Waves (AQA GCSE Physics Combined Science): Revision Notes
Waves
What are waves?
Waves are really important in physics because they transfer energy and information from one place to another without moving any matter. Think about dropping a ball into a pond - you can see the wave energy travel outwards across the water surface, but the water itself doesn't actually move along with the wave.
The pond example is a great way to visualise this concept. While you see the ripples moving outward, the water molecules are actually just moving up and down in place. It's only the wave pattern and energy that travels across the pond's surface.
Wave properties
All waves can be described using these key measurements:
Frequency - This tells us how many waves pass a point each second. We measure frequency in hertz (Hz). A higher frequency means more waves are passing by every second.
Speed - This is how fast the wave travels, measured in metres per second (m/s).
Wavelength - This is the distance between two matching points on the wave (like from one peak to the next peak).
Amplitude - This is the maximum distance a point moves from its normal resting position.
Common Mistake Alert! Amplitude is NOT half the distance from the top to the bottom of the wave - that's a common mistake! Amplitude is measured from the resting position to the maximum displacement.
Period - This is how long it takes for one complete wavelength to pass a point. You can work this out using the relationship:
Where is the period and is the frequency.
Types of waves
Longitudinal waves
In longitudinal waves, the particles in the material vibrate back and forth in the same direction that the wave is travelling.
Sound waves are the best example of longitudinal waves. When sound travels through air, the air particles move backwards and forwards along the same path that the sound is travelling. This creates areas where particles are squashed together (compression) and areas where they're spread out (rarefaction).
Example: Sound Wave Through Air
When you speak, your vocal cords create vibrations that push air particles:
- Compression zones: Air particles are pushed closer together
- Rarefaction zones: Air particles are spread further apart
- The particles vibrate back and forth in the same direction the sound travels
- This pattern of compression and rarefaction moves through the air to reach someone's ears
Transverse waves
In transverse waves, the particles move at right angles (90 degrees) to the direction the wave is travelling.
Good examples of transverse waves include:
- Water ripples - the water moves up and down while the wave travels horizontally across the surface
- Electromagnetic waves (like light and radio waves)
Example: Water Ripples
When you drop a stone into still water:
- The water particles move up and down (vertical motion)
- The wave pattern travels outward horizontally across the surface
- The particle motion is perpendicular to the direction of wave travel
- You can see this clearly by watching a floating leaf - it bobs up and down but doesn't travel with the wave
Key difference
The main thing to remember is:
- Longitudinal waves: particles move along the same direction as the wave
- Transverse waves: particles move across (perpendicular to) the direction of the wave
In both types of wave, it's the wave energy that travels from place to place, not the actual particles or material.
Key Points to Remember:
- Waves transfer energy and information without transferring matter
- Frequency is measured in hertz (Hz) and tells us how many waves pass per second
- Amplitude is the maximum displacement from the resting position (not half the wave height!)
- Longitudinal waves have particles moving in the same direction as the wave (like sound)
- Transverse waves have particles moving perpendicular to the wave direction (like water ripples)