Investigate potential difference, current and resistance for a resistor and a filament lamp; also compare filament lamps in series and parallel. Both parts belong to this core practical.Original labelled apparatus schematic; not to scale. Follow the stated measurements and safety instructions.
Use a safe low-voltage DC supply, switch, leads, ammeter, voltmeter, variable resistor or adjustable supply, fixed resistor and suitable lamps. Never use mains sockets as the experimental supply.
Connect the ammeter in series with the component and the voltmeter in parallel across it. Check ranges and polarity before switching on; an ammeter directly across a supply creates a short circuit.
Switch off between readings where appropriate. Lamps and resistors can become hot; limit current to component ratings and allow cooling before handling.
Current–potential difference method
With the supply off, build the circuit and check connections. Increase potential difference in small steps using the supply or series variable resistor, recording current and voltage pairs after readings settle.
Take a suitable range without overheating the fixed resistor or exceeding lamp ratings. Repeat readings and include zero; if investigating both polarities, switch off before reversing the supply.
For each non-zero pair calculate resistance R = V/I, using volts and amperes. Do not divide by zero at the origin; do not interchange current and potential difference.
Plot current on the vertical axis and potential difference on the horizontal axis to make an I–V graph. At constant temperature, an ohmic resistor gives a straight line through the origin. Its gradient is 1/R, the reciprocal of resistance.
A filament lamp gives a curved graph because its filament heats up and its resistance rises. Calculate resistance at each point using V/I. Do not use the tangent’s gradient to find this resistance: on a curved I–V graph, the gradient is not simply 1/R.
Record whether temperature was controlled or allowed to change. Heating the fixed resistor can alter resistance, so use small currents or short measurement times with consistent settling conditions.
Series and parallel lamps
Use matched lamps and a fixed safe supply voltage. In series there is one path and the same current through each lamp; the component potential differences add to the supply potential difference.Series and parallel lamp connection plans. Connect a safe low-voltage supply across the indicated gap; measure branch and main currents separately.
In parallel, each branch lamp has the supply potential difference across it. Measure branch currents by placing the ammeter in that branch and total current in the main supply path; total current is the sum of branch currents.
Compare brightness only as a qualitative indicator of power, not a direct current measurement. Equal lamps share voltage approximately equally in series; in parallel each can receive the full set supply voltage.
Test the effect of opening one lamp branch with the supply switched off before rearranging. Other parallel branches can still operate; an open series circuit stops current everywhere in that path.
Measure supply voltage under load rather than assuming its setting is exact. Extra lamps can change the load and the real supply may not maintain perfectly constant voltage.
Precision and evaluation
Select meter ranges that give useful resolution without overloading them. Zero/check meters and record stable readings with units; convert milliamperes to amperes before calculating resistance.
Firm connections reduce intermittent contact and unwanted contact resistance. Keep the voltage probe across the intended component, not an extra length of wire or the entire circuit.
Use repeats to assess spread, but distinguish random fluctuations from consistent meter offsets. A stable wrong reading is not evidence of accuracy.
A comparison is fair only when component identity, supply voltage and thermal conditions are appropriate and stated. Explain a changed lamp brightness using voltage, current and power together.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as a voltmeter zero offset.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom