Practical 1: measuring instruments, precision and zero errors

On the morning of the physics practical, Chiamaka walks into the laboratory of her school in Owerri and finds three benches, each set up with different apparatus: a retort stand with a pendulum bob on a thread, a glass block on drawing paper with optical pins, and a circuit board with a cell, a resistance wire and two meters. In WAEC Paper 3 she will answer any two of the three questions in 2¾ hours, and the paper carries 50 marks, so each question is worth 25 marks. NECO calls its practical Paper I (Practical); it also lasts 2 hours 45 minutes and is built on three questions, usually one each from mechanics, optics and electricity. Always follow the instruction on the front of your paper about how many questions to answer. Private candidates sit an alternative to practical paper in the same format: instead of handling apparatus, they are given diagrams of instruments and tables of readings, and must read, tabulate, plot and deduce exactly as if they had done the experiment.

Paper 3 is worth a large share of the total physics mark (Paper 1 has 50 marks, Paper 2 has 60 marks and Paper 3 has 50 marks), so a student who is strong in theory but careless in the laboratory throws away a grade. The good news is that the practical paper is the most predictable paper in the whole examination. The same few experiments come back year after year in slightly different dress, and the same small set of skills earns most of the marks. Those skills are what this unit and the next one teach; units pp3 to pp6 then apply them to the standard experiments.

Examiners describe the skills in four groups, and every mark in a 25-mark question falls into one of them:

• Observing and measuring: setting up the apparatus correctly and taking readings to the precision of the instrument. Most marks for readings are awarded only if the values are within a tolerance of what the examiner expects, so careless reading costs marks directly.
• Recording: a neat table with a heading for every column, each heading showing the quantity and its unit, values given to a consistent number of decimal places, and enough readings (normally five or six sets).
• Graph work: axes labelled with quantities and units, sensible scales, points plotted accurately, a good line of best fit, and a large triangle used to find the slope.
• Deducing and evaluating: calculating the slope or intercept with its unit, using it to find a physical constant, and stating precautions taken and sources of error.

A typical question is worded like this: "You are provided with a metre rule, a knife edge, a 50 g mass and other necessary apparatus. (i) Balance the metre rule on the knife edge and record the balance point G. (ii) ... (iii) Evaluate 1/x for each value of x. (iv) Tabulate your readings. (v) Plot a graph of ... (vi) Determine the slope s of the graph. (vii) State two precautions taken to ensure accurate results." It ends with a short theory part (b), usually 4 to 6 marks, asking for a definition or a small calculation linked to the experiment.

Read the whole question once before you touch any apparatus. Underline what you must measure, what you must calculate for each row (for example T, T², 1/x or sin i), and what you must plot against what. "Plot a graph of T² against l" means T² goes on the vertical axis and l on the horizontal axis. Students who swap the axes lose the graph marks and get the wrong slope.