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Why are wires made of many thin threads?

  1. Jun 25, 2012 #1
    I am guessing that it is easier to produce that way, and gives it more flexibilty. Are these the reasons for wires being mafe of many thin threads, and are there other reasons? Does it allow for better heat dissipation?
     
  2. jcsd
  3. Jun 25, 2012 #2
    It's not easier to produce. The only reason I know of is improved flexability and durability. If you repeatedly bend a solid wire it will break after a reletively small number of cycles. In a stranded wire the strands slide against one another while it bends, reducing stress in the conductor.

    Wiring produced for buildings is installed once and then basicly left alone for life, it is solid copper. Wiring produced for automobiles has to deal with vibration and repeated flexing in places like door hinge areas, it is almost always stranded wire.
     
  4. Jun 26, 2012 #3
    Charge resides on the outside of a conductor.

    Stranded conductors have more 'outside' than inside. Technically they have a greater surface area to volume ratio.

    So you require less copper to carry the same current.

    This is the same underlying reason you cut up potatoes into small pieces to boil them more efficiently.

    Set against this stranded conductors are more expensive to manufacture.
     
  5. Jun 26, 2012 #4
    Thanks for the replies. I did not know that the charge resides on the outside of a conductor. That is another reason why I was confused, as I thought more volume would be better, and stranded wire has less volume.

    Also with the durability reason, that makes sense as well. And if a thread breaks in it, there are still more that can carry the charge, and it can just bridge the gap anyway.
     
  6. Jun 26, 2012 #5
    If all that be true, then why are wires that are installed in applications that don't require fatigue resistance to repeated flexing generally solid strand? The building code in most jurisdictions requires solid strand and forbids multi-strand.

    This is just like wire rope. Mobile cranes have very small wires because they use smaller bending radii on the hoist drum and pullies. Overhead cranes have larger bending radii, and ropes with larger wires. Standing rope has a small number of very large diameter wires. In some applications, they go to a solid rod.
     
  7. Jun 26, 2012 #6
    Do you mean this for DC or low frequency (50-60 Hz).
    Maybe you have in mind the high frequency circuits, where indeed the current is limited to a thin layer. For low frequency circuits the current is quite uniformly distributed over the cross section. There is some surface charge distribution (static), but how would it relate to the maximum current carried by the wire?

    On the other hand, the larger surface area of the stranded wires may result in a better cooling. The maximum current allowed for a given cross section may be actually higher. I did not find some data yet.
     
  8. Jun 26, 2012 #7
    why doesn't charge only flow along the surface of the conducting wires in low frequency AC circuits? Intro to E & M teaches you that the E-field inside a conductor must always be zero (at least in an electrostatic configuration), so why does this change in (certain) electrodynamic cases?
     
  9. Jun 26, 2012 #8

    Nugatory

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    In smallish gauges, solid core is cheaper and easier to produce. It is, however, much more prone to failure from vibration and/or repeated flexing than stranded (mostly because a fatigue crack will propagate through the entire solid wire and break it). So we'd use 14 gauge solid core for a residential 15A 115VAC circuit, but use 14 gauge stranded for an automobile's headlights.

    In very large gauges, stranded is the only practical way to go - a solid-core "wire" 10 cm in diameter isn't a wire, it's a metal rod.
     
  10. Jun 26, 2012 #9
    http://en.wikipedia.org/wiki/Skin_effect
     
  11. Jun 26, 2012 #10

    turbo

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    @OP, you have gotten some very good answers here. I have spend many days repairing, refurbishing, and restoring old guitar amps. Any place that the chassis could experience vibration, the wires were stranded (as were the power cords). The only place that one might expect to see solid copper busses were between potentiometers and ground (chassis). In fact, old Fender tube amps typically had very heavy uninsulated copper wires in such positions. Stranded wires were always used in power cords and in places where vibrations could stress the wires.
     
  12. Jun 26, 2012 #11
    Cool, this does explain the relationship between an AC's frequency, and the current density throughout the wire.

    I suppose I have a more general question as well. The above wikipedia article states that
    So more generally, why would (direct) current be evenly distributed throughout a wire, and not merely just flowing along the surface?
     
  13. Jun 26, 2012 #12

    turbo

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    For reference, this a guitar amplifier built by a good friend of mine. He followed Leo Fender's rules pretty well. Use heavy copper buss connections along the potentiometers, jacks, etc. The thin solid leads to and from capacitors, resisters, etc were soldered to tag-board connecters, and all the connecting wiring under the board is stranded copper. This amp is bulletproof.

    groundbus.jpg
     
  14. Jun 27, 2012 #13
    A wire carrying a current is not a charged conductor because the net charge in any piece of the wire is zero. Electrons are distributed evenly and also flow evenly if in a uniform electric field. In the case of high frequency, the field is not uniform and also a magnetic field exists . These make the flow of electrons nonuniform too.
     
  15. Jun 27, 2012 #14
    In many high current applications, we go to solid copper bus bars, which is about the same thing as a solid rod. This is the best way to handle very high current with minimum voltage drop. We can drop the idea that many small wires do the job better than one solid wire.

    I've seen bus bars with a section of 12 x 36 mm run for hundreds of meters.

    Systems designed for lightening protection are always solid conductors, or if multi strands the strands will be very big. How much more current does any system need to be designed for?
     
    Last edited: Jun 27, 2012
  16. Jun 28, 2012 #15
    It allows you to split a cable into multiple feeds.

    It won't break easily compared to a solid piece.

    More flexible to move around when fitting.

    Easily repaired.

    (And probably somehow cheaper for the manufacturer.)
     
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