A basic electrics question.

[Tim Hall]

My pointing theory is at shiny things and strange objects and going oooooh .

YAProfBrianCoxAICFmillionmillionmillionmillionstars.

 

[Wobblers]

Umm. One hates to quibble. But don't you think that the energy is actually carried, not by the electric field alone, but by Poynting's Vector, which is the vector cross product of the electric and magnetic fields? So when the electric field reverses direction each half cycle of an AC waveform, so does the magnetic field, and hence the energy flow remains in the same direction?

[/one physicist talking to another mode]

Well, yes. For AC: it's essentially an electromagnetic wave being carried down a wire that's acting as a waveguide (I alluded to that in my first post but didn't want to go into more detail as things were getting way too complicated as it was). But that doesn't really apply for DC. (Consider: no energy flows through a DC circuit with a series capacitor in steady state, but it does in an AC circuit - why?) Current and therefore energy flow in a DC circuit is determined by the direction of the electric field down the wire.

PS: "Another mode"?!?

 

[bruce1530]

What nonsense. Everyone knows that it’s little pixies that run down the insides of the wires, carrying little buckets of energy.

 

[Wobblers]

Oi! Stop giving away the Guild secrets!

 

[presta]

A quick google suggests the capacitance of a power cable is of order 100 pF per metre. So as alluded to by @the_mikey , a metre of cable, even if terminating in an open switch with no indicator lamps, would still draw a current, but of less than 0.01 milliamperes

Yes, but capacitance dissipates no power, so you pay nothing for it. That's why the board insist in a minimum power factor, so that they're not pushing current for nothing.

Electrons travel at the speed of light as any fule kno.

The electrons travel at a few microns per second, the wave propagates at typically around 65% of the speed of light in a cable, and depends on the dielectric constant of the insulation (and the relative permeability, but that's usually as near to one as makes no difference).

 

[TheDoctor]

^^That. I used to work on distance-to-fault test gear, and most sorts of cable propagate the wave at about two-and-a-little-bit times ten-to-the-eight m/s2.
And it's a right bugger trying to type scientific notation! About 2/3 of the speed of light.

 

[tyred]

What nonsense. Everyone knows that it’s little pixies that run down the insides of the wires, carrying little buckets of energy.

When you switch on the light they run down the wire with a box of matches and light the candles inside the bulb....

 

[Fab Foodie]

The electrons travel at a few microns per second, the wave propagates at typically around 65% of the speed of light in a cable, and depends on the dielectric constant of the insulation (and the relative permeability, but that's usually as near to one as makes no difference).

I knew that but I didn’t want the truth to get in the way of a good story.....

 

[Fab Foodie]

When you switch on the light they run down the wire with a box of matches and light the candles inside the bulb....

Exackerly....

 

[swee'pea99]

The Poynting vector is just a representation useful to us i.e. (directional) energy flux. The field(s) is very much the more profound quantity and deeper insight. Although the Poynting's Theorem is a profound insight, leading to an appreciation of a local conservation of energy. The actual Poynting Vector is more 'useful' imho for things like NIMs. Poynting's Theorem follows from deeper symmetries though - the conservation of the stress-energy tensor.

I bet you say that to all the girls.

 

[Evenflow]

Electrons travel at a brisk walking pace down a wire
true

 

[marinyork]

Emily Noether was a woman.

 

[presta]

Electrons travel at a brisk walking pace down a wire
true

Not by a very long chalk. As I said above, the speed is on the order of microns per second, not metres per second. For example, with a one amp current in a 1mm diameter wire the electrons are travelling at 92 microns per second, about 17,000 times slower than walking pace.

 

[TVC]

AC and DC transmission is interesting and something I am working on at the moment. Most power transmission is AC, but there are losses in the power lines due to the nature of the mutually generating magnetic and electric fields, leakage and partial discharge. DC transmission is much more efficient as the only factor to account for in Resistance (V=IR from schoolbooks). However only AC is generated, and all loads work on AC so you have to convert AC to DC, squirt it down the line and then change it back to AC at the other end. This conversion involves losses and with the present state of technology it is more efficient to transmit in AC for distances less than 1000km. High Voltage DC (HVDC) systems are used for transmitting power over long distances - for example in thr three gorges project in China where the supply is required 3000km away. HVDC is also used for power transmission under water where the permittivity of water is much higher than air leading to greater losses.The links between the UK and Europe are DC because they travel under water, and because if they were AC you would have to synchronise the phasing of the two systems.

AC to DC conversion is done in huge valve halls and is a bit more complicated than your basic rectifier bridge.

Each of the valves is built around a thyristor bank that is optically switched, I do the saturated reactors that limit the inrush current to the thyristors - they are the cast coil assemblies at each end of the valve. The whole footprint is about the same size as a caravan.

I realise my audience for this information is most likely limited to two people.

 

[mustang1]

Potentially.

Ohm iGod that was brilliant!