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Resistance of a brick - Old coursework question

  1. Dec 27, 2008 #1
    Back when I was doing A-Level physics we were given a coursework assignment that seemed rather ridiculous, which I've been thinking about recently. We were charged with measuring the electrical resistance of a standard household brick at different temperatures, using only equipment we could find in a school lab. Our teacher thought it was insane, as did I. I got a second opinion from my father (he's an electronics engineer that works on diesel-electric locomotives), and he said the only way to do it realistically would be to use a Tesla coil, and that's not exactly something schools just keep in the store room. The solution they were looking for turned out to be a Wheatstone bridge, not that it'd ever work.

    The obvious issues:
    1) Bricks aren't solid and have higher resistance than air, so any current being passed through is more likely to arc round the brick through the air. Only solution - do the experiment in a vacuum.
    2) Heating a brick to 600C uniformly in air isn't an easy task with lab equipment, let alone in a vacuum.
    3) Generating the extremely high voltage required to pass a current through the brick is a big problem.
    4) Measuring the resistance at such high voltages is practically impossible without special equipment.

    I ended up writing that the experiment was impossible using lab equipment - the closest thing even vaguely plausible would be to use an EHT supply with a Wheatstone bridge, with a heat-resistant bell jar to heat the brick in a vacuum, then went on to describe experiments that were more likely to yield a result.

    I know the electrical resistance of bricks has been measured before for lightning safety purposes, but these experiments would have used Tesla coils to generate extreme voltages.

    Anyone think of something we missed?
     
  2. jcsd
  3. Dec 27, 2008 #2

    berkeman

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    Staff: Mentor

    Well, EE technology has advanced a bit since the old locomotive days, so I'd suggest using a laboratory-quality DVM to do your measurements. And to do the measurements versus humidity (DI water chamber), and versus humidity (room air source chamber).

    Plot those data and you will have your answers.
     
  4. Dec 28, 2008 #3
    I think you misunderstood. We were meant to do this experiment with school level lab equipment. Furthermore the variable was heat, not humidity.
     
  5. Dec 29, 2008 #4

    berkeman

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    Staff: Mentor

    Fair enough. Have you calculated the expected results yet? Do you have numbers for the volume resistivity of the brick material? It would help to have a ballpark idea going into the experimental design what kind of numbers you will be measuring.

    The most accurate high-resistivity measurements I've made were done using a "Pico-Ammeter". Do you have such an instrument in your lab equipment? It's a pretty specialized piece of equipment (about the size of a lunch-box DVM), but it's definitely indicated for this type of experiment.
     
  6. Dec 29, 2008 #5
    We did this experiment a few years ago, I'm just wondering if there was any feasible way to do it in a school lab. The science equipment in colleges is pretty poor. The highest voltage that we could generate with standard equipment would be about 50V. Even if I had an extremely accurate pico-ammeter, I'd need to be able to actually pass a current through the brick. The electrical resistivity of brick is much higher than the electrical resistivity of air, so the electric current is much more likely to arc over the brick through the air. To even do such a thing, I'd need like 300kV or something. In a school lab, that's impossible.
     
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