An Introduction to Waves Learning Objectives: Understand that waves transfer energy without transferring the medium. Understand that waves transfer energy without transferring the medium. Recognise common wave terminology. Recognise common wave terminology. Understand and be able to use the wave equation. Understand and be able to use the wave equation. Textbook reference: Section 6.01, pages

Waves Transfer energy through matter or space The energy moves, but the matter which the energy is transferred through stays put.

Rays and wavefronts Waves carry energy outwards from an oscillating source (watch the ripple tank simulation to see what that means) The crest of each wave is called a wavefront, and a “point source” produces circular wavefronts in water – just like dropping a pebble in a pond! However, drawing all the wavefronts can be very time consuming, so instead scientists represent waves (and the direction the waves are travelling) by drawing rays. Oscillation= repeated, regular motion about a fixed point e.g. vibrations, pendulum

Waves To describe waves you need to recognise the common terms used: Wavelength: The physical distance between two adjacent identical points on a wave (usually two adjacent crests). Amplitude: The “half-height” of the wave (i.e. above a line through the middle of the wave), related to wave energy. Amplitude

Types of Waves Waves may be longitudinal Longitudinal wave oscillations are along the direction of energy propagation. Or transverse Transverse waves oscillations are perpendicular to the direction of energy propagation. Energy flow oscillations

Longitudinal Wave Terms If you take a snapshot of a longitudinal wave, you will see areas where the particles are squashed together (called a “compression”) and other areas where they are spread out (a “rarefaction”) If you plot a graph of pressure against distance for the wave, it is easy to see why the wavelength of a longitudinal wave is defined as the distance between two adjacent compressions

Transverse and Longitudinal So the type of wave (transverse or longitudinal) depends on the direction of oscillations producing it compared with the direction of motion of the wave itself. Notice, however, that although the wave (and the energy it carries) is moving, the particles carrying it stay put – they just oscillate around a fixed point.

Water waves Water waves have transverse and longitudinal characteristics. Energy is transferred, but the particles stay essentially in the same place

Waves travel through a medium. Common examples are: water air solids

Wave Properties reflection refraction interference

Wave Properties Diffraction: the bending of a wave around the edge of a barrier.

Sound waves LLLLongitudinal waves NNNNeed a medium to travel through SSSSpeed of sound is 340 m/s in air

Waves and Frequency *bigger frequency faster the oscillations, smaller time it takes to oscillate*  To measure waves properly, we need a way of measuring how fast the oscillations are. To do this, we measure their frequency.  Frequency = The number of oscillations per second( the no. of waves pass per second, measured in Hertz (Hz). Also equal to the number of waves passing a point along the wave per second.  1 Hz = 1 oscillation per second.  Frequency = 1. Time period (the time taken for 1 oscillation) Time period (the time taken for 1 oscillation) Time taken for 1 oscillation Frequency f (Hz) 1 second 1 Hz 0.5 seconds 2 Hz 0.02 seconds 50 Hz 10 seconds 0.1 Hz 1 minute Hz

The Wave Equation  Imagine a wave propagating along a string, and starting a clock when the wave reaches a point (the blue line)  The frequency f tells us how many waves will pass a point per second, so one second later f waves will have passed the blue line.  E.g 3 waves passed per sec. therefore 3 hertz  Since one wave has a length λ (the wavelength!), in one second the whole wave has travelled a distance equal to “f lots of λ”. Or put another way: speed (in m/s) = f λ

The Wave Equation… part 2!  This is called the “wave equation”: (speed) v = f x λ( wavelength) Example: Waves are approaching a beach with a distance of 2m between each crest (i.e. a wavelength of 2m). If 4 waves pass a buoy floating in the water in 1 second, how fast are the waves travelling?

Homework  If not already done, finish all questions from bottom of page 125  If not already done, finish answering question 1 from page 144  Textbook pages  Question 4  Question 5 part a only