Resistive Heating Question

Thanks for the answer. As for the standing wave, I meant to use it as an analogy. But since you raised the point, would it actually be possible to get a standing wave type of phenomena in a very long wire?

OP

I'd guess it would be possible - but you need an answer from someone who knows more about transmission lines & suchlike than I.

Wavelength is "approximately" 16,368,000 feet @ 60Hz, 19,641,600 feet (6,000,000 metres) @ 50 Hz. This is slightly off as this is wavelength measured in a vacuum. There are losses to contend with, but the relative position of the wave periods on the conductor, whether pure or not, are not affected...that is unless copper or aluminum can be used to bend time. That would be on the order of jigawatts, or so I am told.

Anecdote has it that some long power transmission lines in Russia developed standing waves to the point where some serious failures occurred.

The question you are asking is about transmission-line theroy.

Theoretically speaking, a transmission-line will have maximum voltage points (current is minimum) and maximum current points, where the voltage is minimum.

If you grab onto an active transmission-line termination (antenna), 1 KW input, the theory won't matter; because the your body, average, is a one-quarter watt, twenty-five-hundred ohm resistor (if you're dry), and you will get fried. To achieve 1 KW (RF) usually takes, on the order of, 3-4KV at 250-300 ma.

well , i am going to put your question in other similar form,

as we use AC to light up lamp , does it be in off and on condition ??

(as the AC sinosoidal will be vary between on and off).

the answer is yes , but could you feel the varaition which will be vary in time of 50 times per second in 50 HZ? or 60 times per second in 60 HZ??

the same answer we can apply now, yes there is cold spots but the heating will be transfer to that spots from the hot spots which will be distributed over the heating wire equally . so you can'nt feel the difference.

my answer is as i understand the question.

If light in incandescent bulb is due to heating of filament why the light disappears when switched off instead of remaining dim for a little while?. What are the thermal properties of tungsten filament?.

Yes,it remains dim after switching off although for fraction of a second and hence not noticeable under normal condition.But, you can see this in slow motion video similar to those of "Time Warp" programmes telecast in Discovery Channel.

There is a short answer to this, but a lot needs to be said about it.

At 60Hz in the States, or 50Hz here in Australia, mate, you won't be able to notice anything.

Mark N.

Considering the 6 km wavelenght at 50 Hz and you can assume that for lenghts larger than 6 km you can get voltage "cold spots". The thermal cold spots are not there because current does not vary along the conductor (excluding the capacity of the conductor that creates a current path through ground and uneven current in the conductor).

The map is not the territory. The graphical representation is not the wire, the electrons, the voltage... Do you have a sense of what the sinusoidal wave represents? It's Time from left to right, and movement of electrons (if it's current, or flow or amperage, or I) or electrical field force (voltage, or V, or E) that moves the electrons back and forth in the wire. You can have stationary magnetic field location on the wire, but like the man said, they're really far apart at 3100 miles ( at 186,000 miles per second, divided by 60 cycles per second). Are you a student? How old?

I used AC heating - not on wire but on a meander cut from a sheet of stainless steel.

9KW power and very hot spots!

But this was not because AC (or DC) but because current density is not constant.

So inner edges get overheated, melt, oxidise severely, the oxide is no longer carrying current and so the hot spot wanders outward until with some sparks the heater breaks.

The sparks were from a big low voltage transformer used to power this.

This was a one time only experiment - I did not expect these large temperature differences.

Later redone with Thermo-coax wire.

RHABE

cool stuff you got there.

You may experience temperature variation without oxidation (for example in a vacuum) because much heat loss at high temperature is by radiation which varies according to surrounding conditions.

There have been

a. low conductive loss as stainless steel is a bad heat conductor and no heat sinks used,

b. low convective loss albeit high temperature as above around 600°C radiation will emit more energy than lost by convection.

So radiation is the biggest part in vacuum or air but oxidation in air may cause a difference as oxide is much more like a black radiator whereas metal is more like a mirror - so radiating a lot of energy nearly in-plane of the sheet metal.

This was thought to be operated in vacuum for brazing of "impossible to braze" materials. But to achieve a homogeneous temperature distribution in vacuum a heater without cold spots is a necessity, so we had to abandon this idea.

RHABE

We have had similar problems resistance welding in a vacuum or inert gas as shown in the images below. You can see large temperature variations in one image and smaller variation in the other image. It can be done.

The answer to your question is a qualified yes. It is qualified because a wavelength is so long at normal frequencies that the self inductance of the wire dominates. The same is not true in the radial direction. High current bus bars experience noticeable skin effect at power frequencies.

JohnDG is right: the answer is no, there would be no hot spots because the AC voltage and current pass through zero. Each uniform cross section of the conductive element is subject to the same varying current of the AC sine wave, so the heating along the conductive element will be relatively uniform (unless cross sectional area or external heat flow restrictions exist).

Considering commercial and residential wiring and appliances as transmission lines where a standing wave could be predictably established is impractical, not just because of the long wavelength and immeasurable skin effect at this low frequency. Establishing a predictable standing wave requires a structure of reasonably uniform characteristic impedance which controls both the electric and magnetic fields of the conduction path.

Um - so next you will be telling us Nichrome is cold in the middle?

I'm a bit amazed "an engineer" doesn't know "conductor" generally means thermal too.

Oh, for a moment there I thought you were speaking of bus bars - but you mean the guy replaced by tokens and travel passes - and now I'm so crushed, I've gone all teary

So according to your carbon theory; a carbon conductor, like say in a M R Brute arc, is colder in the center than on the outside?

But gee - that would be clever for any mass in air don't you think?

Perhaps when you heard of 'skin effect' you thought it actually meant 'skin' - not a depth - like 8 or 9 mm deep in copper in the 50 - 60 Hz region.

But, by all means, let's pretend we are using 40 or 50 mm diameter single core and high frequency and/or voltage - in our 'resistive heating' question.

You still have the problem of the center not being able to radiate through the 'hot skin', and simple thermodynamics will tell you it will be 'perfectly insulated' and get as hot as the 'under skin' gets.

Unless that is; it's a super cooled hollow conductor, or a burst application. E.g. like the thermal difference a fraction of a second after 1 giant spot weld a day.

Or your argument is valid at 1.15 ^-05 Hz, and provided you use giant single core conductors. But hey - why not rewrite the question, slag the members, and encourage me to point out your level of understanding is as my fake tear is to the ocean?

"Power travels around the outer edge or outer circle of the wire not through the centre and certainly not evenly through the cross section ..."

Sooo ... current carrying capacity is proportional to the circumference of the conductor? Well, thank you. I'll have to advise the Institute of Electrical and Electronics Engineers of their foolish mistakes to date.

May we have your details, so that the Nobel Awards Comittee will know where to send the money?