Why wire experiences force in magnetic field, when net charge is zero.

Smythe

I was doing some reading on current-carrying wires.

In a current-carrying wire, the total net charge in the metal is zero. So why does the wire experience a force when placed in a magnetic field?

Thank you!

TheChizler

The current travelling in the wire creates its own magnetic field which interacts with the external one. Depending on the relative angles of the fields, there will be a resulting force acting on the wire.

Smythe

Thanks TheChizler!

Smythe

Could it also be said the reason for the force is the motion of the electrons, not their charge as such. i.e. a relativistic effect due to their motion.

TheChizler

I'm not entirely sure what you mean by that, could you explain further? By far the predominant effect is the electromagnetic force - Ampere's Law.

Smythe

Well, apparently the non-relativistic way of explaining it is to talk about the interaction of the applied magnetic field with that due to the moving charges.

The relativistic approach is is to talk about the moving charges experiencing a length contraction causing them to appear to have a larger charge density than if they were stationary and then explaining it in terms of the electrostatic force.

FISMA

Smythe wrote: »

The relativistic approach is is to talk about the moving charges experiencing a length contraction causing them to appear to have a larger charge density than if they were stationary and then explaining it in terms of the electrostatic force.

Hmmm, not sure, but I highly doubt it.

Electricity moves slowly, I think on the order or mm/s - or something like that. I'll have to look up drift velocity again, but I know it is slow.

The effect of turning on a switch leaves us with the distinct impression that electrons are moving fast, close to light speed fast. But that's just wrong. Nothing is moving fast at all.

Correct me if I am wrong but when it comes to electricity, no thing is traveling anywhere near the speed of light. Thus, I don't see how length contraction would be applicable.

krd

FISMA wrote: »

Electricity moves slowly, I think on the order or mm/s - or something like that. I'll have to look up drift velocity again, but I know it is slow.

This is something that kind of annoys me - since I've been trying to find out for years.

When a current moves through a copper wire, is there really any movement of electrons at all (in other words, the electrons in the copper orbit their orbit), or simply is a charge imbalance (between protons, and the orbital electrons) been passed from one end of the wire to the other.

I understand with Ampere's Law, the charge resides on the surface - so I would assume the current travels along the surface. But at the same time, for me at least, that would seem to contradict the idea of resistivity in the medium/conductor.

Do the copper atoms reach an agreement on the charge, and then transfer it to the surface atoms, as a summed electric field............Or what is happening? Are electrons things in themselves or are they to be thought of as something like ripples in a pond - though their dechorence value is a specific quanta.


Are all the theories in agreement - or is it a mix of a bit of this here, and a bit of that there.

TheChizler

It is the electrons travelling all right. Semiconductors work by actual movement of electrons into materials. Current in a wire is like pushing marbles through a tube, to paraphrase QI. When you push one in slowly, almost instantly another pops out the other end.
It is true that most charge/free electrons exist on the surface of a conductor, but there is also some distribution through the material.

krd

Where do the electrons come from, when you induce them in a copper coil with a moving magnet?

I looked at wikipedia, and it said electrons are a fundamental particle - likely created at the time of the big bang.

Is there a finite supply of electrons in the universe? When electricity is being generated are the electrons coming from somewhere? Obviously they're coming from somewhere - but I was wondering where the atoms magicing them up in response to changing magnet field.

TheChizler

krd wrote: »

Where do the electrons come from, when you induce them in a copper coil with a moving magnet?

I looked at wikipedia, and it said electrons are a fundamental particle - likely created at the time of the big bang.

Is there a finite supply of electrons in the universe? When electricity is being generated are the electrons coming from somewhere? Obviously they're coming from somewhere - but I was wondering where the atoms magicing them up in response to changing magnet field.

Look up the structure of an atom on Wikipedia. An atom has a certain number of electrons, and sometimes some of these are free to move. A conductor has more free electrons than an insulator. So in a copper wire there are many free electrons that can be easily moved from one atom to another when a potential difference induces them to move.

krd

TheChizler wrote: »

Look up the structure of an atom on Wikipedia. An atom has a certain number of electrons, and sometimes some of these are free to move.

Believe it or not. I have a third level qualification in physics and electronics (not a great one - but I did study it - I may do a little more in the future, if they let me).

I know what I'm asking kind of sound like silly questions. When I was studying electrons/electricity, many of the explanations were incomplete or not even that right. Maybe there isn't a clear answer. My maths is nowhere near up to having a crack at the standard model.

A conductor has more free electrons than an insulator. So in a copper wire there are many free electrons that can be easily moved from one atom to another when a potential difference induces them to move.

I knew the explanation for conductivity in copper - the valency electrons flow - I'm not sure if that description is correct. Copper's valency electrons are not on the the outer shell - one reason it's not that reactive. Two copper atoms side by side - when electricity flows through the wire of the conductor they're in, do actual electrons hop from one copper atom to the other. Are the electron "holes" in the shells with the valency electrons occupied.

On an atomic level, what is potential difference. Why does an atom of copper say to it's neighbour "hey, you're different, have an electron"

Why does a copper atom experiencing a changing magnetic field, feel the need to pass and electron to its neighbour.

TheChizler

My apologies, I didn't know were you were academically, only finishing up my Electrical & Electronic Engineering degree at the moment myself. If you want the more mathematical explanation you'll have to do a bit of reading on Electromagnetic Field Theory, I only paid enough attention to pass the exam . I've always found it effective enough to just know that like charges repel. The valence electrons are pushed/pulled along from atom to atom by unbalanced charge reservoirs at either end, essentially. To use the analogy of a pressurized air tank at one end of a hose, molecules are induced to move from areas of high pressure to low pressure. Essentially electrons are pushed from atom to atom by the 'pressure' of a high charge. The only place they can go is an area of low charge. The potential difference is the relative difference in charges between these two areas.
It's like you have a tube full of magnets with opposite poles pointing at each other, if you put a large magnet at one end it'll push the others through.

krd

TheChizler wrote: »

I only paid enough attention to pass the exam .

I know, I've been in a similar situation, and know a few others who've been there too. It is possible, to do an exam and get every question right, without understanding a single one, just knowing how to give the right answers.

I've always found it effective enough to just know that like charges repel. The valence electrons are pushed/pulled along from atom to atom by unbalanced charge reservoirs at either end, essentially. To use the analogy of a pressurized air tank at one end of a hose, molecules are induced to move from areas of high pressure to low pressure.

For the purposes of most electrical and electronic applications, you can think of electricity as a pressurized fluid. (of course it is not - but it's easier to follow it if you think of it as plumbing).

And the idea of metals like copper being conductors, and plastics being insulators. And the idea of the flow of electricity being the flow of electrons

But it's not that simple. Something like an Ethernet cable - when it's crimped, when the wires are placed in the head, the pins in the head never penetrate the plastic insulation of wires. Apparently. Through quantum tunnelling, the electrons are able to flow across the insulation. What is crossing the gap? Phonons? Electrons?