Required Practicals, Part 1: Planning

This is the first instalment in a short series looking at the qualities that characterise a high-quality report for an A-level Physics required practical. First up is planning, which has to be conducted in advance to ensure that the most appropriate apparatus and methods are used.

Proper planning documentation must identify the variables and the relationship that is being investigated. If the purpose of the practical is to determine a value, such as gravitational acceleration or the Young modulus of a wire, then an appropriate relationship needs to be stated and can be assumed to be true. In both cases (investigation or determination) you must outline the intended method of data collection and estimate the magnitude of measurements that will be taken so the appropriate apparatus can be specified.

All work must be conducted with proper regard to any significant hazards, always keeping in mind the need to collect results that are both precise and accurate.

There is a lot of information given above so here are the key requirements in bullet-point form;

• Identify the variables that will be measured
• State the expected (or assumed) relationship at the heart of the practical
• Summarise the method, including how controlled variables will be managed
• Estimate the magnitude of the measurements
• Specify appropriate apparatus
• Outline any necessary safety precautions

For example, when determining the resistivity of a wire, the measured variables are potential difference and current as these reflect the resistance of the sample. The length of wire and its cross-sectional area must not change, as these values will be used when calculating the resistivity.

Research will reveal a reference value for the resistivity of the wire being tested and this will enable a suitable supply voltage to be chosen so that the potential difference and current have easily-measured values.

It is likely that the wire will get hot so a switch and current-limiting resistor should be used to minimise heating of the wire, which could change the resistivity, therefore distorting the results. Similarly, hot wires can cause burns so the wire should not be touched while the experiment is in progress.

Some aspects of the planning can be particularly tricky, especially discussion of control variables. It is wrong to say “always use the same voltmeter” because all voltmeters (and balances, and micrometers, etc) are assumed to be correctly calibrated and will be checked, by you, before starting the experiment. But you should specify the sensitivity (minimum reading) and range (maximum reading) of the voltmeter – and every other measuring instrument that will be used.

Similarly, it would be wrong to say “keep the mass the same” if what you actually mean is “do not vary the magnitude of the mass”. The mass itself can be changed provided that it is swapped for another mass of the same value (ie, any 1 kg mass will do).

It is also wrong, when determining the EMF and internal resistance of a cell, to list the cell as a control variable. Why? Because every cell with the same specification from the same manufacturer should be identical (within manufacturing tolerances).

It would, however, be correct to say that the condition of the cell must not change: this can be ensured by checking the open-circuit voltage before starting the experiment. The cell condition can then be monitored during the experiment by re-checking the open-circuit voltage periodically, perhaps even between each measurement.

In the same experiment, it is also important to switch-off the circuit immediately after taking a measurement. Unlike in the resistivity experiment, this is not to avoid burns but rather to avoid exhausting the cell, leading to changes in the EMF and internal resistance, which are the very characteristics the experiment is designed to determine.

Some control variables are obvious but easily overlooked. A common example is in the investigation of Charles’ law, where a capillary tube is placed into a water bath and its volume is measured at different temperatures via the height of a liquid bead inside the tube. If the tube is not fully immersed in the water (to the height of the bead) then only part of the column of air will be at the target temperature, resulting in a measured height that is less than it ought to be.

Every control variable should be named and accompanied by a management strategy that includes a description of what the strategy is intended to achieve or what adverse consequence would occur if it were not employed. This can be done in table form.

In the Charles’ law experiment mentioned above, the entries would be; immersion of the capillary tube (control variable) must always be to the height of the bead (strategy) to ensure that the full air volume reaches the required temperature (description).

No credit will be given for stating, as part of the conclusions, adjustments that might have improved the quality of the data retrospectively if these things could reasonably have been anticipated with appropriate thought at the planning stage.

Safety considerations are often best documented in tabular form, with three columns headed; Hazard, Potential Harm and Risk Management. For example, ionising radiation (hazard) can cause biological cell damage (potential harm) so follow the ALARA principles (risk management). You would, of course be expected to detail appropriate steps to achieve ALARA: it is not enough simply to state the acronym.

If a practical involves no significant risks then make that clear; don’t try to invent risks, such as electric shocks when handling 1.5 V cells. Similarly, careless methods are not experimental risks; they are signs of incompetent work. For example, there is no need to say “keep water away from electrical components” if there is absolutely no need for water as part of the experiment.