Required Practical: Measuring Wave Properties (AQA GCSE Physics: Combined Science)

Equipment List

• Resolution of measuring equipment:
  • Metre ruler = 1 mm
  • Stopwatch = 0.01 s
  • Signal generator ~ 10 nHz

Experiment 1: Water Waves in a Ripple Tank

Aims of the Experiment

• To measure frequency, wavelength and wave speed by observing water waves in a ripple tank

Variables

• Independent variable = frequency, f
• Dependent variable = wavelength, λ
• Control variables:
  • Same depth of water
  • Same temperature of water

Method

Set up of ripple tank to investigate wave properties

1. Set up the apparatus as shown and fill the ripple tank with water to a depth of no more than 1 cm
2. Turn on the power supply and the light source to produce a wave pattern on the screen
3. The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts
4. The frequency can be determined by timing how long it takes for a given number of waves to pass a particular point and dividing the number of wavefronts by the time taken
5. Record the frequency and wavelength in a table and repeat the measurements

• An example of the data collection table is shown below:

Analysis of Results

• The speed of the waves can be determined using the equation:

Wave Speed = Frequency × Wavelength

v = fλ

• Where:
  • v = wave speed in metres per second (m/s)
  • f = frequency in Hertz (Hz)
  • λ = wavelength in metres (m)

Experiment 2: Investigating Waves in a Solid

Aim of the Experiment

• To measure frequency, wavelength and wave speed by observing waves in a solid object, such as a stretched string or elastic cord

Variables

• Independent variable = frequency, f
• Dependent variable = wavelength, λ
• Control variables:
  • Same string
  • Same masses attached to string
  • Same length of string

Method

Set up of apparatus to investigate wave properties of a vibrating string

1. Set up the apparatus as shown, then adjust the frequency of the signal generator until a "solid" wave is produced
2. Once the "solid" wave is produced, record the frequency shown on the signal generator
3. Use a ruler to measure the wavelength, the length to measure will depend on the number of "solid" waves produced. Or measure the length of multiple wavelengths, and divide by the number of wavelengths seen
4. Repeat the procedure by adjusting the frequency until another "solid" wave is produced

Guide to measuring the wavelength of waves in a solid

• An example of the data collection table is shown below:

Analysis of Results

• The speed of each wave can be determined using the equation:

Wave Speed = Frequency × Wavelength

v = fλ

Evaluating the Experiment

Systematic Errors:

• It can be difficult to identify the wavefronts while they are moving
  • Use a stroboscope (flashing light) matched to the same frequency of the waves, this will be indicated by the waves appearing to not be moving
  • The frequency can be read from the frequency setting of the stroboscope, and the wavelength will be easier to determine while the waves appear still

Random Errors:

• To improve the accuracy of the wavelength measurement in the ripple tank:
  • Measure across a number of waves (e.g. 5 of them) and then divide the distance by the number of waves

• To improve the accuracy of the frequency measurement in the ripple tank:
  • Measure across a longer time period (e.g. a minute) and then divide the number of waves by the time

• When taking repeat measurements of the frequency of the "solid" wave, the best procedure is as follows:
  • Determine the frequency of the "solid" wave when the largest vibration is observed and note down the frequency at this point
  • Increase the frequency and then gradually reduce it until the "solid" wave is clearly observed again and note down the frequency of this
  • If taking three repeat readings, repeat this procedure again
  • Average the three readings and move on to the next measurement

Safety Considerations

• Care should be taken when working with water and electricity in close proximity
  • Carelessness could lead to electric shock

• No food or drink should be consumed near the experiment 
• If using strobe lighting to see the wavefronts more clearly, ensure no one in the room has photosensitive epilepsy
• Make sure to stand up during the whole experiment, to react quickly to any spills
• Use a rubber string instead of a metal wire, in case it snaps under tension
• Wear safety goggles to protect the eyes in case the string or cord snaps
• Stand well away from the masses in case they fall onto the floor
  • Place a crash mat or any soft surface under the masses to break their fall