Physics question

[brightncheerful]

I am researching information about electromagnetism for an article I'm writing about energy.

At school, I was useless at physics (in 0-level, I got 21 marks out of 250), so my technical knowledge of the subject is poor so I want to be reasonably sure that my understanding of the terminology is broadly correct.

Using the example of an electrical gadget having wire and plug and the electricity mains power supply socket, I have read that while the 'uninformed' (layperson) would think that when the gadget is plugged in to the socket and switched on the electric current flows from the plug through the wire into the gadget, a physicist would think that the energy flow occupies the space around the wire and that the wire itself simply guides or conducts the direction of the flow.

Is that broadly correct and if so then what words would better describe the process so that when I explain to an educated layman what I'm writing about it can be understood, even if the technicalities themselves are beyond the reader's comprehension.

When you reply, (hopefully you will!) I'd also appreciate knowing whether correct to say that the thicker the wire the less the resistance because the resistance itself depends upon how much energy can be steered in a particular direction.

tia
Bnc

[88V8]

Hello from another Physics dimwit... but it is true that current flows around the outside of a cable, not through it. This is why multi-core cable has a higher capacity than single core, because it has a larger perimeter.
http://physics.stackexchange.com/questi ... e-interior

This half-recalled nugget came from an article years ago, about British Rail needing to upgrade its oil-cooled power cables.

It's a great gift, being able to simplify the complex.

V8

[gryffron]

"a physicist would think that the energy flow occupies the space around the wire and that the wire itself simply guides or conducts the direction of the flow. "

I'm not sure where you got this from? But I don't agree. Whether you ask a physicist or a chemist, the energy flow is due to the movement of the electrons in the conductor. So the energy is IN the wire.

If the energy flow was outside the wire, this would imply you could block it with some sort of shield around the wire. Which is clearly not the case.

Thicker wire = more loose electrons available, so they move slower to transfer the same energy. Same as a wider river carries more water.


AC current has an inductive effect. It creates a magnetic field around the conductor which resists the flow of the electricity. This effect is too small to bother about in a domestic appliance. But does have an effect in more extreme cases.

Eg1: Very large supergrid cables carrying thousands of amps have a steel inner cable for strength, and copper around this. Because of the inductive effect there is virtually no current in the centre of the wire. It is all pushed to the edges. Hence the steel part does not affect conductivity. This effect makes it very hard to transmit high current over large distances. You might have thought a wire 10x the surface area would transmit 10x as much current - but (for AC) it won't because no current flows in the middle. Hence you use a transformer - less current, more volts. It is much easier to handle very high volts than very high current.
[EDIT] this is the effect 88V8 is talking about. But it still doesn't mean you are transmitting power outside the wire, just at the outer edges of it[END]

Eg2:When you start talking about very high frequencies, radar, microwaves etc. The inductive effect becomes so significant it can break out of the wire, and continue in a straight line as radio waves. Such high frequencies are forced to use wave guides rather than wires. Which behave more like fibre optic cables than wires, reflecting the energy along their length, rather than conducting it. It still doesn't transmit energy outside the conductor.

gryff
A level physics & chemistry

[GeoffF100]

I speak as someone with high qualifications in physics, but from a long time ago. Electricity is the flow of electrons. The come in through one wire and return through the other. They flow inside the conducting part of the wire. Mains electricity is alternating current. The direction of flow changes fifty times per second in this country. A thicker wire does have less resistance. Resistance = resistivity x length / area. At high frequencies the current flows near the surface of the wire, due to electromagnetic effects.

[Generali]

AIUI, electricity flows through a cable with free electrons in the ions that make up the cable kinda acting like a bunch of billiard balls bouncing off each other but pushing each other forwards.

If you use mains (AC) current then most of the current will flow near to the surface of the wire but still within the wire. The current running through the wire will induce a magnetic field outside the wire but the current still flows within the wire. DC current runs evenly through the wire.

