# Waves

Waves are one of the ways in which energy may be transferred between stores. Waves can be described as , or about a rest position. For example:

• sound waves cause air particles to vibrate back and forth
• ripples cause water particles to vibrate up and down

The direction of these oscillations is the difference between longitudinal or transverse waves. In , the vibrations are parallel to the direction of wave travel. In , the vibrations are at right angles to the direction of wave travel.

Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a to travel through. Electromagnetic waves cause oscillations in electrical and magnetic fields.

It is important to remember that all waves transfer energy but they do not transfer . For example, if a ball is placed on the surface of a pond when ripples move across it, the ball will move up and down but not outwards with the wave. Similarly, when sound waves move through the air to a person’s ear, the air itself does not move - instead the sound is transferred through the vibrating molecules, which stay where they are and vibrate about a fixed point.

## Parts of a wave

Waves are described using the following terms:

• Rest position - the undisturbed position of particles or fields when they are not vibrating.
• Displacement - the distance that a certain point in the medium has moved from its rest position.
• Peak - the highest point above the rest position.
• Trough - the lowest point below the rest position.
• Amplitude - the maximum displacement of a point of a wave from its rest position.
• Wavelength - distance covered by a full cycle of the wave. Usually measured from peak to peak, or trough to trough.
• Time period - the time taken for a full cycle of the wave. Usually measured from peak to peak, or trough to trough.
• Frequency - the number of waves passing a point each second.
Diagram of a wave