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Practical Ohm's law

  1. May 26, 2009 #1

    Strange question, but I have a very practical, if simple, need to apply the Law.

    I need to heat a copper wire to several hundred degrees and I am trying to figure out a rough voltage and amperage to push through it. I am staring at


    trying to figure out what I need to push through 10 or 12 gauge 2m copper wire to get the heat I need.

    For an explanation of what I am trying to do, see below:
    stupid previous owner used construction adhesive to glue paneling to 200 year old hand plastered walls. Ripping to panels off simply rips the plaster off of the wall. The adhesive fails rapidly under moderate heat from a torch, but I can't get the torch behind the panels. Bad alternative plan consists of a using a car battery charger to push 6-12 A at 12V until the copper is nice and hot and then pull the wire down behind the panels to cut through the adhesive.

    The obvious approach is to experiment and see what happens, but I figured I should be applying a very basic scenario of Ohms law, and thought you guys (gals?) might be able to help me out under the assumption I fail.

    suggestions appreciated
  2. jcsd
  3. May 26, 2009 #2


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    Copper would be a poor choice here because
    A) it is a very good conductor and requires a lot of current to make it get hot.
    B) when it is hot is is very weak and would break easily.

    It would be better to look at Nichrome or Konstantin wires. There are used to make heating elements for toasters, room heaters etc.
    You would need about 16 gauge wire for strength and some way to hold it when it is hot.
    This has quite a lot of resistance per meter and stays strong even when it is hot.

    You don't need it to be red hot and a couple of amps should be sufficient to make it hot enough to melt some glue.
    There are different grades of resistance wire and they always show the resistance per meter on the roll. You might be looking for something like 2 or 3 ohms per meter. This would give you 3 amps or 2 amps from 12 volts across a 2 meter length of wire.

    Just to remind you, this is a hot wire and it can burn your fingers or maybe start a fire if you are not careful.

    It can also cut polystyrene foam.
  4. May 27, 2009 #3

    Thanks for the feedback. While I may not have those specific types of wire around the house, your points make a lot of sense: use something not meant to conduct well. I will try some various braided wires first and if they dont work, start looking around for the ones you mention.

    The health warnings are both obvious and appreciated. Never hurts to point those things out and might hurt if you don't.

  5. May 27, 2009 #4
    This sounds like a difficult problem Have you tried a steamer ?
  6. May 27, 2009 #5
    Thanks but a steamer wouldn't get anywhere near the temp needed for the adhesive to fall apart. I also tried using a torch to heat the outside of the paneling and just ended up witha lot of scorched paneling.

    If the wire trick doesn't work, it is off to find way to hook up a big propane tank to a torch attachment.
  7. May 27, 2009 #6


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    Any good electrical supply store should carry Nichrome wire.
    They may call it resistance wire.

    But you would need to look for the resistance per meter to be about right.
  8. May 27, 2009 #7
    To elaborate, the problem with using a copper wire is this: your supply circuit has some internal resistance, likely around 10 ohms. If your load (the wire) has about 0.01 ohms resistance, almost all the voltage will appear across your power supply's internal components, and the power supply itself will get much hotter than your wire.
  9. May 27, 2009 #8
    ah yes, why didn't I think of that? I have an Ohmmeter in front of me. I will just test for the highest resistance.

  10. May 27, 2009 #9


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    Most wires you will probably find will have a lot less resistance than what you need.
    I tested a length of Iron wire as sold in hardware stores and it was a very good conductor.

    Unfortunately, even the type of Nichrome used in domestic 110 volt appliances has too much resistance for your application, so you would probably have to get the right stuff at a suitable store.

    The health warning. I am conscious of the number of "views" on the main page and sometimes throw in something like that, more for general consumption than for the person I am addressing.

    I have seen some abrasive wire used for cutting. It works like a saw and has a handle at each end.
    Like this:
    Last edited: May 27, 2009
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