Measuring wave velocity Revision Notes for AQA GCSE Physics

Measuring wave velocity

What you need to know

You need to know how to work out the speed of sound in air and the speed of ripples on water. These are important skills for your GCSE physics exam.

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Understanding wave velocity measurements is fundamental to physics. You'll encounter these concepts not only in your GCSE exam but also in real-world applications of acoustics and wave mechanics.

Finding the speed of sound in air

There are two main methods you can use to measure how fast sound travels through air. Both methods use different principles but should give you similar results for the speed of sound.

Method 1: Using an echo

This method uses the fact that sound bounces back from walls. The principle relies on measuring the time for sound to travel a known distance and return.

Steps to follow:

Find the distance from where you make the sound to a wall (like a building)
Use a stopwatch to time how long it takes between making the original sound and hearing the echo come back
Work out the speed using: $\text{speed} = \frac{\text{distance}}{\text{time}}$
Remember the sound travels twice the distance (to the wall and back), so use double the distance to the wall

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Critical Point about Distance: The sound travels to the wall AND back to you, so you must double the measured distance to the wall when calculating speed. This is a common mistake in exam questions.

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Making it more accurate: Repeat your experiment several times using different distances. This helps you get better, more reliable results and reduces the impact of measurement errors.

Method 2: Using microphones and an oscilloscope

This method uses electronic equipment to measure sound waves more precisely. It works by comparing wave patterns from two microphones.

Steps to follow:

Set up two microphones in a straight line, with one in front of the other
Connect them to an oscilloscope (a device that shows wave patterns)
Use a loudspeaker to make a sound with a known frequency
Look at the wave patterns on the oscilloscope screen
Move the microphones apart until the waves are exactly one wavelength apart
Calculate the speed using: $\text{wave speed} = \text{frequency} \times \text{wavelength}$

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The oscilloscope method is generally more accurate than the echo method because it eliminates human reaction time errors and provides precise wavelength measurements.

Finding the speed of ripples on water

You can measure how fast waves move across water using a ripple tank. This method is excellent for visualising wave properties and measuring wave speed directly.

Equipment you need:

A shallow tank of water
A vibrating paddle (to make the waves)
A strobe light (flashing light)

Method:

The process involves creating controlled wave patterns that can be measured accurately.

Set the paddle to vibrate at a known frequency to create regular waves
Use the strobe light to 'freeze' the waves so you can see the pattern clearly
Measure the wavelength (distance between two wave peaks)
Calculate the wave speed using: $\text{wave speed} = \text{frequency} \times \text{wavelength}$

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The strobe light is essential because it makes moving waves appear stationary, allowing you to measure wavelength accurately. The flashing frequency should match the wave frequency for best results.

Key formulas to remember

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Essential Equations: There are two important equations you must know for your exam:

$\text{Speed} = \frac{\text{distance}}{\text{time}}$ (measured in m/s)
$\text{Wave speed} = \text{frequency} \times \text{wavelength}$ (where frequency is in Hz and wavelength is in m)

These formulas are fundamental to all wave velocity calculations and will appear frequently in exam questions.

Worked examples

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Worked Example 1: Echo Method

A hand clap is made 480m from a wall. The echo is heard 3 seconds later.

Solution:

Distance to wall = 480m
Total distance travelled = 480m × 2 = 960m (sound travels there and back)
Time = 3 seconds

Using the formula: $\text{Speed} = \frac{\text{distance}}{\text{time}}$

$\text{Speed} = \frac{960\text{m}}{3\text{s}} = 320\text{m/s}$

Note: The distance is doubled because sound travels to the wall and back.

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Worked Example 2: Oscilloscope Method

Two microphones are 35cm apart, representing one wavelength. The frequency is 1000Hz.

Solution:

Wavelength = 35cm = 0.35m
Frequency = 1000Hz

Using the formula: $\text{Wave speed} = \text{frequency} \times \text{wavelength}$

$\text{Wave speed} = 1000\text{Hz} \times 0.35\text{m} = 350\text{m/s}$

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Key Points to Remember:

Sound travels to a wall and back, so double the distance for echo calculations
Wave speed depends on both frequency and wavelength
Use the strobe light to 'freeze' water waves for easy measurement
Always repeat experiments several times for better accuracy
Learn both formulas: $\text{speed} = \frac{\text{distance}}{\text{time}}$ AND $\text{wave speed} = \text{frequency} \times \text{wavelength}$

Measuring wave velocity (AQA GCSE Physics): Revision Notes