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Edexcel IGCSE Physics revision

Waves units questions

Read oscilloscope scales, convert time and wavelength units, and calculate wave speed from practical measurements.

Edexcel IGCSE Physics Subtopic 3.a

Waves units syllabus

Questions on waves units can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

  • 3.1 use the following units: degree (°), hertz (Hz), metre (m), metre/second (m/s) and second (s)

How to answer waves units questions

  1. Write the unit symbol exactly and distinguish quantities that use similar-looking units.
  2. Convert prefixes and compound units before substituting values into an equation.
  3. Check that the unit of the final answer matches the quantity asked for.
  4. Use the context of a diagram or calculation to reject units with the wrong dimensions.

Waves units example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Calculating sound speed from a trace

January 2023 · Paper 2P · Question 7(b)(ii)

Question 1

A student uses this method to investigate the speed of sound in air.

  • set up an oscilloscope to detect and display a sound wave
  • use a computer and a speaker to produce a sound of known wavelength
  • use the oscilloscope to determine the frequency of the sound wave
  • use a formula to calculate the speed of sound

The diagram shows the oscilloscope screen and the oscilloscope settings.

Oscilloscope screen showing a sinusoidal wave over a grid; text at right gives y direction: 1 square = 5mV and x direction: 1 square = 0.5ms.

The wavelength of the sound wave is 27cm.

Calculate the speed of sound.

speed of sound = ________ m/s

Final answer

Using the measured frequency of 1250 Hz, the speed of sound is 340 m/s (337.5 m/s before rounding).

Mark scheme points

  1. M1 Use the relationship speed = frequency × wavelength.
  2. M2 Substitute the frequency and wavelength consistently in SI units: 1250 Hz and 0.27 m. Follow-through from the frequency found in the previous part is allowed.
  3. M3 Evaluate to give 337.5 m/s or approximately 340 m/s.

Explanation

Use the frequency determined from the oscilloscope and convert the wavelength from centimetres to metres before calculating the wave speed.

λ = 27 cm = 0.27 m
v = f × λ
v = 1250 Hz × 0.27 m
v = 337.5 m/s ≈ 340 m/s

Common mistakes

  • Using 27 directly instead of converting the wavelength to 0.27 m gives an incorrect speed in m/s.
  • The examiner report notes that candidates who stated their calculated frequency generally used it correctly in this calculation. Write the frequency explicitly before substituting it into the formula.

Example 2: Investigating sound speed and temperature

November 2020 (R) · Paper 2P · Question 4(b)(ii)

Question 2

A student investigates the speed of sound in air.

The student investigates how the speed of sound in air varies with temperature.

He places several Bunsen burners in the space between the microphones, as shown in diagram 2.

Diagram 2: microphones A and B with several heat arrows between them.

He uses this method

  • light all the Bunsen burners on a low heating flame
  • measure the air temperature in three places between the microphones using three temperature sensors
  • use the datalogger to record the time taken for a sound wave to travel from microphone A to microphone B
  • determine the speed of sound

The student repeats this method as the air temperature increases.

The table shows the student’s results.

Temperature in °C Time
in ms		 Speed
in m/s
sensor 1 sensor 2 sensor 3 mean
20.3 20.2 19.9 20.1 4.37 344
29.9 31.3 30.0 30.4 4.29 350
39.7 41.0 39.0 39.9 4.22 358
51.0 50.2 49.3 50.2 4.15 362
59.8 61.5 58.6 4.08 368

Calculate the mean temperature for the last row in the table.

Give your answer to a suitable number of decimal places.

mean temperature = ________ °C

Final answer

The mean temperature for the final row is 60.0 °C.

Mark scheme points

  1. M1 Correctly calculate the average of the three sensor readings.
  2. M2 Give the answer to 1 decimal place.

Explanation

Add all three temperatures, divide by 3, then round to 1 decimal place to match the precision of the table.

Mean = (59.8 + 61.5 + 58.6) ÷ 3
     = 179.9 ÷ 3
     = 59.966...
     = 60.0 °C (to 1 d.p.)

Common mistakes

  • The examiner report notes that students sometimes leave out a sensor reading when calculating the mean, or give it to an unsuitable number of decimal places. Check that all three readings are included and round the result to 1 decimal place.
  • Do not truncate 59.966... to 59.9; rounding to 1 decimal place gives 60.0 °C.

Practise Waves units questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.

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