The first time I taught: Waves

Catherine is a Chartered Teacher and has been teaching physics for 30 years. She vividly remembers the first time she taught waves. A salt pendulum transformed her approach – and revolutionised her pupils’ understanding

saltpendulum

This was the plan

When I was planning that first lesson, I looked at a book called “Physics is Fun” and wondered how I could make teaching the wave equation last a lesson!

I remember thinking I would draw a transverse wave on the board and label the wavelength. Then, I’d talk about the number of waves produced per second being the frequency, I’d discuss distance travelled per second and go on to derive the wave equation. After that, I planned to get them to work through some problems.

Here’s how it went

I soon found out it would take much longer to get across basic concepts that I took for granted. And in retrospect, I would not recommend this book as it gave no insight into teaching – but I didn’t realise at the time that that was my problem.

Period was not understood even when I told them it was the time for one oscillation. It turned out they did not know what an oscillation was. I explained the terms so that the students could substitute them into the equation. But eventually I realised that their grasp of the basic concepts was lacking.

How I teach this topic now

Realising that engagement helped understanding was key to how I changed my approach.

My lesson on waves now begins by asking the students to write down all they know about waves. I go round looking at what they have written and pick out the relevant points and then link them to topical events. For example, the fact that waves transfer energy can lead to a discussion about tsunamis.

I then introduce the word period and the class finds the period of a pendulum I have set up on the front bench.

The salt pendulum

I found this pendulum in a great book, “The world of physics” by Avison

The pendulum is about one metre in length with a bob made from the top third of a two litre carbonated drinks bottle. The bottle is inverted and has four holes made about 1cm from its rim so it can be suspended and swing freely. The cap has a 3mm hold drilled in it which is covered with sellotape and the bottle is half-filled with salt. Each student times the time for 10 oscillations and then divides by 10 to find the period (T). This introduces the terminology in a meaningful way.

Having written down the period on the board, I then start the pendulum swinging again. This time I take the sellotape off the bottom of the pendulum so salt runs freely down to fall onto some black paper about 20cm below it. The black paper is the start of a roll of paper so I can pull a length of it under the pendulum. I make sure the pendulum is drawing out a line at 90° to the edge of the paper that I am going to pull on.

I tell the pupils to stand so they can see what happens as I pull on the black paper so that it moves with a constant speed beneath the swinging pendulum. I get some pupils to time me as I pull the paper and generally stop after three complete waves have been drawn out. I ask them to tell me when a wave is drawn and can they relate it to the period.

Here’s how the pupils react

They are generally amazed that a wave results. They see very clearly one wave is produced for each complete swing of the pendulum, so they appreciate one wave is generated in a period. I tell them this is an example of a transverse wave as the oscillation is at 90° to the direction of travel of the wave. Again this point is very clearly illustrated as the pendulum is oscillating at 90° to the direction of travel of the wave.

I ask them to write down how to work out the speed of an object. We measure the distance the paper has moved and note the average time the pupils recorded I pulled for on the board. I then ask them to work out the speed of the wave – and I walk round checking their working.

I then say we can do the calculation a different way. I get a student to measure the wavelength and someone else to check their measurement. I ask: if it takes T seconds (I use the value not the symbol for the period to keep things very clear) to produce one wave, how many waves would be generated every second?

I tell them the number of waves generated per second is called the frequency. I then say if the wavelength is multiplied by the number of waves produced per second, can anyone tell me what I have worked out?

I find students are engaged and they are busy using mathematics they are very familiar with as they are all aware speed=distance/time even if some do not remember it!

I write down the wave equation and define each term and give the units for each term before the lesson ends.

Something else I have found helpful

I have also improved on slinky. I use coloured cotton reels joined together by springs. It make it so easy to show year 9 the longitudinal wave and it is also excellent for standing waves, showing nodes and anti-nodes. I developed this when a primary teacher asked me to explain the difference between two types of wave and they had not basic feeling for particles and movement.

I did decide to let a class try to produce their own wave train but would not recommend that! Skill is required and they are involved timing and measuring.

 

Links to related  IOP resources

Key Stage 3

Supporting Physics Teaching: Light (scroll down to “Frequency, speed and wavelength”)

Supporting Physics Teaching: Sound (scroll down to “Period and frequency”)

Key Stage 5

Teaching Advanced Physics: Progressive waves  

All Key Stages

Practical Physics: Waves

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