Re: Is charge conserved between frames?

From: Androcles (dummy_at_dummy.net)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 09:00:57 GMT

"sal" <pragmatist@nospam.org> wrote in message
news:pan.2004.10.11.02.32.56.102726@nospam.org...
> On Sun, 10 Oct 2004 10:48:15 +0000, Androcles wrote:
>
>>
>> "sal" <pragmatist@nospam.org> wrote in message
>> news:pan.2004.10.10.03.42.17.661724@nospam.org...
>>> On Sat, 09 Oct 2004 11:08:51 +0000, Dummy wrote:
>>>
>>>
>> So you'll duck out with some excuse, but I stated the wrong excuse. It
>> would be natural for anyone to change the excuse to avoid being the
>> subject of my prediction.
>>
>>> If he weren't a Bush support I'd be willing to take more time with him.
>>
>> I thought bushes were supported by roots... Whatever.
>
> Oops should have said "Bush supporter". Dropped an "er".
>
>
>>>> Let's make it really easy. Wind a coil of wire around a cylindrical bar
>>>> magnet, tightly, so that it cannot slip. Connect the ends of the wire
>>>> to an LED. Now spin the whole contraption, any axis, any direction. Why
>>>> doesn't the LED glow?
>>>
>>> Well, it obviously doesn't. And obviously none of the electrons in the
>>> wire gets accelerated at all (by the B field, I mean). So the answer
>>> should be obvious, right?
>
>> Obvious to physicist, but not obvious to a relativist.
>
>> "It is known that Maxwell's electrodynamics--as usually understood at the
>> present time--when applied to moving bodies, leads to asymmetries which
>> do
>> not appear to be inherent in the phenomena. Take, for example, the
>> reciprocal electrodynamic action of a magnet and a conductor. The
>> observable phenomenon here depends only on the relative motion of the
>> conductor and the magnet, whereas the customary view draws a sharp
>> distinction between the two cases in which either the one or the other of
>> these bodies is in motion. For if the magnet is in motion and the
>> conductor at rest, there arises in the neighbourhood of the magnet an
>> electric field with a certain definite energy, producing a current at the
>> places where parts of the conductor are situated. But if the magnet is
>> stationary and the conductor in motion, no electric field arises in the
>> neighbourhood of the magnet. In the conductor, however, we find an
>> electromotive force, to which in itself there is no corresponding energy,
>> but which gives rise--assuming equality of relative motion in the two
>> cases discussed--to electric currents of the same path and intensity as
>> those produced by the electric forces in the former case." -Albert "fool"
>> Einstein.
>>
>> Conclusion: Something is wrong with Maxwell's electrodynamics.
>>
>> Your mission, should you decide to accept it, is to discover what is
>> wrong. This tape will self destruct in five seconds.
>
> As far as I know the asymmetry Einstein objected to is still there. In
> the case you mentioned, with a coil and magnet moving together, if you
> want to actually figure the EMF from q(E+VxB) and the computed B and E
> fields in the lab frame, you can do it, but the result is unappealing: In
> the lab frame, the moving magnet causes an E field which has exactly the
> right magnitude and direction to cancel the force exerted on the charges
> in the wire due to their motion through the B field of the magnet.
>
> Again, q(E+VxB) contains _no_ term for "motion of the field" -- the E and
> B fields "just are"; they don't "move". They can weaken in one place and
> strengthen in another, but they can't "move" from one point to another ...
> from the point of view of Maxwell's equations!
>

>> And you can show by conservation of angular
>>> momentum that it has to be that way. But...
>>>
>>> But the Lorentz force law is f = q(E + VxB) where V refers to the
>>> charged particle. There's NO TERM in the law for the "velocity of the
>>> field" -- it doesn't matter whether the magnetic field is "moving" or
>>> not. The law works regardless.
>>>
>>> So why _can't_ you arrange it so the electrons accelerate in the wire,
>>> when the wire moves with the magnet?

Suppose the conductor is a flat ribbon, wound around a bar magnet like a
spool of film for a movie projector. Let us place this disk in the y-z
plane.
To induce a current, slide the magnet through the centre along the x-axis.
OR: put a current through the conductor and we have a simple solenoid,
motion along the x-axis.
Take the bar magnet out, wind a wire spiral around soft iron and replace.
Using AC, we have a simple transformer. As you know, DC doesn't work, but we
would have an electromagnet. Switch DC on, and the magnetic field
"instantly" appears. Except it cannot be instantly at the extremity of the
spool. The field can only travel at c, radially.
Switch off and the field collapses, but at c.
We induce a (quite large) potential across the spool.
Note that we can make the diameter of the spool arbitrarily large.
Now replace the bar magnet and spin everything about the x-axis.
The spool is a spiral, like the groove in an old vynil record, and the
needle
of the gramophone will move radially. So the conductor is moving radially
through the magnetic field of the bar magnet, just as the magnetic field
moved through the conductor when we had a transformer setup. By increasing
the spin, we can cause this radial motion to be c. Why doesn't
the LED glow if it is connected across the spool? :-)

