Applied Science A6 Materials and performance technicians notes
Supplementary notes for technicians
All through these supplementary notes, remember that you must carry out your own risk assessment and take suitable precautions.
Activity AA6.7 'Cycling helmets'
For demonstration 2, make a switch with a sandwich-like structure: the ‘filling’ is a non-conducting mesh, with small openings (e.g. a 10 cm x 10 cm square from a string vest). The ‘bread’ is two pieces of aluminium foil cut 9 cm x 9 cm, that is, slightly smaller than the string vest square. At this point the sandwich analogy breaks down!
When a mass falls onto this make-shift switch (the ‘sandwich’), the two sheets of foil will temporarily make contact with each other through the string vest.
The mass and height it falls from should be such that the force of impact causes the foil to make contact, but not so large that the foil continues to make contact after the mass comes to rest.
Use this switch with a timer which can be set to run when a circuit between its two terminals is complete, and to stop when continuity is broken. A suitable arrangement will enable you to measure impact times.
AA6.13 'Thermal conductance of different fabrics
The temperature probes will need to be flat to ensure they don't hold the plates and samples apart. Use a bare wire 'bead' thermocouple rather than one built into a probe.
It is not necessary to glue the items in place, because the glue may provide insulation and distort the results. Simply clamp the 'sandwich' together.
For the heating element: you could use any of the following:
- a cup warmer/heater designed to run off a car 12V supply
- a lab hot plate as used for heating beakers with stirrers (uses mains electricity, but designed for laboratory conditions, and some come with a thermostat)
- the cheapest option would be a home-made heating element. The output from a 1-metre length of 24 swg nichrome wire will be about 36 watts when connected to a 12 V supply. Wind the wire around a piece of stripboard. Remove the copper strips from the stripboard if necessary- these may short-circuit the heating circuit. See photos at the bottom of this web page.
Thank you to John Ellis (CLEAPSS) for the first two ideas, and Tim Henderson (Heston Community School) for the home-made version.
At the bottom of this page are Tim Henderson's photos of his kit.
Here is Tim Henderson's
graph of results (64 KB)
AA6.19 'The roadie has a problem'
Preparing the multicore cables Use Cat 5e standard network cable. Designed for computer local area networks, each of the strand pairs is colour coded and has the great advantage of being a solid copper conductor rather than strands. Gently score the insulating sheath at some point along the cable length. Bend and pull the sheath apart to expose one or two inner strands, at random. Snip the exposed copper wire with scissors. Push the sheath together to cover the gap and then use heatshrink over the top to cover it. Although the area of damage will be visible, students cannot see which conducting path has been damaged.
With thanks to Tim Henderson, Heston Community School.
AA6.24 'Reflect or absorb?'
The length of drainpipe is not critical. Anything in the range 10 - 20 cm is long enough to make a reasonable thickness of the sample material.
A speaker works perfectly well as a detector in this application. We suggest 4 ohm speakers with diameter about 60 mm.
Using an empty carrier as a control: the empty sample carrier also reveals something about the detection system itself (e.g. sound carried by the pipe). The air too has an effect, and the result would be different if you were able to evacuate the pipe.
We are grateful to John Hawcock of Acland Burghley School, Camden for his suggestions.
AA 6.24 Reflect or absorb? photo Ailwyn Holmes
AA6.24 Reflect or absorb? signal photo Ailwyn Holmes
A6.13 photos
AA6.13 Thermal conductance of different fabrics: home-made kit by Tim Henderson of Heston Community School 
