1.1.2 Implementing Procedures | OCR Physics H556

What Implementation Skills Are Tested

In written papers, you'll be asked about HOW to carry out measurements correctly. This includes choosing appropriate equipment, describing techniques to reduce uncertainty, and explaining how to avoid common errors.

💡 Common Implementation Questions

Describe how to measure [quantity] accurately
Explain how to reduce the uncertainty in this measurement
State one way to improve the precision of this experiment
Describe how to avoid parallax error when using [equipment]

Core Implementation Concepts

1. Equipment Setup & Calibration

**Calibration = checking equipment against known standards** **Why calibrate?** - Ensures equipment reads correctly - Identifies systematic errors - Improves accuracy of results **How to calibrate:** Metre rule: Check 0 cm mark aligns with edge Balance: Zero the balance before use (tare function) Thermometer: Check reads 0°C in ice water, 100°C in boiling water Voltmeter: Check reads 0V when not connected **Exam question style:** "Explain how to check that a voltmeter is calibrated correctly." [2 marks] Answer: Connect voltmeter across a known voltage source / Check reading matches expected value

2. Measurement Techniques

**Ruler/Metre Rule (±0.5mm uncertainty)** - Always measure from 0 cm mark OR from a clear reference point - Avoid parallax: Eye perpendicular to scale - Use smallest division for uncertainty **Vernier Calipers (±0.01mm)** - Close jaws gently on object - Read main scale first, then vernier scale - Take multiple measurements at different positions **Micrometer (±0.001mm)** - Zero the micrometer before use (check for zero error) - Turn ratchet until it clicks (constant force) - Read main scale + thimble scale - Repeat 3+ times, calculate mean **Digital Stopwatch (±0.01s)** - Time multiple oscillations (e.g., 10 swings of pendulum) - Divide total time by number of oscillations - Reduces percentage uncertainty - Start timing when object passes reference point **Avoiding Parallax Error:** Position your eye directly perpendicular to the scale Use a plane mirror behind scale - your eye and reflection should align

3. Reducing Uncertainties

**1. Take Repeat Measurements** Measure 3-5 times, calculate mean Reduces effect of random errors State range or standard deviation **2. Increase the Measured Quantity** Example: Time 10 oscillations instead of 1 Percentage uncertainty = (absolute uncertainty / measured value) × 100% If you measure 10T instead of T, % uncertainty reduces by factor of 10 **3. Use More Precise Equipment** Micrometer (±0.001mm) instead of ruler (±0.5mm) Light gate (±0.001s) instead of stopwatch (±0.1s) **4. Control Temperature** Many quantities depend on temperature (resistance, length) Allow apparatus to reach thermal equilibrium Use insulation or water bath **5. Eliminate Zero Errors** Micrometer: Check reads 0.00mm when closed Balance: Use tare function Ruler: Start measurement from 1cm mark if 0 is damaged

4. Safety & Practical Tips

**Using Light Gates:** - Ensure beam is perpendicular to card/object - Card should be opaque (blocks beam completely) - Measure width of card with vernier calipers - Start object from rest (check v₀ = 0) **Using Oscilloscopes:** - Set time base correctly (seconds/division) - Set Y-gain (volts/division) - Adjust trigger level to get stable trace - Measure peak-to-peak voltage for AC **Using Ammeters & Voltmeters:** - Ammeter in SERIES (measures current through) - Voltmeter in PARALLEL (measures potential difference across) - Check polarity (+ and - terminals) - Select appropriate range (don't exceed maximum) **Common Mistakes:** ❌ Not zeroing equipment before use ❌ Reading scale at an angle (parallax) ❌ Not repeating measurements ❌ Using damaged equipment (e.g., bent ruler) ❌ Not allowing time for thermal equilibrium

Worked Examples

Example 1: Measuring diameter of a wire

[5 marks]

Question:

A student wishes to measure the diameter of a copper wire. The diameter is approximately 0.5mm. Suggest suitable equipment and describe how to use it to obtain an accurate measurement.

