Absolute Reference Frame

[Prosoothus]

If you took two identical parallel wires, seperated by a small distance, and you sent the same amount of direct current through both of them (in the same direction), would the wires attract each other, repel each other, or have no effect on each other? Why would they react that way?

 

[przyk]

I know almost nothing about electromagnetism (I've recently started studying it), but I think its quite well known that the magnetic fields created by like charges travelling parallel to one another cause them to attract one another, so the wires attract one another (I've seen this for myself).

Usually you'd consider this in a reference frame in which the wires were stationary. In a reference frame in which the electrons are stationary and its the wires that are moving, the magnetic fields that pull the wires together is created by the moving protons (if that's what this thread is getting at).

 

[Prosoothus]

przyk,

Usually you'd consider this in a reference frame in which the wires were stationary. In a reference frame in which the electrons are stationary and its the wires that are moving, the magnetic fields that pull the wires together is created by the moving protons (if that's what this thread is getting at).

So, if you have negatively charged wires with currents flowing through them, they wouldn't attract each other since there would be an equal amount of protons and electrons moving, right?

What if you replace the two wires with two parallel beams of electrons travelling through a vacuum. Would they attract each other, or not?

 

[przyk]

So, if you have negatively charged wires with currents flowing through them, they wouldn't attract each other since there would be an equal amount of protons and electrons moving, right?

Just checking: you're suggesting a wire with no net charge or current, and then adding extra electrons flowing through it?

You'd get the repulsive effects of the net charges of the wires in this case. They'd be the only effects in a frame that "followed the current" (the wire is moving). You'd get the magnetic attraction competing with this in a frame attached to the wire. I suppose this is compensated for in SR by length contraction increasing the charge density, or something like that.

What if you replace the two wires with two parallel beams of electrons travelling through a vacuum. Would they attract each other, or not?

If the beams were stationary, they'd repel (because negative charges repel). Once the beams are moving you get the magnetic effects in addition, which reduces the repulsion. Presumably (I'm guessing about theories I don't really understand yet), the magnetic attraction only becomes strong enough to cancel the electric repulsion at c. If this canceling of the electric and magnetic fields happens before the electrons reach c, you've got a contradiction in SR, otherwise its just time dilation.

 

[Prosoothus]

przyk,

If the beams were stationary, they'd repel (because negative charges repel). Once the beams are moving you get the magnetic effects in addition, which reduces the repulsion.

How would there be magnetic effects since the electrons in the first beam are stationairy relative to the electrons in the second beam? Are you suggesting that magnetic fields created between two moving electrons are not due to their relative motion, but to some sort of absolute motion?

 

[przyk]

No, I'm suggesting the magnetic field is frame dependent. As far as I know it all boils down to the forces between charges depending on their velocities in any given frame. I don't know if the magnetic field is anything more than just the deviation velocities give from electrostatic forces - I've heard magnetism described as the relativistic effects of electricity. Apparently its supposed to fit quite neatly into SR, and I'm curious to see how this works.

Gonna have to see what people who know more about this than I do have to say... (Pete? Dale? Help!)

 

[cato]

this might help:
http://en.wikipedia.org/wiki/Relativistic_electromagnetism

edit: this also:
http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/magnetism/wiresmagnetism.asp

 

[DaleSpam]

You are correct przyk, the magnetic and electric fields are frame dependent. In fact, the kind of "two wires" experiment that Prosoothus is proposing is the typical way that relativistic electromagnetic concepts are introduced. The basic result is that different frames will disagree on the electric and magnetic fields, but they will agree on things like the magnitude of forces etc.

-Dale

 

[Prosoothus]

DaleSpam,

You are correct przyk, the magnetic and electric fields are frame dependent. In fact, the kind of "two wires" experiment that Prosoothus is proposing is the typical way that relativistic electromagnetic concepts are introduced. The basic result is that different frames will disagree on the electric and magnetic fields, but they will agree on things like the magnitude of forces etc.

Do you care to answer my question? If you replace the two parallel wires, with two streams of moving electrons in a vacuum, would there be an attractive force between the two streams?

In cato's link, it shows how the wire problem is solved in relativity using the protons in the wire . In the electron stream problem I provided, there are no protons.

 

[James R]

If you replace the two parallel wires, with two streams of moving electrons in a vacuum, would there be an attractive force between the two streams?

Yes.

 

[DaleSpam]

Do you care to answer my question? If you replace the two parallel wires, with two streams of moving electrons in a vacuum, would there be an attractive force between the two streams?

In cato's link, it shows how the wire problem is solved in relativity using the protons in the wire . In the electron stream problem I provided, there are no protons.

