9 Experimental skills
- Syllabus
- 2024
- Section
- 9
- Level
- —
Problem-solving and application
- Syllabus
- 2024
- Topic
- —
- Level
- —
Turn practical measurements into the required result
A practical problem is solved by translating the measurements into the quantity required, then choosing an operation or physics relationship that represents that quantity. The apparatus story provides context; the requested quantity decides which data matter.
Use this sequence: identify the required quantity and unit → label the relevant measurements → write the relationship in words or symbols → convert units if necessary → calculate with meaningful intermediate working → check that the result has the requested unit and is physically possible.
Example: a container has mass 75 g and the container plus liquid has mass 149 g. The liquid alone has mass 149−75=74 g. A listed volume of 88 cm³ is not needed because the target is mass, not density.
Do not automatically use every number. A combined measurement is not the same as the part being asked for, and a numerical answer without the correct unit or a context check may describe the wrong physical quantity.
Connect a practical situation to the physics that explains it
Applying physics in a practical context means using a relevant principle or relationship to explain what the apparatus, variables and observations represent. The context may be unfamiliar, but the physics link should be explicit.
Build the link in four moves: identify the system and what is measured → select the physics idea that connects those quantities → state how a change or observation follows from that idea → use the given evidence to support the result or explanation.
For the liquid data, subtracting the empty container gives the liquid mass, 74 g. If the liquid's density is required, the relevant relationship is density = mass ÷ volume, so the measured mass and 88 cm³ volume now both matter. The same data can therefore serve different purposes depending on the physics quantity requested.
Naming a law, formula or piece of apparatus is not yet an application. The answer must connect the physics to the specific measurement or observation; details that do not affect that connection should not be forced into the reasoning.
Investigation planning and methods
- Syllabus
- 2024
- Topic
- —
- Level
- —
Plan an investigation from the physics relationship
A useful investigation plan turns a physics relationship into a repeatable comparison. It states what will be changed, what will be measured, what must stay constant and which technique will produce the required measurement.
Plan in this order: define the relationship to investigate → choose a sensible range and several values for the quantity changed → select apparatus that measures the response → identify conditions that could also affect the response and keep them constant → describe how each reading is taken → repeat readings and calculate a mean when repetition is appropriate.
Example: to investigate how magnetic field strength changes with distance from a coil, use a ruler to set several distances and a Hall probe to measure field strength at each position. Distance is changed, field strength is measured, and current plus coil geometry are kept constant. Repeating each field reading and finding a mean makes the comparison more dependable.
An apparatus list is not a complete plan. Each item must have a stated purpose, and the procedure must reveal how the selected technique tests the physics relationship. Changing several relevant conditions together would make the cause of any measured change unclear.
Choose a safe method that measures the intended quantity
An appropriate experimental method uses apparatus that measures the intended physical quantity and a technique that produces meaningful readings without exposing people to an avoidable hazard.
| Measurement task | Appropriate method | Essential technique |
|---|---|---|
| mass of an object or container | balance | zero the balance, then record mass in its stated unit |
| weight or another force | newton meter | check the zero and read the scale in newtons while the force acts along the meter |
| corrected radioactive count rate | GM tube with counter and timer | measure background count several times, calculate its mean, then subtract the mean background from the source reading |
Safety must follow the chain hazard → possible harm → control. Liquid nitrogen can cause harm through extreme cold and splashing, so use eye protection and suitable protective gloves and pour carefully to avoid splashes. The control must match the actual hazard; writing only “be careful” does not describe a safe technique.
Similar-looking instruments are not interchangeable: a balance measures mass, while a newton meter measures force. A method is skilful only when it says how the apparatus is used, not merely what it is called; a safety precaution is valid only when it reduces a stated risk in that method.
Observations, measurements and variables
- Syllabus
- 2024
- Topic
- —
- Level
- —
Measure, record and present only the precision supported
A measurement is appropriately precise when the instrument and reading method can genuinely support the digits recorded. A measurement needs a numerical value and unit; an observation should describe exactly what was detected without adding an unsupported explanation.
Use this sequence: choose an instrument with a suitable range and resolution → find the value of one smallest division → align the instrument correctly → read from the correct viewing position → record the value with its unit and only the supported precision.
Example: if 20 kPa to 30 kPa contains five equal spaces, each smallest division is 2 kPa. A pointer at the second division after 20 reads 24 kPa. For a vertical height, fix the metre rule with zero at the reference height, keep it vertical with a set square or plumb line, and read at eye level.
Record repeated results methodically in a table. Put quantity and unit in the heading, such as “power / W”, then enter only numbers in that column. Measurements from the same instrument should normally use consistent decimal places.
More digits do not automatically mean greater precision. Do not invent decimal places beyond the scale or display, attach units separately to every table entry, or read an analogue pointer from an angle that shifts its apparent position.
