A Current-Carrying Conductor

[Viona]

Hello,
In the section of Magnetic Force on a Current- Carrying Conductor in the book of College Physics by Serway, it is written that the Current- Carrying Conductor in a magnetic field deflects because the magnetic force on the electrons transfers to the bulk of the wire due to the collisions between the electrons and the atoms. I am wondering why this does not happen when an electric current passes in a wire? Why the electric force does not transfer to the bulk of the wire due to the collisions of the electrons with atoms and as a result the wire moves in the direction of the electric force?

 

[DaveE]

Summary:: Why a current-carrying wire does not move in the direction of the electric force?

Hello,
In the section of Magnetic Force on a Current- Carrying Conductor in the book of College Physics by Serway, it is written that the Current- Carrying Conductor in a magnetic field deflects because the magnetic force on the electrons transfers to the bulk of the wire due to the collisions between the electrons and the atoms. I am wondering why this does not happen when an electric current passes in a wire? Why the electric force does not transfer to the bulk of the wire due to the collisions of the electrons with atoms and as a result the wire moves in the direction of the electric force?

It does experience that force. However, it's a small effect and it is unheard of to have an unsupported wire carrying current, so there's no motion to see. In a solid metal, the electrons aren't really colliding with atoms much at all, It is more like they are shared in the "outer" conduction band, passed from atom to atom with essentially no disturbance.

 

[willem2]

Why the electric force does not transfer to the bulk of the wire due to the collisions of the electrons with atoms and as a result the wire moves in the direction of the electric force?

If the wire is neutral, there will be an opposite force on the rest of the wire, and the net force will be 0, so no movement anywhere. If the wire is negatively charged. The force on the electrons will be bigger than the force on the force on the rest of the wire, and the electrons will pull along the rest of the wire.
This is a different situation than with magnetic forces. because with magnetic forces, because the charges in the rest of the wire do not move with the current, so there will be no magnetic force on them.

 

[Lord Jestocost]

Summary:: Why a current-carrying wire does not move in the direction of the electric force?

I am wondering why this does not happen when an electric current passes in a wire? Why the electric force does not transfer to the bulk of the wire due to the collisions of the electrons with atoms and as a result the wire moves in the direction of the electric force?

Moving charges transfer momentum to the conductor that carries the current. Macroscopically, this effect might be to too small to notice it at all when using conventional wires. Microscopically, the momentum transfer due to moving charges can manifest itself in the electromigration phenomenon.

 

[sophiecentaur]

It does experience that force. However, it's a small effect

I have thought of this problem before and I was wondering about what might happen in a trough of mercury. I would imagine that the bulk of the material, which would consist of positive mercury ions would experience a force towards the negative end (incredibly small field , though) and the electrons would, of course, be attracted to the positive end. Question is would it be possible to measure the slope of the displaced volume of the liquid, held down by its weight.
This effect could be created (even observable) if a suitable conducting fluid with higher resistance and hence a greater PD / field across it.

 

[phyzguy]

I am wondering why this does not happen when an electric current passes in a wire? Why the electric force does not transfer to the bulk of the wire due to the collisions of the electrons with atoms and as a result the wire moves in the direction of the electric force?

It does. One of the failure modes in modern integrated circuits is that over time collisions of current-carrying electrons with metal atoms in the wire eventually move enough atoms to create a gap in the wire and cause an open circuit. This effect is called electromigration.

 

[DaveE]

I have thought of this problem before and I was wondering about what might happen in a trough of mercury. I would imagine that the bulk of the material, which would consist of positive mercury ions would experience a force towards the negative end (incredibly small field , though) and the electrons would, of course, be attracted to the positive end. Question is would it be possible to measure the slope of the displaced volume of the liquid, held down by its weight.
This effect could be created (even observable) if a suitable conducting fluid with higher resistance and hence a greater PD / field across it.

Or a plasma, perhaps. This is a thing with DC excited plasmas, like the gas ion lasers I worked on. However, I'm a little fuzzy on the ballistic (momentum) issues since they are really small compared to the strong E-field effects. We called it "gas pumping" and it was a significant engineering issue with the design of high power laser tubes. BTW, not really my job, I was an EE, not a physicist.

 

[DaveE]

It does. One of the failure modes in modern integrated circuits is that over time collisions of current-carrying electrons with metal atoms in the wire eventually move enough atoms to create a gap in the wire and cause an open circuit. This effect is called electromigration.

I suspect the "migration" part of the movement is really due to the imposed E-filed. I think the ballistic part is the disruption of the mechanical (xtal) structure that allows the ions to move. OTOH, I definitely could be wrong. There are significant forces to keep the ions in place that don't apply as much to the electrons, so there would be a greater imbalance (+ vs - charges) in the momentum than you would see in a liquid or gas.

This is also why DC incandescent lamps don't last as long as AC lamps.

 

[sophiecentaur]

Or a plasma, perhaps.

The low density would make a big difference because the velocity and mobility of charge carriers would be (could be) higher.

 

[DaveE]

The low density would make a big difference because the velocity and mobility of charge carriers would be (could be) higher.

Yes, plus the number of charged particles is much greater. Sort of like a bunch of separate billiard balls, not really like a metal at all.

 

[phyzguy]

I suspect the "migration" part of the movement is really due to the imposed E-filed. I think the ballistic part is the disruption of the mechanical (xtal) structure that allows the ions to move. OTOH, I definitely could be wrong. There are significant forces to keep the ions in place that don't apply as much to the electrons, so there would be a greater imbalance (+ vs - charges) in the momentum than you would see in a liquid or gas.

This is also why DC incandescent lamps don't last as long as AC lamps.

No, I don't think so. It is primarily momentum transfer from the electrons to the metal ions that eventually move the metal ions off their lattice sites. It is one of the main reasons why modern ICs have transitioned from the use of aluminum to copper interconnects. The copper atoms are heavier and the binding forces between the copper atoms are stronger, so the electromigration lifetimes are much longer.

 

[sophiecentaur]

the number of charged particles is much greater.

Fewer than in a metal, where we're talking in terms of 10²³ - in a lump of metal. That's where the mm/s drift speed comes from.

 

[DaveE]

Fewer than in a metal, where we're talking in terms of 10²³ - in a lump of metal. That's where the mm/s drift speed comes from.

Yes. I phrased that wrong. I meant the percentage. A higher proportion of free charged particles in plasma collisions. More discharge than conduction. Usually higher fields and greater mean free path so momentum is greater. Also ions are more free to move too. It's really a different thing, kind of off-topic, I think, since the OP question was about conductors.

 

[Viona]

It does experience that force. However, it's a small effect and it is unheard of to have an unsupported wire carrying current, so there's no motion to see. In a solid metal, the electrons aren't really colliding with atoms much at all, It is more like they are shared in the "outer" conduction band, passed from atom to atom with essentially no disturbance.

Thank you for the useful explanation.

 

[Viona]

It does. One of the failure modes in modern integrated circuits is that over time collisions of current-carrying electrons with metal atoms in the wire eventually move enough atoms to create a gap in the wire and cause an open circuit. This effect is called electromigration.

This new information for me! Thank you!

 

[Viona]

Thank you all for the explanation and useful discussion.