[quelquod]

The energy flow is within the conductor. This creates an electromagnetic field around the conductor dependent on the amount of current flowing, if the current alternates so does the field and radiates as radio waves. At 50Hz mains frequency and normal household currents the field is insignificant but more to the point in mains cables there are 2 conductors which carry current in opposite directions. In this case the field from each is also opposite and substantially overlaps and for practical purposes cancels so there's nothing much outside the cable. Interestingly for power lines, substations and the like the currents and fields are much stronger and some people claim to be affected by them.

The tendency for alternating current to crowd to the outside of a conductor at right angles to the direction of flow is commonly known as 'skin effect' since the conductor behaves as though it had a more highly conducting skin. The depth of the effect (ie the useful part of a conductor) shrinks as the frequency rises, varies with the material of the conductor, and is significant at radio frequencies. At 50Hz in copper the skin is around 9mm deep and so for conductors thinner than around 2cm thick it can be ignored which generally covers domestic cables. As noted earlier though for really thick cables the effect needs to be accounted for even at 50Hz and there's no disadvantage in using a stronger though less conductive core in such cables.

As a by-the-way, at terrestrial TV frequencies the skin effect is only around 3 microns deep (there are 1000 microns in a millimetre) so the effect can be very troublesome.

Because Power = Current x Voltage, and current is affected by the capacity of the cable, our power grid runs at a high voltage to keep the current requirements and thus the losses lower for a given amount of power transferred.

[csearle]

I agree entirely with quelquod's description of it. As an aside, these days we are obliged to install metal consumer units in domestic premises (when near escape routes). We have to ensure that the two live conductors of any given circuit pass through the same hole in the box so that the external alternating fields around the individual conductors don't induce eddy currents into the metal enclosure, which would waste energy in the form of heat.

Regards,
Chris

[Lootman]

Thicker wire = more loose electrons available, so they move slower to transfer the same energy. Same as a wider river carries more water.

Yes I always found it helpful to understand electricity in terms of water flowing through a pipe.

Voltage = water pressure
Current = actual rate of flow
Resistance = loss due to friction and laminar drag
Wire - pipe

The analogy collapses at some point but, as the saying goes, it's good enough for government work.

[Alaric]

The analogy collapses at some point but, as the saying goes, it's good enough for government work.

The underlying mathematics of fluids is very similar to that of electromagnetism.

[Hardgrafter]

There is also a field around Direct Current conductors. For Field read ' Energy' of some sort. Very small compared to AC conductors, but its there. Tha'ts how we measure DC Current over a few Amps using a 'Clamp Meter' that you put around the cable. It uses something called the 'Hall Effect'. AC current is also measured that way, but I know its not called 'Hall Effect'. What it is, is something else. Look up Wikipedia? This month they want subscribers, so chip in if you can.

[quelquod]

The field strength around a DC conductor is the same as the instantaneous field strength around an AC conductor for a given current. It's a static equation. The only reason that it doesn't 'radiate' in the normal RF sense is that being static there's no information in it.
I thought that AC clamp meters were nothing more than a 1-turn current transformer?

[quelquod]

The analogy collapses at some point ...

I suppose one point of collapse is that fluids generally flow more poorly at the extreme edges of their conductor whereas (AC) electrons seem to do better there.

[brightncheerful]

The energy flow is within the conductor. This creates an electromagnetic field around the conductor dependent on the amount of current flowing,

Fascinating and very helpful, thank you. I only wish I could grasp understand the terminology but I daresay once I continue writing my article more will sink in.

I have highlighted the quote because I'm wondering if that's correct. although it might've been answered by ensuing comment(s).

