This practice outline estimates a cell's emf and an effective internal resistance over a stated discharge range. It supports development of a locally approved method and electrical risk assessment; it is not a complete validated procedure. Check current Pearson documents for course-specific requirements.
Aim and scientific principle
Vary an approved external load and measure current and terminal potential difference. Interpret a straight-line model only over conditions where emf and effective internal resistance are approximately constant. Emf is energy supplied per unit charge, measured in volts, not a mechanical force.
Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.
Apparatus and setup
Use cell, variable resistor or resistance box, switch, ammeter, voltmeter and connecting leads. Consider measuring range, resolution, uncertainty, calibration and suitability for the sample, not the instrument name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.
Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.
Method
Begin with the load circuit open and an approved current-limiting load setting. Connect the voltmeter directly across the cell terminals, avoiding inclusion of external lead/contact drops in the voltage measurement. Record the ammeter position and whether it measures total cell current or only load current.
Vary the external load within cell, resistor and meter ratings. Record paired current and terminal voltage using a stated, consistent delay after each connection. Allow an appropriate instrument response while limiting discharge and heating; arbitrarily quick readings can capture a different transient response.
Open the load circuit between readings, record recovery time and repeat reference conditions to check drift. A high-input-resistance voltmeter still draws some current; an open load switch does not establish zero total cell current. Use total cell current in the model, or justify neglecting the voltmeter current.
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Plot terminal voltage vertically against discharge current horizontally. If the constant-parameter model is supported over the sampled range, estimate emf from the zero-current intercept and internal resistance from the negative of the gradient. Inspect residuals and repeated reference points before interpreting the fit.
Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.
Variables and controls
Independent variable: deliberately selected external load resistance; current is the measured horizontal-axis quantity used in the terminal-voltage model.
Important controls: cell, temperature, reading delay, meter ranges and connection resistance.
For each control, state how it is maintained or monitored. Simply writing "keep temperature constant" is incomplete if the method has no bath, probe, equilibration time or other means of doing so. Choose a range wide enough to reveal the relationship but within safe and scientifically valid limits. Choose the number, range and spacing of values for the expected response and analysis. Concentrating values near an uncertain threshold or optimum can be more useful than uniform spacing; no fixed count establishes an adequate design.
What is distinctive about this practical
For positive current defined as discharge current leaving the positive cell terminal, the model is V=E−Ir. If emf and r are constant, a terminal-voltage versus current graph has intercept emf and gradient −r. These signs and slope units depend on the stated axes and current convention. A current versus voltage graph instead has gradient −1/r when r is non-zero; it does not directly give r.
The fitted r is an effective parameter for the sampled current, temperature, state of charge and reading-time conditions, not necessarily a universal constant of the cell. Polarisation, discharge, heating and instrument loading can undermine the simple fit. A single unloaded voltage reading cannot determine both parameters. A high-resistance voltmeter approximates emf only when its current produces a negligible internal drop and the cell state is appropriate; opening the load switch alone does not guarantee this.
Recording and processing results
Record load setting, paired current and voltage, meter connections/ranges, reading delay, recovery time and repeated reference readings. Fit terminal voltage against total cell current, or state why neglected voltmeter current is insignificant. Quote intercept in volts and slope in volts per ampere, equivalent to ohms. Inspect curvature and time-dependent drift; do not force a straight line or conceal a changing cell state. Compare a high-resistance-meter unloaded reading with the fitted intercept within the measurement and model uncertainties, not as an exact independent emf standard. Put units in column headings, record raw readings consistently with instrument resolution and the method and distinguish repeats from calculated means. Do not delete an anomaly merely because it is inconvenient. Recheck the condition, look for a procedural explanation and state any exclusion rule.
Choose a fitted line, curve or ruled joins according to the data, scientific relationship and question; do not connect or smooth points mechanically. A gradient should use a large triangle and values read from the fitted line. When a relationship is linearised, explain the transformation and connect gradient or intercept to the requested physical quantity. When observations are categorical, define the category rule before seeing the result.
