Measurements & Their Errors — A-Level Physics Revision

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What is Measurements & Their Errors?

Measurements in physics are never perfect and always have a degree of uncertainty. This topic explores how to identify and quantify these errors, distinguishing between random errors, which cause readings to be scattered around the true value, and systematic errors, which consistently shift readings by the same amount. Understanding how to calculate absolute, fractional, and percentage uncertainties is a fundamental skill for evaluating the validity of experimental results.

Board notes: Covered by all major boards (AQA, Edexcel, OCR) as a fundamental introductory topic. The depth of uncertainty combination rules can vary slightly, with AQA often requiring a more detailed treatment in practical assessments.

Worked example

Common mistakes

• 1Confusing random and systematic errors. A common mistake is to believe that repeating measurements will reduce systematic errors, whereas it only reduces the effect of random errors.
• 2Incorrectly calculating percentage uncertainty. Students often divide the uncertainty by the measured value but forget to multiply by 100, or they use the wrong value for the uncertainty itself.
• 3Forgetting to state the unit with the uncertainty. The uncertainty in a measurement has the same unit as the measurement itself, and this must always be included.

Measurements & Their Errors exam questions

Exam-style questions for Measurements & Their Errors with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the difference between precision and accuracy in A-Level Physics?

  Precision refers to how close repeated measurements are to each other, indicating the level of random error. Accuracy refers to how close a measurement is to the true value, indicating the level of systematic error.

• How do I combine uncertainties in calculations?

  When adding or subtracting quantities, you add their absolute uncertainties. When multiplying or dividing quantities, you add their percentage uncertainties. For a quantity raised to a power, you multiply its percentage uncertainty by that power.