5 ms·
How does line-sag lead to increased resistance? My understanding is that the mean-free-path of conduction electrons in even OFHC copper is quite short -- scatt
by ISL 6y ago
How does line-sag lead to increased resistance?
My understanding is that the mean-free-path of conduction electrons in even OFHC copper is quite short -- scattering on impurities is the dominant loss mechanism, not the boundaries of the conductor. Is the theory advanced above that the morphology of the wire matters, or is there another mechanism at work?
- etimberg 6y agoThe resistance is not constant and is in fact a function of temperature [0]. Line sag is a proxy measurement for line temperature, since things expand as they heat up. From [1], most materials have positive alpha coefficients so the resistance increases with temperature. 0. https://en.wikipedia.org/wiki/Electrical_resistivity_and_conuctivity#Temperature_dependence https://en.wikipedia.org/wiki/Electrical_resistivity_and_con... 1. https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/temperature-coefficient-resistance/ https://www.allaboutcircuits.com/textbook/direct-current/chp...
- ISL 6y agoThe statement referred to heating driving sagging, which increased resistance. The increase of resistance with temperature is well-known -- I was curious if there is some effect that is directly related to the wire sag (changing morphology, somehow-altered surface area/skin-depth, changing kinks at the connections, some subtlety with increased length (cross-sectional area also increases), etc..).
- Scoundreller 6y agoMy thought is that after each expansion/contraction cycle, contraction is never exactly back to its previous state. Like bending a piece of metal back and forth, it weakens each time.
- hanniabu 6y agoTheoretically I would thing the sag itself should increase resistance since resistance increases with both length and smaller wire diameter. When the line lengthens the diameter should decrease (similar to stretching a piece of gum or putty).
- leni536 6y agoI don't think it should. Heat expansion should slightly increase the diameter.
- mensetmanusman 6y agoNot if the length increases to counteract it.
- sharpneli 6y agoIt doesn’t ”counteract” it. All dimensions increase based on the same coefficient. Same percentage increase on all axis. See https://en.m.wikipedia.org/wiki/Thermal_expansion https://en.m.wikipedia.org/wiki/Thermal_expansion and check the coefficient of expansion.
- function_seven 6y agoIt's not being stretched by an external force, though. It's expanding due to heat. The diameter would also expand some negligible amount I think.
- Scoundreller 6y agoMy thought was that the gauge of the wire goes down as it sags. The same weight of wire across a longer distance. Maybe I’m wrong and this kind of wire stretching doesn’t impact resistance?
- etaioinshrdlu 6y agoI think sag means voltage sag.
- ericbarrett 6y agoIn this case sag is meant literally. https://www.atcllc.com/blog/why-do-power-lines-sag-on-hot-days/ https://www.atcllc.com/blog/why-do-power-lines-sag-on-hot-da...
- fencepost 6y agoOne of the more impressive things I've seen with electricity is watching a transmission line into a local neighborhood turn orange and stretch to drape itself across a pickup truck parked in the wrong spot. Took a minute to realize what I'd just watched.
- andrewflnr 6y agoYou glowing orange from heat?
- fencepost 6y agoYep, only thing that made sense. There was a heavy storm going through and I was driving by at night, during the day probably wouldn't have been visible at all.
- morei 6y agoThe volume expands as it heats, so it's not a given that the diameter (gauge) goes down. But the length is longer and that does increase the total resistance.
- thescriptkiddie 6y agoJust FYI, transmission lines are aluminum not copper. Also, both copper and aluminum have positive temperature coefficients of resistance.