Here are some related links where scenarios involving moving charges are considered, not just wires with both kinds of charge carriers:
http://instruct.tri-c.edu/fgram/web/E&M&Rel.htm
http://www.newton.dep.anl.gov/askasci/phy00/phy00993.htm
http://www.phys.unsw.edu.au/einsteinlight/jw/module2_FEB.htm
http://hepth.hanyang.ac.kr/~kst/lect/relativity/x850.htm

My favorite summary is, of course, in Wikipedia: http://en.wikipedia.org/wiki/Magnetic_field

"A thought experiment one can do to show this is with two identical infinite and parallel lines of charge having no motion relative to each other but moving together relative to an observer. Another observer is moving alongside the two lines of charge (at the same velocity) and observes only electrostatic repulsive force and acceleration. The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an attractive force, in a classical physics context, that reduces the electrostatic repulsive force and also that is increasing with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context."

I am sure you can find more if you look around.

-Dale

 

[CANGAS]

About twenty five years ago I was trying to jump start my 73 Transam which was not enthusiastic about turning over. A little rust in the bores? Maybe.

KNOWING WHAT I WAS DOING ( don't, don't, don't do it unless you know too ), I re engineered the Transam battery connections so as to safely jump from another battery and present 24 volts to the starter. I was momentarily amazed to see the jumper cables writhe like snakes fighting. A brief analysis made me remember the physics involved, having been learned(?) in freshman class.

The concept that currents moving parallel make wires attract is in fact the basis of the definition of the Ampere, whereby a specific current within wires a specific distance apart and a certain length result in a specific force observed between the wires.

In conductors such as copper jumper cables the electric wave travels surprisingly close to speed of light in vacuum. However the actual velocity of the moving electrons ( not the drift velocity ) is impressive but very often not a substantial percent of c. The attraction or repulsion of parallel or antiparallel electric currents is very active and measurable at velocities far below c.

Does there seem to be a paradox between observed currents and their forces when analyzed according to Special Relativity? I certainly think so. Stationary relative to an interested observer, the 24 volt cables jumped apart like scared snakes, with electrons moving far slower than c.

If altering the situation a bit to have parallel current, the jumper cables should equally violently leap upon each other. If the observer were moving at electron speed, far less than c, he or she would see the cables move apart or sit still? If the observer were moving at electron velocity, the ionic nuclei "stationary" in the lattice would be relatively moving with respect to the "actually" moving observer and so the observer would see an attraction between ionic nuclei in the two parallel cables.

But, if we consider two parallel beams of electrons sans conductor, traveling at a modest part of c, in the opinion of the observer, there are no ionic nuclei to muddy our water. If the two cohorts of electrons are "really" stationary in the form of static charges they will repel. But what if the observer is moving by the stationary electrons at the SAME velocity of the electrons as if they were in a cable, a very modest part of c, and bereft of the complicating issue of ionic lattice nuclei? The still observer will first see a repulsion, then at a certain speed see no mutual force, then at higher speed see an attraction?

Is this really the observation of laws of physics working the same way at all relative velocities? Repulsion, then stillness, then attraction, all depending only on the VELOCITY OF THE OBSERVER?

 

[DaleSpam]

Does there seem to be a paradox between observed currents and their forces when analyzed according to Special Relativity? I certainly think so. Stationary relative to an interested observer, the 24 volt cables jumped apart like scared snakes, with electrons moving far slower than c.

If you think there is a paradox here I strongly encourage you to work out the math. Either you will be able to demonstrate conclusively that there is a paradox or you will see exactly why you were mistaken.

-Dale

 

[Prosoothus]

DaleSpam,

The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an attractive force, in a classical physics context, that reduces the electrostatic repulsive force and also that is increasing with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context.

"Observes the repulsive acceleration of the lines more slowly"????

"reduction of repulsive acceleration can be though of as an attractive force"???

Oh God, that must be the most far-fetched piece of crap I've read on sciforums in the last four years. It would even make a crackpot blush.

If relativity doesn't work, don't abolish it. Just patch it up with wierd pseudo-forces. :bugeye:

 

[DaleSpam]

What's wrong with it? Have you never heard of vector addition?

---> = -----> + <--

If you understand the concepts but don't like the wording then take it up with Wikipedia. I personally thought it was clear and well-expressed.

Btw, you apparently misunderstood the paragraph. The "pseudo-force" presented here is to explain things in a classical context (in terms of separate electric and magnetic fields) and is not necessary for the more unified SR approach.

-Dale

 

[Prosoothus]

DaleSpam,

The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees.

The author is complicating the issue to satisfy relativity. He/she included motion when that's not the issue. The issue is the force of attraction between the electrons, and not the acceleration or motion caused by the force.

Plus, he/she describes how the repulsive force seems to decrease for the observer due to time dilation, but never explains how time dilation can cause the repulsive force to become attractive.

 

[DaleSpam]

The author is complicating the issue to satisfy relativity. He/she included motion when that's not the issue. The issue is the force of attraction between the electrons, and not the acceleration or motion caused by the force.

How is motion not an issue in discussing magnetism? Magnetism is a force exerted specifically on moving charges and caused by moving charges. In any case, saying that the force is reduced is equivalent to saying that the acceleration is reduced for any massive particle (and vice versa).

