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12.8 Required Practical: Waves

GCSE Physics · AQA · Chapter 12: Wave Properties

A free GCSE Physics lesson for AQA (8463) from OBA School: measuring the frequency, wavelength and speed of waves in a ripple tank and on a stretched string. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Choose suitable apparatus to measure the frequency, wavelength and speed of water waves in a ripple tank
  • Measure the wavelength and speed of waves on a stretched string using a vibration generator
  • Identify the variables and the hazards in each experiment and how they are controlled
  • Process results with v = f λ and graphs, and evaluate the methods with improvements

Key words

  • Independent variable: The variable you change on purpose, e.g. the frequency of the vibration generator.
  • Dependent variable: The variable you measure, e.g. the wavelength.
  • Control variable: A variable kept the same so the test is fair, e.g. the depth of water or the tension in the string.
  • Resolution: The smallest change an instrument can show, e.g. 1 mm on a metre ruler.
  • Random error: Unpredictable scatter in readings, e.g. from judging where a moving wavefront is. Reduced by repeating and averaging.
  • Systematic error: An error that shifts every reading the same way, e.g. shadows on the paper being bigger than the real waves.
  • Node: A point on a vibrating string that does not move. Neighbouring nodes are half a wavelength apart.

Part A: waves in a ripple tank

Aim: to measure the frequency, wavelength and speed of water waves and to choose the most suitable apparatus for each measurement.

Method

1. Fill the tank with water to a depth of about 5 mm and check it is level so the depth is the same everywhere.

2. Switch on the lamp and the motor. Adjust the bar so it just touches the water and makes plane waves.

3. Frequency: count the number of waves passing a mark on the paper in 10 s, then divide by 10. Repeat and find a mean. (Or read it from the power supply if it shows frequency.)

4. Wavelength: place a metre ruler on the paper, at right angles to the wavefronts. Measure across 10 wavelengths (11 bright lines) and divide by 10. Taking a photo with the ruler in view “freezes” the pattern.

5. Speed: calculate v = f λ. Check it by timing one wave travelling a measured distance across the tank: speed = distance ÷ time.

6. Change the frequency of the motor and repeat steps 3–5.

Exam tip: Measuring across 10 wavelengths makes the ruler’s resolution (1 mm) a much smaller fraction of the length measured, so the percentage uncertainty in λ is ten times smaller.

Part B: waves on a stretched string

Aim: to measure the wavelength and speed of waves on a stretched string (waves in a solid).

Method

1. Fix the vibration generator to the bench and pass the string over a pulley clamped at the other end. Hang masses (e.g. 200 g) from the string to give a constant tension.

2. Switch on the signal generator. Adjust the frequency slowly until a clear, steady pattern of loops is seen. Read the frequency f from the signal generator.

3. Use a metre ruler to measure the length across as many loops as possible (from the first node to the last node). Divide by the number of loops to get the length of one loop, then λ = 2 × loop length.

4. Calculate v = f λ.

5. Increase the frequency to get more loops and repeat. The speed should stay the same while the tension and string are unchanged.

Variables and safety

  • Independent variable; Ripple tank frequency of the motor; Stretched string frequency of the signal generator
  • Dependent variable; Ripple tank wavelength (and speed); Stretched string wavelength (number and length of loops)
  • Control variables; Ripple tank depth of water; same tank and bar; Stretched string tension (mass hung); type of string; vibrating length
  • water spilt near the lamp and mains sockets; Control use a low-voltage lamp; keep the water away from the power supply; mop up spills straight away
  • slipping on spilt water; Control mop up spills; keep bags off the floor
  • masses falling on feet; Control clamp the pulley firmly; keep the masses low and feet clear
  • string snapping under tension; Control do not overload the string; wear eye protection

Processing and evaluating results

Worked example: ripple tank results

Waves counted in 10 s: 33, 31, 32 → mean 32, so f = 32 ÷ 10 = 3.2 Hz
Length of 10 wavelengths on the paper = 18.5 cm, so λ = 1.85 cm = 0.0185 m
v = f λ = 3.2 × 0.0185 = 0.0592 m/s

Answer: v ≈ 0.059 m/s (5.9 cm/s)

For the string, plot frequency (y-axis) against 1/wavelength (x-axis). Because f = v × (1/λ), the graph is a straight line through the origin and its gradient is the wave speed.

  • loops on 1.50 m; 40 2; 60 3; 80 4; 100 5; 120 6
  • λ (m); 40 1.50; 60 1.00; 80 0.75; 100 0.60; 120 0.50
  • v = f λ (m/s); 40 60; 60 60; 80 60; 100 60; 120 60
  • fast-moving ripples are hard to count; Improvement film with a phone (slow motion) and count frame by frame, or use a stroboscope to “freeze” the waves
  • shadows on the paper are bigger than the real waves (lamp is a point source) – a systematic error; Improvement put an object of known size in the tank to find the scale factor, or measure the ratio using a photo
  • hard to see exactly where a node is; Improvement measure across many loops; view from directly in front to avoid parallax
  • reaction time when timing one wave; Improvement time over a longer distance or use v = f λ instead

Common mistakes

  • Taking one loop on the string as one wavelength. Two loops make one wavelength.
  • Measuring a single wavelength with a ruler. Measure across many and divide.
  • Changing the tension (or water depth) during the experiment – this changes the wave speed.
  • Forgetting to convert cm to m before calculating a speed in m/s.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the best way to measure the wavelength of ripples in a ripple tank? (Choose from: measure across 10 wavelengths on the shadow pattern with a ruler and divide by 10, measure one wavelength with a ruler held above the water, time one wave across the tank with a stopwatch, read it from the power supply.)

Answer: measure across 10 wavelengths on the shadow pattern with a ruler and divide by 10

Working: Measuring across many wavelengths and dividing reduces the percentage uncertainty from the ruler’s resolution.

Question 2. In the stretched-string experiment, what is the independent variable? (Choose from: the frequency of the signal generator, the wavelength, the mass hanging on the string, the length of the bench.)

Answer: the frequency of the signal generator

Working: You change the frequency and measure the wavelength (the dependent variable).

Question 3. In the stretched-string experiment, which must be kept constant so the wave speed does not change? (Choose from: the tension in the string, the frequency, the number of loops, the wavelength.)

Answer: the tension in the string

Working: Keep the same hanging masses (tension) and the same string; then v stays constant as f changes.

Related lessons

Continue in the full course

This lesson is section 12.8 (AGCPH1208) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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