Am Wed, 02 Apr 2008 10:45:08 +0100 schrieb Invisible:

> volition anyway; it's not like it requires a *force* to drive them or
> anything...

Well, strictly spoken no. Newtons first law applies to electrons too... 
(Think of Vacuum tubes, particle accelerators, synchrotron storage ring 
although with these electrons lose energy simply by getting accelerated 
*radially* [to remain on cirular course], and by some magic not covered 
by mechanics, they lose energy through radiation, which is the whole 
point of synchrotrons)

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On 2 Apr 2008 06:34:59 -0500, Michael Zier <mic### [at] mirizide> wrote:

>Am Wed, 02 Apr 2008 10:45:08 +0100 schrieb Invisible:
>
>> volition anyway; it's not like it requires a *force* to drive them or
>> anything...
>
>Well, strictly spoken no. Newtons first law applies to electrons too... 
>(Think of Vacuum tubes, particle accelerators, synchrotron storage ring 
>although with these electrons lose energy simply by getting accelerated 
>*radially* [to remain on cirular course], and by some magic not covered 
>by mechanics, they lose energy through radiation, which is the whole 
>point of synchrotrons)

I think Invisible was being ironic.
But why do you say "synchrotron storage ring although with these
electrons lose energy simply by getting accelerated *radially*"?
Why lose energy not gain energy? They are being accelerated is it
considered a negative acceleration?

-- 

Regards
     Stephen

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scott wrote:

> Maybe trying to 
> teach you about complex AC circuit theory and power electronics was a 
> bad starting point :-)

Hmm... maybe.

> This explains why on old cars the radio switches off momentarily as you 
> start your car.

What do you mean "old cars"?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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scott wrote:

> Capacitors and inductors can STORE ENERGY!

No - really?

> What if I connected a vertical pipe several metres high to your pipe 
> network in the middle somewhere?  As you applied some external force 
> through your system, I opened up a valve for a second to let water flow 
> into my vertical pipe, then closed it.  You would notice a slight 
> increase in force needed. Then, once you switched off your external 
> force later, I open my valve and let my water flow back into the system 
> and it would force more water out the other end.
> 
> Would you be puzzled that after you switched off your force water kept 
> coming out for a bit longer?

No - because there is positive pressure at the value you just opened, 
and this pressure is what is pushing the water along.

Similarly, a capacitor can store charge such that when you turn off your 
external current source, the capacitor is still presenting a voltage to 
the system, and hence current is still flowing. There is no mystery in 
that. The mystery is in saying that there is actually "no voltage", and 
the current is just magically flowing all by itself.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Am Wed, 02 Apr 2008 12:50:58 +0100 schrieb Stephen:

> But why do you say "synchrotron storage ring although with these
> electrons lose energy simply by getting accelerated *radially*"? Why
> lose energy not gain energy? They are being accelerated is it considered
> a negative acceleration?

I said "radially" not "tangentially". They must be accelerated towards 
the center, otherwise  they'd fly straight-line (it's done by bending 
magnets, so it's not a circle but a N-gon with rounded corners). In spite 
of having the radial force perpendicular to the flight path, yielding 
zero for the scalar product Work=Force \times distance and therefore 
can't change the energy of the electrons, the will lose energy in form of 
electromagnetic radiation (depends on the facility, but most will produce 
X-Rays for all kinds of useful experiments). That comes from 
electrodynamics: accelerated charges emit radiation.

That caused great confusion about how atoms could be stable and resulted 
in the advent of quantum mechanics. How could electrons circle around the 
atom's core, without radiation emission and finally plunging into the 
core? Bohr's model of standing particle waves as electrons was the first 
explanation of atoms, that didn't involve accelerated charges (and as we 
know today, was not quite correct).

Yes, I admit it, I'm a physicist. Now you know it...

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Am Wed, 02 Apr 2008 07:03:37 -0500 schrieb Michael Zier:

> Am Wed, 02 Apr 2008 12:50:58 +0100 schrieb Stephen:
> 
>> lose energy not gain energy? They are being accelerated is it
>> considered a negative acceleration?

Ahh, and yes: I consider a body being accelerates whenever it's velocity 
vector changes. That includes cases, where only the magnitude changes 
(either + or -) or where only the direction changes (and of course 
combinations).

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On 2 Apr 2008 07:05:43 -0500, Michael Zier <mic### [at] mirizide> wrote:

>Am Wed, 02 Apr 2008 07:03:37 -0500 schrieb Michael Zier:
>
>> Am Wed, 02 Apr 2008 12:50:58 +0100 schrieb Stephen:
>> 
>>> lose energy not gain energy? They are being accelerated is it
>>> considered a negative acceleration?
>
>Ahh, and yes: I consider a body being accelerates whenever it's velocity 
>vector changes. That includes cases, where only the magnitude changes 
>(either + or -) or where only the direction changes (and of course 
>combinations).

As an engineer I agree, so it must be true :)
Does an electron have less energy in a circular path than a straight
one. You are pumping energy into the system so I would have thought
that it would be a gain in energy for the electron and a loss of
energy to the magnetic field.
BTW I also have problems with credit and debit when working on
financial systems ;)
-- 

Regards
     Stephen

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> The mystery is in saying that there is actually "no voltage", and the 
> current is just magically flowing all by itself.

Well, the quick way to explain is to look up the formula for an inductor:

V(t) = L * dI / dt

So, the voltage is proportional to the rate of change of current.  This is 
because a magnetic field is generated by the flowing current, and when the 
current changes the magnetic field generates a reverse-voltage to try and 
cancel out the current change.

Nowhere does it say that a voltage of zero means no current flows.

If you've got 5 V and 5 A flowing through an inductor and you then suddenly 
switch the current to 10 A, it's quite possible that the inductor will 
generate a back-emf of -5V (so giving a sum of zero across the inductor) to 
try and keep the current at 5A.  During the split second that the voltage is 
zero, it is behaving exactly like a piece of wire with no resistance.

Or for a capacitor:

I(t) = C * dV / dt

This says that the current is proportional to the rate of change of voltage. 
Nowhere does it say that no current can flow if the voltage is zero.  But, 
note, the rate of change of voltage can't be zero, otherwise no current 
would flow.

But for a resistor:

V(t) = I(t) * R

In this case, I=0 means V=0 whatever is going on (for non-zero R)

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Am Wed, 02 Apr 2008 13:19:09 +0100 schrieb Stephen:

> 
> As an engineer I agree, so it must be true :) Does an electron have less
> energy in a circular path than a straight one. You are pumping energy
> into the system so I would have thought that it would be a gain in
> energy for the electron and a loss of energy to the magnetic field.

No. If there was no such thing as "synchrotron radiation", the electrons 
would neither gain nor lose energy in a circlular path, not different 
from a stone tied to a teather and swung around in a circle (neglecting 
the friction forces for a moment) or the earth orbiting the sun.

BUT, we have syncrotron radiation, so after one traverse of the storage 
ring, the electrons have a little less energy than before. To maintain a 
steady operation, the lost (or better: transferred to the experiment 
hall) energy must be given back to the electrons, and that's why in the 
storage ring there's a acceleration section (this time truly tangential) 
which adds energy to the travelling electrons (exactly the amount emitted 
by the radiation).

> BTW I also have problems with credit and debit when working on financial
> systems ;)

Well, as long as you know which one must be on your account... ;)

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Stephen wrote:

> BTW I also have problems with credit and debit when working on
> financial systems ;)

Double entry book keeping FTW! :-D

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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