 # Waves  ### You need to KNOW: that a progressive wave transports energy from one place to another that mechanical waves use particles to transfer energy (neighbouring particles bump into each other and set their neighbours moving). Eg. sound waves use air particles so it cannot travel through a vacuum (a vacuum is empty space with no particles in it at all!). Sound travels faster through solids than liquids than gases because the particles are more closely packed. that electromagnetic waves don't use particles (in fact particles interrupt their progress through a material) and can therefore travel through a vacuum. e.g. light

### Physical Quantities and their units distance, displacement, wavelength, amplitude - all measured in metres (m) frequency measured in hertz (Hz) time, period - measured in seconds (s) speed - measured in metres per second (m/s)

# v = s/t

speed = distance/time

# v = fλ

wavespeed = frequency x wavelength

NB - be careful with the case of the letters!

### Definitions The vibrations in a longitudinal wave are parallel to the direction in which the energy is travelling. Eg. sound.  The vibrations in a transverse wave are perpendicular (at right angles) to the direction in which the energy is travelling. Eg. light.  Wavelength ( - lamda a Greek letter 'l') is the shortest distance between two particles that are oscillating in phase. (Distance between two crests on a displacement/distance graph). It is measured in metres (m) Period (T)is the time taken for one complete oscillation of a particle in the wave. Distance between two crests on a displacement/time graph. It is measured in seconds (s). Amplitude (A) is the maximum displacement from the mean position.

To the top of a crest or bottom of a trough from the middle line in either graph). Frequency (f) is the number of oscillations per second.

It cannot be read directly off a graph. You need to find (T) from the displacement/time graph and then find its reciprocal (f =1/T) Phase difference (A Level topic) - see here Intensity of a progressive wave (A Level topic) - see here

See the diagrams below Analogue signals contain all the information as a continuously varying wave Digital signals are a series of pulses - either high or low - on or off - sometimes expressed as binary code 1s and 0s.

See Analogue vs. Digital notes

### You need to UNDERSTAND that:- An oscilloscope is a voltmeter that shows how electrical signals vary with time on a screen. It can show sound waveforms if a transducer such as a microphone is connected to it. This changes sound energy into electrical energy and shows the vibrations on the screen. In sound : high frequency means high pitch of note (lots of waveforms on the screen), low frequency (few waveforems) means low pitch of note (deep note). In light: high frequency means the blue end of the spectrum, low frequency means red and of the spectrum. In sound: big amplitude means loud note, small amplitude means quiet (not low as this can be used for pitch too) note. In light: big amplitude means bright light, small amplitude means dim light. to make high pitched sounds you use short, thin strings or short pipes and vice versa. to make loud notes you blow hard into wind instruments and pluck hard at string instruments. that sound travels much slower than light does.

### For the examination you need to BE ABLE TO:-

Label wave diagrams :-  Here is a Java simulation to play with...  Try this useful learning tool to help you learn this topic and/or test your knowledge... This image is a rock formation - click on it to find out more...