On the right wave length

22nd April 2005 at 01:00
Stephen Hearn describes how working with a newly developed composite challenged his AS-level students to achieve outstanding results

Recently I set up some AS-level sensor coursework in physics, making use of the new material quantum tunnelling composite (QTC), which is available from Peratec.

The physics of this material is unusual. QTC is a metal-filled polymer with special properties. Unlike most non-metallic composite materials that can conduct electricity, the electrical conduction mechanism in QTC is not via electrons meandering along tangled paths of relatively low resistance, so-called percolation. In QTC, quantum mechanical laws account for how tiny metal spheres that never come into physical contact nevertheless are "connected" by probability waves.

These waves describe, among other things, the position and momentum of the electrons. (It is not useful to think of electrons as little balls to understand this material.) The electrical connection improves as the spheres are pushed closer together and the probability of the electrons making the jump between spheres increases. So-called quantum mechanical tunnelling occurs. Electrons sneak through a classically forbidden region of insulation and drift through the material as electrical current.

I gave a small group of AS-level students some QTC sample pills and the Peratec website address. I asked them to design some sensing circuits as part of their coursework for AS-level advancing physics. The next five hours of laboratory time were exciting and challenging for all of us. For a short while - and quite unusually these days - I forgot about coursework mark grids. The experience was refreshing - I recommend it.

The students diligently and sensibly set about investigating the resistanceload characteristic for the material (see figure right). At this stage they were all thinking that this would be a simple and quick experiment (good coursework, easy and lots of marks). The results they obtained bore little resemblance to published graphs on the web. Panic set in temporarily. I reassured them that solving the current problems with their experiments would attract lots of marks. Their spirits perked up and they rose to the challenge.

Each student had discovered that the resistanceload characteristic was critically dependent on how the load was applied. Some of them started to design and build little pieces of apparatus. Impromptu conferences took place in class, they shared data and argued about how to interpret results and explain what was happening in their individual experiments. They were communicating effectively with each other and working well as a group. I was no longer worried about coursework marking. I could sort that out later.

Slowly, each student started to get reproducible results. They each did more work to check data points carefully. It was all looking very good.

They were on track for top marks. Then, one by one, they found that the material "aged" with continuous loading unloading cycles and they became anxious again. It was as if the electrical resistance of the material continually evolved. This was not a typical AS-level laboratory experience.

However, nobody worried about getting the textbook answer anymore. The students were now thinking that problems were there to be solved and to some extent enjoyed.

The end of the allotted time arrived too soon. Some students started to explore other avenues. One thought maybe they could make a better temperature sensor from the material. Another redesigned their apparatus to create an angle of rotation transducer. This was done by squeezing the material between bolted washers. Apparatus was cobbled together and rough trials conducted. Work was quickly evaluated and decisions made on how their sensing system performed in the light of certain evaluation criteria.

By the end of the week my small group of researchers wanted to do experiments and solve problems. They needed to understand physics ideas well so they could design sensible circuits for testing the performance of their sensors.

At the end of the five hours' laboratory time, I had to stop them working.

Of their own accord, each student carefully evaluated their work. They reached conclusions as to why their sensor circuits had failed. For the first time they had produced coursework that reached tentative conclusions and reflected a real investigation experience.

This is what investigation in science should be like in school, at least some of the time. Get some QTC and try some investigation work with your Year 11 or ASA2 groups now!

* Two free samples of QTC will be distributed to every maintained secondary school in the UK in late Mayearly June: one set to the head of science and one to the head of design and technology.

The samples will comprise a large number of QTC pills, which change resistance from the perfect insulator to an almost perfect conductor under modest pressure, and small samples of QTC sheet, which changes resistance according to pressure applied on the surface. The samples will be contained in a DVD case with printed information and a CD-Rom providing animations of QTC behaviour, applications and investigations.

The QTC materials are also on sale for educational establishments from Middlesex University Teaching Resources Centre, Unit 10, The IO Centre, Lea Road, Waltham Cross, Hertfordshire, EN9 1AS.

Tel: 01992 716052 Email: sales@muventures.co.uk


* Further information about QTC visit www.peratech.co.uk


Stephen Hearn is head of science at Charterhouse School, Wiltshire

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