Safety, ethics and responsible practice
Avoid short circuits, use suitable resistors and stop if the cell or components heat. A useful safety statement names the hazard, identifies who or what could be harmed and gives a control that reduces likelihood or severity. "Take care" does not specify an action that reduces the identified risk.
Worked application: from raw evidence to a defensible conclusion
This is a constructed constant-model check, not measured data. For emf 1.50 V and effective internal resistance 0.40 ohm, a discharge current of 0.50 A gives an internal drop of 0.20 V and terminal voltage 1.30 V. On a terminal-voltage versus current graph the gradient is -0.40 V per ampere, so the inferred internal resistance is positive 0.40 ohm, not a negative resistance.
Using a load-current reading while a non-negligible voltmeter current is omitted means the current in the model is wrong: Icell=Iload+Imeter. These constructed values do not establish actual cell stability, meter loading or uncertainty. In real data, retain measurement times and reference repeats, investigate drift and state the range over which the effective model is defensible.
This chain is stronger than a memorised recipe because every step answers the practical question. If the graph contradicts the prediction, report the evidence honestly and investigate method, range, controls and model assumptions before claiming a new scientific law.
Evaluation: limitation, effect and improvement
Limitation: cell emf and internal resistance drift as the cell discharges or warms.
Likely effect: systematic discharge during an ordered series can lower later terminal-potential readings and distort both the intercept used for emf and the gradient used for internal resistance.
Targeted improvement: use a justified low-current range and consistent reading/recovery delays, repeat reference loads and monitor temperature where practical. Varying load order may expose a trend with time but does not correct drift. A fresh cell is not proof of stable emf or internal resistance; document the observed stability and restrict or revise the model when it fails.
Separate random variation, systematic bias and model limitation. Repeats help reveal random variation but do not remove a zero error. Calibration establishes the relation between reference values and indications; appropriate adjustment or correction can address an identified offset. It does not make biased sampling representative. Finer resolution and greater repeatability are distinct properties, and finer resolution may improve an estimate only when the reading is a material source of uncertainty.
Common misconceptions and corrections
Repeats automatically make a result accurate. Repeats estimate random variation; they do not correct a biased method or calibration error.
A control variable is just a list item. Explain the apparatus or procedure that keeps it stable.
The line must pass through every point. Use the overall pattern and a best-fit relationship, then investigate justified anomalies.
Percentage error and percentage uncertainty are interchangeable. Error compares a measured value with a suitable reference. Uncertainty characterises dispersion of values attributed to the measurand; it may be expressed as a standard uncertainty or an interval with stated coverage, and includes more than resolution alone. State the reference, denominator and uncertainty convention used.
A successful endpoint proves the theory. The observation supports a conclusion only within the method, range and assumptions tested.
More sophisticated apparatus is always better. The improvement must address the dominant limitation and be practical in the stated context.
Assessment guidance
Pearson assesses practical understanding throughout the science papers and concentrates it in written Units 3 and 6. Planning answers need an operational independent variable, a measured dependent variable, a justified range, controls, repeats, safety and a stated analysis. Data answers need units, consistent precision, sensible significant figures and transparent calculations. Graph answers need labelled axes, a scale using the grid well and line treatment appropriate to the data and question. Evaluation answers should use a limitation-effect-improvement chain specific to the apparatus. Do not claim that a memorised core-practical recipe is the only acceptable method when the question supplies a different valid context.
Retrieval practice
Extend or adapt this practical to answer a related question. Where the design uses a continuous variable, justify its planned values. List apparatus with range and resolution where relevant, control methods, a raw record layout, the expected analysis and one hazard-control pair. Then identify one random uncertainty, one systematic bias and one model limitation. Explain which can be reduced by repeats, which needs calibration followed by appropriate correction or adjustment, or a redesign, and how your conclusion would change if the anomaly were confirmed.
