2.1 (p)
Contents
Demonstration 1
Firstly it is advisable to start with a couble of demonstrations to help explain how a standing wave is formed and apply the terms nodes and anti-nodes. Two obvious demonstartions of standing waves are:
- to get the pupils to make standing transverse waves with the slinky
- to observe standing waves on a string using a strobe
Resources
An nice demo is available on the University of Southhampton web site.
Analysis
As well as identifying the major terms and the processes that occur to form a standing wave, it is an opportunity to explain how stinged instruments work and describe the names of the different frequencies e.g. 1st Harmonic
Equipment
Safety
Demonstration 2
Demonstrations of standing waves can also be used to calculate the wavelength of these waves. It is possible to set up two speakers facing each other projecting a single frequency and use a microphone to detect the positions of nodes and anti-nodes using an oscilloscope. Secondly a microwave emitter can be set up opposite a metal plate that reflects the waves, creating a standing wave. The wavelength can be measured from twice the distance between two nodes.
Resources
Analysis
As well as measuring the wavelength of these waves, it is worth discussing the strength of the standing waves and the sharpness of the nodes at different locations between the speakers or the microwave emitter and metal plate. The difference in amplitude of the two waves is the cause.
Equipment
Safety
Demonstration 3
The subject of standing waves is a nice opportunity to explain how a microwave heats up food. This time, use a spread of choc. chips on a stationary microwave plate and heat for a very short period of time.
Resources
Analysis
Measure the distance between melted chips to find half a wavelength and find the frequency of the microwaves from the sticker on the machine and calculate the speed of light.
Equipment
Safety
Demonstration 4
Kundt's tube. Vary the distance between the speaker and reflector to obtain a standing wave. This can also be done by changing the frequency.
Resources
Analysis
This is an oportunity to explain how wind instruments work and how a standing wave in a tube is different to that on a string and how it relies on pressure. It is also possible to measure the speed of sound from this experiment.