Model Answer:

**Equipment: Micrometer screw gauge (±0.001mm)** **Why micrometer not ruler?** - Diameter is very small (~0.5mm) - Ruler has uncertainty ±0.5mm (too large - could be 100% uncertainty!) - Micrometer has resolution 0.01mm, uncertainty ±0.001mm - This gives percentage uncertainty < 1% **Method:** 1. Zero the micrometer (close jaws, check reads 0.00mm) - If zero error present, note it and correct readings 2. Open jaws, place wire between anvil and spindle 3. Turn ratchet (not barrel) until it clicks - This ensures constant force, prevents compression 4. Record reading: main scale + thimble scale 5. Repeat at 5 different positions along wire - Wire may not have uniform diameter 6. Calculate mean diameter **Example readings:** d₁ = 0.48mm, d₂ = 0.49mm, d₃ = 0.48mm, d₄ = 0.50mm, d₅ = 0.49mm Mean d = (0.48 + 0.49 + 0.48 + 0.50 + 0.49) ÷ 5 = 0.49mm ± 0.001mm

Mark Scheme:

✓ States micrometer [1] ✓ Justification: diameter too small for ruler / micrometer more precise [1] ✓ Describes zeroing / checking for zero error [1] ✓ Mentions using ratchet [1] ✓ Takes multiple readings at different positions + calculates mean [1]

Example 2: Improving timing accuracy

[4 marks]

Question:

A student times the period of a pendulum using a stopwatch. The period is approximately 1.5s. Suggest two ways to improve the accuracy of the time measurement.

Model Answer:

**Method 1: Time multiple oscillations** - Time 10 complete oscillations instead of 1 - Divide total time by 10 to get period T - Example: Total time = 15.2s → T = 15.2 ÷ 10 = 1.52s **Why this helps:** Absolute uncertainty in stopwatch = ±0.1s (manual) or ±0.01s (digital) For 1 oscillation: % uncertainty = (0.1 / 1.5) × 100% = 6.7% For 10 oscillations: % uncertainty = (0.1 / 15) × 100% = 0.67% Percentage uncertainty reduced by factor of 10! **Method 2: Use light gate + datalogger** - Light gate has resolution ±0.001s (much better than manual timing) - Attach card to pendulum bob - Light gate detects when card passes through beam - Computer calculates period automatically - Eliminates human reaction time error **Method 3: Video analysis** - Film pendulum with high-speed camera (frame rate = 100 fps) - Each frame = 0.01s - Count frames for multiple oscillations - More precise than stopwatch **Exam answer structure:** State the method [1 mark] Explain why it improves accuracy/precision [1 mark]

Mark Scheme:

✓ Time multiple oscillations [1] ✓ Reduces percentage uncertainty [1] ✓ Use light gate / photogate [1] ✓ More precise / eliminates reaction time [1]

Interactive Tool: Measurement Accuracy Trainer

Practice using different measurement tools and learn to avoid common errors

Features:

✓Animated equipment (micrometer, vernier calipers, oscilloscope)
✓Identify and correct measurement errors
✓Calculate uncertainties in real-time
✓Compare precision of different instruments

🚧 Simulation coming soon • ID: measurement-trainer

Exam Practice Questions

Question 1

[1 mark]

Multiple Choice

A student measures the length of a metal rod using a metre rule with mm divisions. The length is 45.6cm. What is the absolute uncertainty in this measurement?

A: ±0.05cm

B: ±0.1cm

C: ±0.5mm

D: ±1mm

Show Answer & Mark Scheme

Answer:

A

Explanation:

For a ruler with mm divisions, uncertainty = ±0.5mm = ±0.05cm (half the smallest division)

Question 2

[2 marks]

Short Answer

Explain how to avoid parallax error when measuring the length of a spring with a ruler.

Show Answer & Mark Scheme

Answer:

✓ Position eye perpendicular to the ruler [1] ✓ OR use a set square / plane mirror behind ruler to check eye position [1]

Question 3

[6 marks]

Extended Response

A student uses a micrometer to measure the diameter of a ball bearing. Describe how the student should use the micrometer to obtain an accurate value for the diameter. Include reference to: zero error, technique, and repeats.

Show Answer & Mark Scheme

Answer:

**Checking for zero error [2 marks]:** ✓ Close the micrometer jaws completely ✓ Check if the reading is 0.00mm ✓ If not zero, note the zero error and subtract from all readings **Measurement technique [2 marks]:** ✓ Place ball bearing between anvil and spindle ✓ Turn the ratchet (not the barrel) until it clicks ✓ This applies constant force and prevents over-tightening ✓ Read main scale and thimble scale **Taking repeats [2 marks]:** ✓ Rotate ball bearing and measure diameter at different orientations ✓ Take at least 3 measurements ✓ Calculate mean diameter ✓ Ball may not be perfectly spherical

Implementing Procedures - Key Takeaways

•Always zero equipment before use and check for zero errors
•Use the most precise equipment available for the measurement
•Avoid parallax by viewing scale perpendicular to eye
•Time multiple oscillations to reduce percentage uncertainty
•Take 3-5 repeat readings and calculate the mean
•Use ratchet on micrometer to apply constant force
•Light gates (±0.001s) are more precise than stopwatches (±0.1s)
•Percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%