Identify variables by the job they do in the investigation
Variables are identified from the investigation aim and method, not from the type of physical quantity. The independent variable is deliberately changed, the dependent variable is measured as the response, and control variables are kept constant so other influences do not change the response.
| Variable role | Identification question | Gamma-penetration example |
|---|---|---|
| independent | What is deliberately changed? | number of lead sheets |
| dependent | What response is measured? | count measured by the detector |
| control | What else could affect the response and must stay constant? | source-detector distance and time used for each count |
A useful shortcut is to rewrite the aim as “the effect of X on Y”: X is usually the independent variable and Y the dependent variable. Then use the physics to find other factors that could also change Y; those become control variables. Keeping them constant makes the comparison fair because a change in Y can be linked to X.
A control variable is not the same as a control group, and it is not simply any quantity that happens not to change. It must be a relevant factor whose change could affect the dependent variable. If such a factor varies, the experiment cannot show which change caused the response.
Analysis, communication and evaluation
- Syllabus
- 2024
- Topic
- —
- Level
- —
Turn data into a conclusion the evidence can support
A sound conclusion is the last link in an evidence chain: identify the pattern, test it with data, compare the results, then state whether the evidence supports the proposed relationship. The conclusion must match both the direction and the strength of the evidence.
Use this sequence: inspect the full table or graph for trend and anomalies → state the proposed relationship precisely → select at least two well-separated data points → perform the relevant calculation for each point → compare the calculated values → give a qualified conclusion.
Example: test whether distance is proportional to time squared. At 5.0 s, distance/time² = 15/25 = 0.60 m/s². At 10.0 s, distance/time² = 60/100 = 0.60 m/s². The equal values support the proposed relationship over the measured range. Show the substitution, value and unit so the comparison is traceable.
A curve that falls as x rises shows a negative relationship, but does not by itself prove inverse proportionality. One point cannot test constancy, and an anomalous point should be identified and checked rather than silently deleted. Say “supports” rather than “proves” because experimental evidence is limited to the data collected.
Communicate findings so the evidence is easy to follow
Scientific communication lets another reader see what was measured, how it was processed and how it supports the conclusion. Use precise technical language, display calculations in a checkable order and choose a graph treatment that represents the data rather than forcing a preferred pattern.
| Evidence feature | Clear communication |
|---|---|
| axes | label each quantity and unit; use a sensible scale |
| plotted data | mark points accurately and identify any anomaly |
| best fit | use a straight line for a linear trend or a smooth curve for a curved trend, with points reasonably balanced around it |
| calculation | write the relationship, substitute values, give the result and unit |
| conclusion | name the trend and cite the comparison that supports it |
For continuous measurements, use a line graph. Decide between a straight line and smooth curve from the overall distribution of points, not by joining every point dot-to-dot. A justified anomalous point need not pull the best-fit line or curve away from the main pattern.
A graph without quantity-and-unit labels is ambiguous, and a calculation with only a final number cannot be checked. Technical language should increase precision, not hide the reasoning: every reported finding should still connect visibly to a plotted pattern, table value or calculation.
Assess reliability by checking consistency
Reliability is about consistency: would repeated measurements made under the same conditions give similar results? It is judged from repeat readings and their spread, not from whether a single value looks plausible or agrees with an expected answer.
For each condition, take repeated readings → compare their spread → repeat any suspicious result → identify an anomaly only when the repeats provide evidence → calculate a mean from the valid readings. State both the action and its purpose: repeats reveal variation, while the mean reduces the influence of random variation.
Suppose three readings at one setting are 4.8, 4.9 and 7.2. Do not discard 7.2 immediately. Repeat the measurement at that setting and check the method. If the new readings cluster near 4.8–4.9, identify 7.2 as anomalous, exclude it with that justification, and report the mean of the consistent readings.
Repeating once is not enough to establish a pattern of consistency, and averaging an unexamined anomaly can make the result less representative. Repeats and a mean improve reliability, but they do not remove a systematic offset that shifts every reading in the same direction.
Evaluate accuracy and validity as different questions
Accuracy asks how close a measurement is likely to be to the accepted or true value. Validity asks whether the method and evidence genuinely test the intended relationship. A useful evaluation names a specific limitation, explains its effect, proposes a targeted improvement and states why that improvement helps.
| Question | Possible limitation | Targeted improvement |
|---|---|---|
| accuracy | zero error, parallax or scale divisions too coarse | zero or calibrate the instrument, read square-on, or use finer resolution |
| validity | another relevant variable changes | control that variable so only the independent variable changes |
| validity of a claimed graph shape | too few readings or large gaps hide the shape | take more readings at smaller intervals, especially between existing points |
If the evidence cannot distinguish a straight line from a curve, “take more readings” is incomplete. Specify smaller intervals in the uncertain region: the extra points reveal whether the gradient stays constant or changes, so the conclusion about the relationship is better supported.
More repeats mainly test reliability; they do not automatically correct a zero error or make an uncontrolled comparison valid. Likewise, a wider range or smaller intervals can strengthen the validity of a relationship claim without making each individual instrument reading more accurate. Match every improvement to the limitation it addresses.