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This lesson covers the Required Practical on measuring the speed of waves in a solid and the speed of waves in water, as required by the AQA GCSE Physics specification (4.6.1). This practical is commonly examined and you must know the method, equipment, variables, sources of error, and how to improve accuracy.
AQA requires you to carry out experiments to measure the speed of waves in different contexts. The two main experiments are:
Both experiments use the wave equation: v = f x lambda
To find the wave speed, you need to measure the frequency and the wavelength of the wave, then substitute into the equation.
Exam Tip: The required practical on waves is one of the most commonly examined practicals. You must be able to describe the method, identify the variables, explain sources of error, and suggest improvements. Even if you did not do the practical in class, you can still be examined on it.
| Equipment | Purpose |
|---|---|
| Ripple tank (shallow tray with water) | Produces water waves |
| Motor with dipper (vibrating bar) | Creates regular, continuous waves |
| Lamp / strobe light | Illuminates waves from above to project shadows on paper below |
| White screen or paper (below tank) | Shows wave pattern as shadows |
| Ruler | Measures the distance covered by several wavelengths |
| Stopwatch | Measures the time for a set number of waves |
| Signal generator (optional) | Controls the frequency of the dipper |
Set up the ripple tank on a level surface and fill with water to a depth of about 5 mm.
Attach the dipper (a straight bar) to the motor and position it so it just touches the water surface.
Switch on the motor to produce regular, parallel water waves.
Place a lamp above the tank to project wave shadows onto the white paper below.
Adjust the strobe light (if available) so the wave pattern appears to "freeze" — this makes measurement easier.
Use a ruler to measure the distance occupied by a known number of waves (e.g. 10 wavelengths) on the white paper below the tank.
Calculate the wavelength by dividing the total distance by the number of waves:
lambda = total distance / number of waves
Use a stopwatch to time how long it takes for a set number of waves (e.g. 20 waves) to pass a fixed point.
Calculate the frequency:
f = number of waves / total time
Calculate the wave speed using:
v = f x lambda
| Variable Type | Variable | Details |
|---|---|---|
| Independent variable | Frequency of waves | Changed by adjusting the motor speed or signal generator |
| Dependent variable | Wavelength | Measured using the ruler on the projected wave pattern |
| Control variables | Water depth, temperature, size of ripple tank | Kept the same throughout the experiment |
Exam Tip: When describing this practical in an exam, always mention measuring multiple wavelengths and dividing by the number of waves. This is the key technique for reducing percentage error in wavelength measurement. Simply saying "repeat and take a mean" is not enough — you need to explain why this reduces error.
| Equipment | Purpose |
|---|---|
| Vibration generator | Creates waves on the string |
| Signal generator | Controls the frequency of vibration |
| String / elastic cord | The medium through which waves travel |
| Pulley and masses | Provides tension to keep the string taut |
| Ruler / metre rule | Measures the wavelength |
| Bridge / clamp | Fixes one end of the string |
Set up the string between the vibration generator and a pulley at the other end, with a mass hanging over the pulley to provide tension.
Connect the vibration generator to the signal generator.
Switch on the signal generator and gradually increase the frequency until a standing wave pattern appears on the string.
When you see a clear standing wave with well-defined nodes (points of no vibration) and antinodes (points of maximum vibration), measure the distance between two adjacent nodes.
The distance between two adjacent nodes equals half a wavelength:
lambda = 2 x (distance between adjacent nodes)
Read the frequency from the signal generator display.
Calculate the wave speed: v = f x lambda
Change the frequency to find different standing wave patterns and repeat the measurements.
| Variable Type | Variable | Details |
|---|---|---|
| Independent variable | Frequency | Changed using the signal generator |
| Dependent variable | Wavelength | Measured from the standing wave pattern |
| Control variables | String length, tension (mass), type of string | Kept constant throughout |
| Hazard | Risk | Precaution |
|---|---|---|
| Water spillage (ripple tank) | Slipping on wet floor | Wipe up spills immediately; keep water level low |
| Electrical equipment near water | Electric shock | Keep all electrical connections away from the water; use a low-voltage power supply |
| Vibration generator | Fingers caught in moving parts | Keep fingers away from the vibrating components |
| Hanging masses (string experiment) | Masses falling on feet | Place a soft landing pad (e.g. cushion) beneath the masses |
| Strobe light | May trigger seizures in people with photosensitive epilepsy | Check for epilepsy before use; provide warning |
Exam Tip: Safety questions are commonly worth 1-2 marks. Always state the hazard, the risk (what could happen), and the precaution (what you do to reduce the risk). For the ripple tank, the most important safety point is keeping electrical equipment away from water to prevent electric shock.
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