Describing Waves

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Describing Waves

Waves are moving energy. Light energy moving from the computer screen to your eye moves as light waves. Sound energy moving from a radio to your ear moves as sound waves.

Waves can move along ropes, strings or across the surface of water. Some waves can even travel through space. When waves move along, they make the surface or object move in regular patterns often called wave disturbances.

The important thing to notice is that no 'matter' is moved with the wave. Water waves, which move the water particles up and down on the spot don't actually move any water along with the wave. Only the energy travels along.

All waves can be put into two groups:

  • Transverse waves
  • Longitudinal waves

In transverse waves the particles vibrate at right angles to the movement of energy.

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Examples are light waves, water waves and all electromagnetic waves.

In longitudinal waves the particles vibrate in the same direction as the movement of energy.

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Examples are sound waves and seismic waves.

To compare waves we need to be able describe their characteristics, these include:

  • Amplitude
  • Wavelength
  • Time period
  • Frequency
Describing Waves
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The amplitude of a wave is the height of the wave from the middle to a peak or trough. This is often called the maximum displacement of the wave. (The displacement of a particle is the distance a particle moves from the centre.)

Watch out. Many students make the mistake of measuring from a peak to a trough that gives double the correct answer.

The wavelength is the length of one complete wave. It can be measured, on a distance graph, from any point to the next similar point on the wave. Remember one complete wave includes a peak and a trough.

Describing Waves

The time period of a wave is the time it takes for one complete wave.

The frequency of a wave is the number of waves that travels past a point in one second. Frequency is measured in Hertz (Hz). 1 Hz = 1 wave per second.

There is a simple relationship between frequency and time period. The lower the frequency is the longer the time period will be.

Drag and drop the frequencies and time periods into the correct boxes:

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There is a simple relationship between speed, frequency and wavelength.

Speed = Frequency x Wavelength

This can be put into an equation triangle:

Describing Waves

You must use the correct units for each value

  • Speed is in m/s (not km/s)
  • Wavelength is in m (not km)
  • Frequency is in Hz.

Worked examples:

 

1The speed of sound in air is 330 m/s. A note played on an instrument has a frequency of 110 Hz. What is the wavelength of the note?
 Wavelength = Speed / frequency
 = 330/110
 = 3 m
2The frequency of a water wave is 2 Hz and it has a wavelength of 0.02 m. What is the speed of the wave?
 Speed = Frequency x wavelength
 = 2 x 0.02
 = 0.04 m/s

 

Always remember to put units on your answers.

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