Physics, Waves and WPD, 5-8

Waves

Progressive Waves

Progressive waves transfer energy without transferring any matter. There is no net movement of the medium, although each particle does oscillate around its equilibrium position.

Transverse Waves

In a transverse wave, particles oscillate at right angles to the direction of propagation.

Longitudinal Waves

In a longitudinal wave, particles oscillate parallel to the direction of propagation. These have compressions and rarefactions.

Mechanical and Electromagnetic Waves

Mechanical waves transfer energy through a medium by the oscillation of particles, and can be transverse or longitudinal.

EM waves are not mechanical, and no not require a medium to propagate.

  • Linked electric and magnetic fields

  • Regions of space when an electric charge would experience a force

  • As an EM wave passes, the electric and magnetic fields oscillate

  • Electric field changes, inducing a magnetic filed perpendicularly

  • Magnetic field varies, changing electric field

  • Wave is self perpetuating

  • EM waves are always transverse

Wave Properties

  • Displacement is measured from the equilibrium position, and can be positive or negative

  • Maximum displacement is equal to the amplitude

  • The amount of energy transferred by a wave is dependent on its amplitude

  • Wavelength sis the distance between two consecutive points with identical displacement and velocity

Phase

  • Two points exactly one wavelength apart are in phase - they oscillate in step with each other

  • Two points half a wavelength apart are in antiphase

  • Phase difference is dependent on the fraction of a wavelength between two points

  • One wavelength is represented by 360°

  • Two points with a phase difference of 360°are in phase

  • Two points with a phase difference of 180°are in antiphase

  • One radian is equivalent to 180°

  • \(360^{\circ}=2\pi\)

Frequency

The time taken for one complete wave to pass a certain point is its period, \(T\). The phase difference can then be described as the fraction of a period between two points.

\begin{gather*} f=\frac{1}{T} \\ T=\frac{1}{f} \\ \end{gather*}

Wave Speed

The speed of a wave depends on the properties of the medium. For a mechanical wave, it depends on:

  • The size of the forces between oscillating particles - the elasticity of the medium

  • The intertia of the vibrating particles - how easy or difficult it is to accelerate each particle

  • Sound travels faster through solids because of the stronger forces between adjacent particles

  • \(v=f\lambda\)