Plus, he/she describes how the repulsive force seems to decrease for the observer due to time dilation, but never explains how time dilation can cause the repulsive force to become attractive.

You need to think things through a bit before you just revlexively dismiss SR.

In classical physics when the particles are stationary there is only a repulsive electric force, as the particles begin to move there begins to be an attractive magnetic force. This magnetic force is initially weaker than the electric force so that the net force is repulsive. At some velocity the forces balance. The force becomes repulsive only at greater than that velocity.

In the SR view is substantially the same. At rest there is a repulsive electromagnetic force. As the particles move faster the repulsive electromagnetic force becomes weaker until, at a particular velocity, they are equal. What is that velocity? As you might suspect that velocity is c. You could certainly, in theory, get an attractive electromagnetic force between two like-charged particles traveling faster than c.

-Dale

 

[CANGAS]

In our everyday observations of electric currents and such like, the conductors are considered by scientists to be composed of the atoms locked into their lattice positions and freely roaming electrons. It is considered that each lattice atom donates one electron to an electron "gas" in which an individual electron flits from atom to atom with a velocity called a thermal velocity. This is about 10EX6 meters per second, and in every random direction. Under the influence of an EMF, such as the 24 volt electric wave provided by a battery of 12 volt car batteries, the flitting electrons gain a very small velocity vector governed by the direction of the electric wave, or, field. This small additional velocity is called the drift velocity. It is about one meter per hour in everyday situations like jumping 24 volts into a 73 Transam starter. The net, drift, velocity is the pertinent velocity when calculating induced magnetism. This is proven by referring to any physics textbook on the planet.

The official definition of the ampere is based exactly upon the very measurable force exerted by one current, at drift velocity, upon another current, at drift velocity, in a parallel wire. The spectacular observation of jumper cables jumping like excited snakes is based exactly upon the very observable force exerted by one current, at drift velocity, upon another current, at drift velocity, in a parallel wire.

So, we have static charges which, in two adjacent objects, repel when the objects themselves are static ( pun fully intended ). And when the same amount of charge is moving at one meter per hour ( how close to c is one meter per hour? ) in parallel direction in two objects, the objects attract strongly and measurably enough to be the official definition of the ampere.

By changing the velocity from zero to one meter per hour we change from observing charges repelling each other to where the same amount of charges are attracting each other.

Pardon me if I'm missing something important, but, what is the Relativistic connection between drift velocity at one meter per hour and c at something faster?

 

[DaleSpam]

Hi CANGAS,

Over the course of my academic carreer I worked thousands of problems. Today I remember very few of those individual problems. One of them that I do specifically remember went something like this:
Consider two 1mm diameter droplets of water. One percent of the water molecules in one droplet have an extra electron, and one percent of the water molecules in the other have one electron too few. Calculate the force between the droplets in metric tons.

There were two things that surprised me while working out this problem. The first was how many charge carriers there actually were in this case. Even though the droplets were only 1mm diameter and even though only 1% were ionized there were still a huge number of ions, way beyond trillions. The second was the enormous force that resulted. The result was in the thousands of metric tons.

So how is this relevant to your point? Well, as discussed here already magnetic forces are simply electrostatic forces taking into account the relativistic effects of the moving charges. Although the drift velocity may be slow there is still some time-dilation and length-contraction resulting in a small fractional increase in the number of moving charge carriers per unit length in the non-moving frame. Though this fraction is small the total number of charge carriers may still be surprising large. In addition, the electric force due to even such a slight charge imbalance can be tremendous.

So, having said all of that I would really like to re-iterate my previous post. I want to encourage you once again to actually sit down and do the calculations yourself. That is really the best way to convince yourself that SR is wrong or to understand why you are wrong. I think you will be surprised at the magnitude of the effect even at velocities that are small relative to c.

-Dale

 

[CANGAS]

Dale:

There are several issues of interest in this thread and so it is too easy to accent one issue at the expense of another.

I am familiar with the math of the forces involved in these matters. In fact, my knowledge of this is what aroused me to point out that enourmous velocity is not necessary in order to enjoy great force between charged particles.

One point of contention was that someone seemed to say that charged particles having the same sign, such two electrons, had to travel at c before their intrinsic mutual electrostatic repulsion changed to mutual magnetic attraction; I found that saying to be counter to all of my study in the matter and so intended to present my case for much lower velocity to be sufficient.

Another point of contention, poorly presented in my post, is that Special Relativity allows the observer to consider themself to be the prime, or, stationary, observer, in which case the charges could be actually initially stationary while the observer(s) moved past the charges. A group of observers having varying velocities, including zero, would then see different charge movement ranging from repulsion, to nonmoving equillibrium ( the two charges maintaing the same distance apart ), to attraction. The paradox I find is that, in my opinion, the same two charges cannot be in three contradictory states of movement "simultaneously". This paradox is no trivial factor to my thinking, and I find it to indicate a fatal flaw in SR.