>>> Hmmmm....
>>
>> There can be no force between one body.
>
> Right -- and if any current were induced, energy, angular momentum, or
> both would not be conserved. So, it can't be.
>
> You can also "explain" it directly from Maxwell's equations, as I said
> above, or you can explain it using the "cutting field-lines" argument --
> the field lines of the magnet move with the magnet, as do the wires, so
> the wires cut no B lines and no EMF is generated by the B field. But that
> point of view has limitations.
>
> For an example of a case that seems really hard to handle by arguments
> like "field-line cutting", try finding the magnetic field generated by a
> single charged particle which travels past you. Alternatively, force the
> charged particle to move in a circle, and find the B field.

That's an easy one. The conductor constrains the motion of the charge
to a circle, the voltage provides the force. It's a solenoid.

> (Assume it's
> going much slower than C so propagation delays within the lab can be
> ignored.) Both of these are easy to do if you start with the E field in
> the rest frame of the particle and then transform it to the lab frame.
> Any other approach seems likely to prove difficult, however.
>
>
>> "Thus one would have to imagine that the motion of a solid body [...]
>> through the resting aether exerts upon the dimensions that body an
>> influence..." HA Lorentz, Dover p5.
>>
>> Who was this idiot Lorentz that thought the pressure of aether acting on
>> molecules wouldn't slow down and stop the moving object?
>>
>> What is the nature of the influence he's rabbiting on about? Pushing the
>> electrons to the far end of the rod, perhaps? I'd call that a magic
>> wand, better suited to Harry Potter than physics. Lorentz contraction my
>> arse.
>>
>> Y'know, you can purchase for pennies a cigarette lighter that operates,
>> not with flint and steel, but by exerting a force and producing a spark
>> of sufficient energy to ignite the flame. Go to your local 7-11 or
>> Stop'n'Go and buy one.
>> Hint: Etymology: Greek piezein, to press.
>
> Cool. I haven't seen those.

LOL! (Yes, I really am laughing!)
I'll tell you where else you can see one. At the needle end of an old
record player. You'll see some thin wires leading away from it to
the input of the amplifier.

>
> Along the same lines, I've got something called a "Zerostat" which fires a
> stream of ionized air when you squeeze the trigger. When you release the
> trigger, it fires another stream of ions, of opposite polarity. The
> "streams" are carried by air currents, which are propelled off the end of
> a sharp needle by the charge on the needle, which ionizes molecules in the
> air. The ion streams are actually strong enough to be felt as a draft on
> one's palm. It's intended for getting static off of LP records (remember
> those?), but it does some other cool tricks too, like lighting one of
> those miniature neon bulbs if you shoot it at one leg. (Leg of the bulb,
> not leg of the observer.)
>
>
>> "Hmmmmm" that and ponder this.
>> Force always acts between two bodies, the gun kicks back, E = 1/2 mv^2 +
>> 1/2Mu^2
>> where E is the energy of the chemical charge in the cartridge, m is the
>> mass of the projectile, M is the mass of the weapon, v and u you can
>> figure out. Total energy of the charge is E = mv^2.
>
> Um ... only if M=m and u=v, which implies an awfully heavy bullet or an
> awfully light gun ... I think?

Mu = mv by conservation of momentum, no 'um's allowed. The energy of
the charge is mv^2.

> For a typical gun, which is much heavier
> than the bullet, relatively little of the energy goes into the recoil and
> I'd expect you to have E ~ 1/2 mv^2.

The same amount of energy goes into the recoil. See Newton's laws.
I think it was Geiss that said I was an idiot that didn't understand
physics...
but then, like you, he's a relativist, not a physicist. :-)
Look at it this way: take a steel tube with a 0.22 bore, place two bullets
facing opposite ways at the centre, separated by the contents of a
firecracker, and with a small hole drilled in the side. Light blue touch
paper and retire immediately. Now you have your equal masses and
equal velocites, the tube stays where it is. E = mv^2.

> Momentum divides evenly between the gun and the bullet, but the square
> on the velocity term in the kinetic energy means the body that moves
> slower gets short-changed.
>
>
>> The projectile leaves the "stationary" weapon at u+v, doesn't it?
>
> Yes, sure does.

Einstein would have said (u+v)/(1+uv/c^2).
I'm telling Dinky van de Torquemada of your blasphemy.

Hey Dinky! Look, an immoortel fumble!
LOL!

Androcles.

>
>
>> Am I off topic? No. Dig into what is happening when a chemical charge
>> fires and you'll find it is the same subject. Mass, E-field, B-field,
>> relative motion, energy.
>
> --
> I can be contacted through http://www.physicsinsights.org
>

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