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Resistance of a brick PLEASE HELP

  1. Apr 18, 2005 #1
    here's a question we got given from our teacher (AS level)

    Design a laboratory experiment to investigate how the resistance of a house brick varies with temperature in the range 20 to 800 degrees centegrade. it is known that the reststance on the brick is very high. the brick is of a non-uniform shape, and the resistance is to be measured from the two end faces.

    personally i dont know where to start. or go on to.

    any ideas people?!
     
  2. jcsd
  3. Apr 19, 2005 #2
    Got given the same thing yesterday, infact "working" on it now here in my physics lesson.

    :rofl:
     
  4. Apr 19, 2005 #3
    Perhaps something involving a blow torch and a control....I'll see where my thinking takes me. :smile:
     
  5. Apr 19, 2005 #4
    misskitty,

    That will help with the temp dependence if the resistance (at high or low temp) can be measured at all. But in order to measure resistance you have to get a measurable amount of current through the brick. That's going to take a very big voltage, or a very sensitive ammeter.
     
  6. Apr 19, 2005 #5
    This is true. It was more of just throwing out an idea of what could help. :redface: It wasn't my intention to sound like an imbecile. Where would you even get an ammeter that sensitive? My dad is an electrician who does a lot of work with high voltage machinery. He doesn't even have on that sensitive.
     
  7. Apr 19, 2005 #6
    Is using a smaller sample allowed? It would make the measurements much easier.

    If you can find a way to cut a smaller piece of the material maintaing some sort of a nice geometry for it it'll make the measurements easier. You won't need that big voltages and the heating times for high temp measurements will be lower. From the resistance of a smaller piece you can calculate the resistivity from that calculate the whole brick's resistance.
     
  8. Apr 19, 2005 #7
    That's good thinking Inha. :smile: I know very little about resistance (haven't studied this yet...:blushing:). How much electrical resistance does something develop when it is heated? How does it happen?
     
  9. Apr 19, 2005 #8
    Here's a pretty simple explanation:

    For metals the resistivity increases linearly with temperature at high (around room temp and above) temperatures. The vibrational energy of the atoms increases with with increasing temperature which increases the scattering of conduction electrons (hinders their mobility) which means the resistance is increased.

    For semiconductors it decreases because increased temperature allows more electrons to partake in conduction processes.

    For insulators as the brick I'm not sure. Increasing temperature allow more electrons to partake in conduction processes again but I'm not sure what the effect of the scattering processes is. I'm leaning on the semiconductor like behaviour but maybe some of the CM physicists here can help.
     
  10. Apr 19, 2005 #9
    misskitty,

    "It wasn't my intention to sound like an imbecile."

    You didn't sound like an imbecile, and I certainly didn't mean to make you feel like one! Getting the temp up to 800C is half the problem; your idea of using a blow torch might work. I was just thinking about the other half of the problem, measuring very high resistance.
     
  11. Apr 19, 2005 #10
    Thanks Inha. Is there such a thing as maximum resistance?
     
  12. Apr 19, 2005 #11
    OH!! I'm sorry! I misunderstood what you typed Jdavel. :redface: Its hard to interpret how some things are intendend on the internet. Friends? :smile:

    I think your idea of the ammeter would work, but I don't know where you'd gain access to one that would be that sensitive. The local telephone company perhaps? :rolleyes:
     
  13. Apr 19, 2005 #12
    I just thought of something that might get the brick up to 800 degrees. If we can't heat the brick that high with a blow torch, perhaps and autoclave might work. Placing the brick into an autoclave would raise it to the required temperature, or perhaps a few degrees higher, after the brick is heated, it can be removed and the ammeter can be placed on the end faces of the brick to measure the resistance of the brick. The question now is how much relative error would be produced if we were to use that proceedure? If the relative error is greater that 10% then we need to come up with a new proceedure with less error.
     
  14. Apr 19, 2005 #13

    Gokul43201

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    For most insulators that I'm aware of, the increse in resistivity from (electron-phonon) scattering hardly offsets the decrease from thermal activation across the band gap (in fact there will be several closer lying donor and acceptor levels, which will contribute at significantly lower temperatures than T ~ Eg/k). I too would imagine that the resistance of the brick will drop with increasing temperature.

    To make the resistance of the brick "reasonable", use a thin slice with a large cross-section area. By making L/A small, you are reducing the resistance (recall the formula for resistance in terms of resistivity). Even so, this is a formidable task. I would recommend using an oven/furnace instead of a blow-torch. Also, I recommend you first bake-out the sample (brick) before doing the real measurement, else you may get significant contributions from moisture in the pores - or you could do a DC measurement.
     
  15. Apr 19, 2005 #14
    Just found out the sensitivity of the meter is not going to pose an issue. A normal Ohm-meter used by electricians is sensitive enough to measure the resistance produced by heating the brick. Fluke makes these meters. So they are pretty easy to obtain. I agree with Goku, using a furnice or oven might work. Gaining access to an autoclave could prove to be rather challenging. Where as a furnice or kiln can be accessed at an art or pottery studio.
     
  16. Apr 19, 2005 #15

    Gokul43201

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    I agree. Even at a resistivity of about 100 M.Ohm-cm, you can make the resistance be a mere 1 M.Ohm, with an L/A ~ 1/100 (cm-1). Handheld multimeters can handle such numbers with ease.

    Just remember not to put the voltmeter in the furnace as well. :wink:
     
  17. Apr 19, 2005 #16
    :eek:Oh NO; Don't do that! Bad things will happen if you do that! :surprised: Make sure to take the brick out of the furnace before testing the resistance with the Ohm-meter.

    What a mess that would be trying explaining what happend to the meter if someone put it inside the furnace with the brick and then tried to figure out why it wouldn't work. :bugeye: It would be rather funny though...:rofl:
     
  18. Apr 19, 2005 #17

    Gokul43201

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    Actually, it's not necessary at all to take the brick out of the furnace. You can have stainless (or other suitable metal) leads coming out of the furnace from the plates on the brick, and you can clip onto these leads and measure the resistance. Leaving the brick in the furnace ensures that it's at the temperature that the furnace/oven is set for. This way, you can measure resistance virtually continuously as a function of temperature, without having to take the brick out each time.
     
  19. Apr 20, 2005 #18
    hi
    im doing the same plan
    ive decided to cut the brick up and use a kiln from the art department and use a metal plate either side of the brick and weld the wires to it
    however the wires will melt at like 800C dur so i was wonderin like does any one have ideas about how to insulate them lol. i was thinking ceramic rings but not sure
    how r we supose to fit this all in 500 words arhhhh help me sob
    lol xx
     
  20. Apr 20, 2005 #19
    Wouldn't the metal have a different resistance than the brick though?
     
  21. Apr 20, 2005 #20

    Gokul43201

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    This is roughly how I'd do it too.

    The wires will not melt if they're (stainless) steel wires. You can use ceramic beads (aka thermocouple beads) to (try to) insulate them, but that will only work for a short while. Just make sure you also evaluate the temperature dependence of the resistance of the lead wires, lest that affect your data.
     
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