Paper 5 - Controls, safety and electronic measurements
Learn to specify meaningful controls, calibrate instruments and match precautions to actual hazards. Use oscilloscopes, light gates and data loggers to measure changing electrical and mechanical quantities, with clear limits on what each instrument records.
Making a control physically effective
If a temperature sensor gives perfectly repeatable readings, must those readings be correct? A constant offset can survive every repeat. Reliable planning needs both control of the physical conditions and checks on what the instrument reports.
A control is a quantity held constant to isolate the intended dependence. State how it remains constant and, where needed, how you will check it. In a wire-resistance experiment, using the same wire helps keep material and diameter constant, but it does not keep temperature constant if the current heats the wire. Use a small current, brief readings and cooling intervals; check a reference length again to detect drift. In an oscillation experiment, use the same spring and a fixed small release displacement, and check that the spring returns to its original length.
For cooling of an insulated object, monitor room temperature with a separate thermometer, avoid draughts and use the same room away from changing heat sources. Fix the insulation thickness and sensor contact. A probe must touch the object's surface beneath the insulation if that is the temperature required. A probe outside the insulation measures a different temperature. An infrared thermometer cannot see a hidden metal surface through ordinary opaque insulation.
Calibration compares an instrument's response with known reference values. A zero check is one useful test, but a correct zero does not establish the scale over the full range. Use several reference values spanning the intended range, wait for thermal equilibrium where appropriate, and plot sensor output against the reference quantity. Check whether a straight calibration line is justified rather than assuming one.
Worked example 1: use a calibration rather than the display alone
Question. A sensor gives at and at . Independent intermediate checks support linear response. What temperature corresponds to ?
Plan. Find the output change per temperature interval and interpolate inside the calibrated range.
Check. The output is five eighths of the way between its reference values, so the temperature is five eighths of the way across the interval. Equal kelvin and Celsius intervals permit this calculation. The calibration is supplied information, not a data-sheet formula.
Watch out
Repeats reveal scatter and can reduce its effect on a mean. They do not remove a shared zero offset, an incorrect calibration or a consistently misplaced probe. Identify the mechanism before proposing the improvement.
Precautions matched to mechanisms of harm
A useful safety statement connects a hazard to a way of reducing harm. “Be careful” does not explain which action would prevent an injury. Choose details that fit the actual apparatus rather than copying a list from another experiment.
| Arrangement | Relevant hazard and mechanism | A specific precaution |
|---|---|---|
| Heated metal or hot-water bath | Contact with hot surfaces or splashed water can burn skin | Use tongs or suitable heat-resistant gloves to handle the metal, place the bath on a stable heat-resistant surface and let it cool before dismantling |
| Suspended mass or heavy oscillating bar | A stand can topple or a support can fail, allowing the mass to strike someone | Clamp the stand securely to the bench, check the support and keep feet/hands clear of the falling path |
| Stretched spring | A released or broken spring can strike eyes | Keep within the elastic range and use a suitable screen or eye protection for the release path |
| Low-voltage wire circuit | Excessive current can make the wire hot or overload a component | Use a suitable current limit, check component ratings and switch off before changing connections |
A precaution must also preserve a workable measurement. A screen should not obstruct a motion sensor. Securing a stand should not clamp the part that must oscillate. With an optical experiment, arrange the beam so it cannot enter anyone's eye, and do not use a reflective object to search for it near eye level. Use the source specified and the supervised laboratory's safety procedure.
A preliminary run is useful for more than choosing a graph range. It can identify excessive heating, an unstable support, a release that hits the stop too hard or a sensor lead that catches moving apparatus. Revise the method before collecting the full set of data. Written risk assessment prepares the work; the teacher must supervise actual laboratory practice.
Exam tip
Name the hazard and the relevant action together: for example, “clamp the stand to prevent the suspended bar toppling it”. A generic precaution earns no automatic safety mark in every experiment.
Continue this lesson.
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