AQA AS Level Physics Paper 2, November 2021: Question 3

✅ Correct Answer

Reading: 9.65  |  Unit: mm

💡 Key Knowledge

• Main scale: Read the furthest visible main scale line past the thimble edge (shows 9.0 mm ).
• Micrometer scale: Find which horizontal line on the rotating thimble aligns with the central datum line of the main scale ( 0.65 mm ).
• Total calculation: 9.0 mm + 0.65 mm = 9.65 mm .

❌ Common Errors

• Forgetting to include the correct unit ( mm ).
• Misreading the main scale by missing the half-millimetre graduation mark below the central line.
• Confusing the thimble scale reading (e.g. writing 9.15 instead of 9.65 ).

🧠 Exam Technique

Always inspect the scale closely: note that the main scale features graduations above and below the horizontal reference line, where lines below represent additional 0.5 mm increments.

✅ Correct Answer

Any two of the following valid procedural steps:

• Close the micrometer jaws using the ratchet (without the pencil) to determine any zero error.
• Add or subtract the zero error value from each subsequent measurement reading.
• Take multiple readings along the length or around different orientations of the pencil and calculate the mean.

💡 Key Knowledge

• Systematic Error: A zero error is a systematic uncertainty that causes all readings to be shifted by a constant amount.
• Ratchet usage: Always use the ratchet mechanism to apply a consistent closing force and prevent damage to the instrument or sample.

❌ Common Errors

Students often lose marks by simply stating "take multiple readings" without specifying where or how (e.g., at different points along/around the pencil), or by failing to mention that the ratchet must be used when checking for zero error.

🧠 Exam Technique

When an exam question mentions a "zero error", your default reflex should be to explain how to check the instrument at zero (closed jaws) and how to apply a correction to the final data. Combine this with standard averaging techniques to secure full marks.