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Name: Kristine
Status: student
Grade: other
Country: Philippines
Date: Winter 2009-2010



Question:
Do bends in a wire affect its electrical resistance? How do dislocations change the resistance?


Replies:
Hi Kristine,

Bending a copper wire repeatedly causes an effect called "work hardening", which (as you refer to) causes fracturing and dislocation of the copper's crystalline structure. Copper's conductivity is usually stated as measured as a percentage of the IACS standard (International Annealed Copper Standard). This is a very old standard, and because the purity of commercially available copper has increased slightly since this standard was established many years ago, commercially pure annealed copper has a conductivity of 101% IACS. That means its conductivity is 1% better than the original IACS standard.

With that as background, work hardening annealed copper by bending, or other methods of cold working, results in a slight decrease in conductivity. In fact, normal copper wire is not fully annealed. Generally, before the last "draw" though the final die, the wire is annealed. Pulling it through the last die to draw down it to its final diameter, causes a small amount of work hardening. To preserve the finished wire's polished surface (that results from drawing through the hardened, polished die), a final annealing is not done after the final "draw". As a result, normal commercial copper wire typically has a conductivity of 98% or 99% IACS. That is, the "work hardening" (and resulting crystal fracturing and dislocation) caused by the "draw" though the final die, has increased the copper's resistance slightly.

So, your suspicion is correct: bending a wire (or any other form of "work hardening") does cause a small increase in resistance (or decrease in conductivity) of copper wire.

Regards,

Bob Wilson


Hi Kristine,

The answer largely depends on what is happening at the atomic level. A "bend" in a wire may seem like a large distortion to the naked eye, but at the atomic level may have no effect whatsoever to the movement of electrons. However, if we bend a wire enough (or twist it back and forth) we can affect the crystal structure, the large structures that represent the organization of atoms. If the crystal fractures or there are errors at the grain boundaries (the line where one crystal pattern merges with another crystal pattern - think of a jigsaw puzzle), then electrons motion are likely to be affected and resistance will go up.

Another way to think about this -using the "electron sea" or "conducting band" model- is that a metal can be viewed as a bunch of atoms that are held so close together that their electrons become delocalized or are shared/spread across many atoms. When a potential difference is felt across a bulk sample, electrons move toward the "lower" potential energy (the positive end). If the crystal lattice is imperfect or the grain boundaries do not match well, the conduction band is skewed toward higher energies, the electron sea becomes discontinuous. Again, we experience this as resistance.

Greg (Roberto Gregorius)
Canisius College



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