AQA GCSE Combined Science: Trilogy Physics Paper 2 (Foundation), November 2021: Question 4

✅ Model Answer (Level 2: 3–4 Marks)

To measure wavelength:

• Place a metre rule on or beside the screen, aligned perpendicular to the wave fronts.
• Take a photograph of the shadow waves on the screen alongside the ruler to "freeze" the motion.
• Count the number of complete waves over a set distance (e.g. across the whole screen) and calculate wavelength by:
  wavelength = total distance ÷ number of waves .

To measure frequency:

• Mark a reference point on the screen.
• Use a stopwatch / stop clock to time how long it takes for a fixed number of waves (e.g. 10 waves) to pass that point.
• Calculate frequency using:
  frequency = number of waves ÷ time taken (s) .
• Alternative method: Record the screen using a video camera with a timer in shot, replay in slow motion, and count the waves passing a point in 1 second.

🧠 Exam Technique & Mark Scheme Rules

• The "Both" Rule: The mark scheme explicitly states that you must describe both frequency and wavelength to reach Level 2 (3–4 marks). If you miss one completely, the maximum you can score is 2 marks.
• Name the equipment: Always state what tool is used for each measurement (e.g. metre rule, stopwatch, camera).
• Averaging over multiple waves: Water waves are small and fast. Measuring 10 wavelengths and dividing by 10 is far more accurate than trying to measure a single tiny moving wave.

💡 Key Knowledge

• Wavelength (λ): The distance from one wave crest to the next adjacent crest (or one shadow line to the next).
• Frequency (f): The number of complete wave cycles passing a fixed point each second (measured in Hertz, Hz).
• Freezing motion: Because ripple tank waves move constantly, taking a photo or using a stroboscope is the standard scientific way to eliminate measurement uncertainty.

❌ Common Errors

• Trying to measure a single moving wave with a ruler directly in the tank: This causes parallax error and water disturbance. The ruler belongs on the screen.
• Forgetting the stopwatch: Stating "count the waves that pass" without mentioning a timer or timing period gives 0 marks for the frequency calculation.
• Confusing period and frequency: Timing just 1 wave gives the period (T), not the frequency ( f = 1/T ).

📐 Step-by-Step Calculation

1. Identify the three repeat readings:
   12.8 Hz , 12.4 Hz , 12.3 Hz
2. Add the three values together:
   12.8 + 12.4 + 12.3 = 37.5
3. Divide by the number of readings (3):
   37.5 ÷ 3 = 12.5

🧠 Exam Technique: Anomalies

Always inspect repeat data before calculating a mean:

• Check if any reading is an anomaly (a result that does not fit the pattern).
• Here, all values (12.8, 12.4, 12.3) are close together, so no anomaly is discarded.
• Give your answer to a sensible number of decimal places matching the raw data (1 decimal place).

✅ Correct Answer

Tick the first box:

☑ To reduce the effect of random errors. [1 mark]

💡 Understanding Error Types

• Random errors: Caused by unpredictable variations (e.g. human reaction time when starting/stopping the timer). Repeating measurements and taking a mean smooths these out and reduces their effect.
• Systematic errors: Cause readings to differ from the true value by a consistent amount each time (e.g. faulty scale calibration). Repeats do not eliminate or reduce systematic errors!
• Zero errors: A specific type of systematic error where an instrument gives a non-zero reading when the true value is zero. Requires tare/correction, not averaging.

📐 Step-by-Step Calculation

1. Identify values: Frequency f = 20 Hz .
2. Substitute into formula:
   period = 1 / 20 [1 mark]
3. Calculate and evaluate:
   1 ÷ 20 = 0.05 [1 mark]

❌ Common Errors

• Inverting the division: Calculating 20 ÷ 1 = 20 instead of 1 ÷ 20 .
• Missing unit awareness: Period is measured in seconds (s). While the unit was provided on the answer line, always double-check that you converted any millihertz (not needed here, but common).

📐 Step-by-Step Calculation

1. Identify given values:
   Frequency f = 20 Hz
   Wavelength λ = 0.012 m
2. Substitute into the equation:
   wave speed = 20 × 0.012 [1 mark]
3. Calculate final value:
   wave speed = 0.24 [1 mark]

🧠 Unit Check

Always check whether the wavelength needs converting:

• Standard unit for wave speed is m/s.
• Frequency is in Hz (cycles per second).
• Wavelength was already given in metres ( 0.012 m ). If it had been in centimetres ( 1.2 cm ), you would first divide by 100!