smallest change the display can show. | Smallest detectable input change = resolution ÷ sensitivity. |
| A data logger makes any measurement accurate. | It removes reaction time, but its sensor can still have a zero error or poor calibration, and too low a sampling rate misses fast changes. | Zero and calibrate the sensor; choose a suitable sampling rate. |
| An anomalous reading should be rubbed out. | It may reveal a real problem, and removing it hides the evidence. | Repeat it if possible; otherwise keep it in the table, marked, and leave it out of the line and the mean. |
How this topic connects
6.2 Uncertainties and logarithmic graphs, 6.3 A2 core practicals
After the readings: combining uncertainties, error bars, worst-fit lines and log graphs. Every A2 core practical uses the planning skills on this page.
3.1 Planning at IAS
Verniers, micrometers, zero errors, parallax and the basic plan checklist.
Units 4 and 5
The physics behind the contexts: \(F=BIl\) and capacitor discharge (Unit 4); simple harmonic motion, gas laws, thermal energy and radioactivity (Unit 5).
Key equations
| Equation | Use | In the formulae list? |
|---|---|---|
| corrected count rate = count rate − background count rate | every radioactivity experiment | no — learn it |
| sensitivity = Δ(output) ÷ Δ(input) | comparing sensors and instruments | no — learn it |
| smallest detectable change = resolution ÷ sensitivity | a sensor read by a meter | no — derive it |
| percentage uncertainty = (uncertainty ÷ value) × 100% | finding the biggest source of uncertainty | no — learn it |
| \(T=2\pi\sqrt{l/g}\) | pendulum; \(l\) to the centre of the bob | yes |
| \(V=V_0e^{-t/RC}\) | judging the time scale of a discharge | yes |
| \(F=BIl\sin\theta\) | current balance | yes |
Check your understanding
1. A 470 µF capacitor discharges through 47 kΩ. Is a stopwatch suitable for timing the discharge?
\(RC=470\times10^{-6}\times47\times10^3=22.1\ \text{s}\). A 0.2 s reaction time is under 1% of this, so hand timing with readings every 5 or 10 s is reasonable.
2. A GM counter records 4.40 counts per second near a source; the background is 0.20 s⁻¹. What count rate is due to the source?
\(4.40-0.20=\) 4.20 s⁻¹. Always subtract the background before analysing.
3. A sensor's output changes by 25 mV for each kelvin. It is read with a voltmeter of resolution 0.01 V. What is the smallest temperature change it can detect?
\(0.01\ \text{V}\div0.025\ \text{V K}^{-1}=\) 0.40 K.
4. Five values of the speed of sound are 331, 332, 331, 330, 332 m s⁻¹; the accepted value at that temperature is 343 m s⁻¹. Describe the results.
Precise (spread only ±1 m s⁻¹) but not accurate (about 12 m s⁻¹ low): there is a systematic error, which repeating will not remove.
5. Why is a fiducial mark placed at the centre of an oscillation rather than at one end?
The object moves fastest at the centre, so the instant it passes is sharply defined; at the ends it is momentarily at rest and the turning point is hard to judge.
Specification checklist
P1–P11 are our labels for the Unit 6 practical skills in the specification.
- (P1) I can choose the most appropriate apparatus for each measurement and give its range, resolution and the exact dimension measured.
- (P2) I can describe zero checks and calibration, including measuring and subtracting a background count rate.
- (P3) I can describe measuring techniques for A2 experiments, including when and how to use a data logger.
- (P4) I can identify the control variables and say how each is kept constant.
- (P5) I can decide the number, range and spacing of readings and whether repeats (or longer counts) are needed.
- (P6) I can identify hazards, including radioactive sources, high voltages and hot liquids, and give precautions that deal with them.
- (P7) I can explain how the data will be used: the graph, its expected shape and what the gradient or intercept gives.
- (P8) I can record results in a table with quantity / unit headings and consistent significant figures, and correct faulty headings and units.
- (P9) I can identify anomalous readings and explain how to deal with them.
- (P10) I can suggest improvements to an experiment, including additional apparatus such as set squares, fiducial marks and data loggers.
- (P11) I can use the terms precision, accuracy, resolution and sensitivity correctly.
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