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AS & A-Level Physics 26 — Practical Skills and Experimental Reasoning

AS & A-Level Physics 26 — Practical Skills and Experimental Reasoning

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Independent Deckloop revision aligned with the Cambridge International AS & A Level Physics (9702) syllabus, 2025–2027. Not affiliated with or endorsed by Cambridge International Education. Chapter 26 of 26: Practical skills and experimental reasoning. Concepts, worked applications and misconception checks.

Physics EN AS & A-Level
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Instrument Selection, Resolution, and Zero Checks

When conducting experiments, selecting the right measuring instrument is crucial for obtaining reliable data. The suitability of an instrument depends on the quantity being measured and the required precision. Each instrument has a specific resolution, which is the smallest change in the quantity it can detect. For example, a standard metre rule has a resolution of 1 mm, while a micrometer screw gauge can measure to 0.01 mm. Before taking measurements, it's essential to perform a zero check to identify any systematic zero error. A zero error occurs when the instrument does not read exactly zero when it should. This could be a positive zero error (reading above zero) or a negative zero error (reading below zero). Correcting for zero errors ensures accuracy by removing a consistent offset from all readings.

Key points

  • Instrument suitability depends on the quantity and desired precision.
  • Resolution is the smallest measurable increment of an instrument.
  • Zero checks identify systematic zero errors (positive or negative).
  • Correct zero errors by adding or subtracting the error from raw readings.

Worked example

Question

A student uses a digital ammeter to measure current. Before connecting it, the ammeter displays -0.02 A. When connected to a circuit, it reads 0.45 A. What is the actual current?

Solution

1. Identify the zero error: The ammeter reads -0.02 A when no current flows, indicating a negative zero error of -0.02 A.
2. Understand that a negative zero error means the measured reading is consistently lower than the true value.
3. Correct the measured reading: Actual current = Measured reading - Zero error.
4. Calculate: Actual current = 0.45 A - (-0.02 A) = 0.45 A + 0.02 A.

The actual current is 0.47 A.

Common pitfalls

  • Ignoring zero errors: Failing to perform a zero check or correct for a detected zero error will lead to systematically inaccurate results. Always check for zero errors and apply corrections.
  • Using an instrument with insufficient resolution: Using a metre rule to measure the thickness of a human hair will yield highly uncertain results. Always select an instrument whose resolution is appropriate for the scale of the measurement.

Prerequisites

  • Understanding physical quantities and units is fundamental to interpreting instrument readings.
  • Knowledge of systematic errors, including zero errors, is essential for performing and understanding zero checks.
  • Distinguishing between precision and accuracy helps in selecting instruments with appropriate resolution.
Further resources