My wonder is that surely the electromagnetic field is all around regardless. Since (as I understand) the Sun is the primary source of electromagnetic energy on planet Earth, surely the whole of the Earth is affected? That would explain, not necessarily in a scientific way. why some people are sensitive to electromagnetic energy in the (human) atmosphere. Without wanting to give much away, the project I'm writing the article for concerns the impact of positive and negative vibes between human beings. I am working on the assumption that our natural ability to conduct (electromagnetic) energy can be used positively or negatively depending upon the circumstances and our deliberation. Since 'deliberate' is to do with removing freedom, the freedom that electromagnetic energy would enjoy if left to its own devices can be interfered with.

[csearle]

The analogy collapses at some point...

Yes for example if a pipe breaks you get a wet floor.

Chris

[csearle]

Another effect of the electromagnetic fields around wires is that if the current in the wire is interrupted, e.g. a filament (remember them?) in a lamp breaks, then the fields collapse and in doing so induce a current back into the wires. This causes a build-up of voltage at the break, which can (and often does) cause an arc across the break. This arc bridges the gap with a resistance lower than the filament, which makes the remainder of the filament glow brighter. This process continues progressively (and very rapidly) causing a flash and the circuit breaker to trip through excessive current.

Regards,
Chris

PS That is my take on it. I'd be happy to accept corrections if any are needed because I've been preaching this for quite a while now and don't really want to be wrong.

[gryffron]

My wonder is that surely the electromagnetic field is all around regardless. Since (as I understand) the Sun is the primary source of electromagnetic energy on planet Earth, surely the whole of the Earth is affected? That would explain, not necessarily in a scientific way. why some people are sensitive to electromagnetic energy in the (human) atmosphere.

Loads of things generate electromagnetic fields. The sun, the stars, the earth itself, radio transmitters, every electrical conductor to some extent. Millions of radio waves are bombarding you right now. Some of these are significant, some tiny. Mostly, humans are good at detecting only one source of electromagnetic energy - visible light. By and large, the rest go right through us unnoticed.

Humans of course, being made up of conductive fluids, and using electrical signals for nerve impulses, generate very small electromagnetic fields ourselves. Some of them large enough to measure with the right equipment. The heart (ECG) being the most obvious example. But you can measure other muscle commands, and even brainwaves (EEG).

Some creatures, most notably sharks, can detect other living creatures by this electromagnetic field, and could certainly find a large creature like a human by such means. I don't believe anyone has ever demonstrated such an ability in humans, to even detect the presence of another, let alone read anything from variations. But hey, it's your story.

gryff

[stewamax]

As far as AC is concerned, a good analogy is a pipe full of billiard balls. If, at one end, you move the exposed ball backwards and forwards quickly, the impulse will travel from one ball to another and the ball at the other end will vibrate. But the average position of all the balls doesn’t change. Unlike DC - where the equivalent balls would travel down the pipe – power (and information) travels down the pipe without the electrons themselves doing so.
And if you vary the amount or frequency of shaking the ball, you send amplitude modulated (AM) or frequency modulated (FM) information to the ball at the other end.

[swill453]

Without wanting to give much away, the project I'm writing the article for concerns the impact of positive and negative vibes between human beings. I am working on the assumption that our natural ability to conduct (electromagnetic) energy can be used positively or negatively depending upon the circumstances and our deliberation. Since 'deliberate' is to do with removing freedom, the freedom that electromagnetic energy would enjoy if left to its own devices can be interfered with.

Do you have any scientific evidence for your assumption at all?

Is the assumption affected by the knowledge that your understanding in the opening post turns out to be entirely wrong?

Scott.

[brightncheerful]

Thank you for your comments, all very helpful.


OT:

Do you have any scientific evidence for your assumption at all?

Why would i need any?

[gryffron]

Do you have any scientific evidence for your assumption at all?

You don't need evidence for a good story. You can start with any crazy concept, like there are wizards living amongst us, or there's another world on the other side of a mirror. All great stories.

You just need to be consistent. Whilst the world's greatest scientists can still be entertained by silly concepts like the above, even small children can spot inconsistencies in your made up world. And they quickly ruin the story, even for kids.

gryff