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Message |
Fred Abse
Guest
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 Posted:
Sat Mar 19, 2005 2:18 am Post subject:
Re: Guitar signal |
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On Thu, 17 Mar 2005 07:20:51 +0100, Roger Johansson wrote:
| Quote: | Think of the ignition in a car. A voltage from the battery "causes" a
current in the ignition coil. The power supply is suddenly cut off, the
current continues to flow through the coil,
|
No, it doesn't, the current stops (more or less) instantly. In this
example, there's no capacitor across the contact breaker, to simplify
things.
| Quote: | that is the nature of coils.
This current "causes" a very high voltage to appear at the ignition plug
and results in a spark.
|
No, it works like this, in inductors, the relationship between voltage and
current is:
V = -L di/dt
If current is cut off suddenly, di/dt is very high. in a perfect world,
infinite, but we're not in a perfect world, so there is finite decay time.
L is fixed, hence V is also very high.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
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Active8
Guest
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| Posted:
Sat Mar 19, 2005 6:10 am Post subject:
Re: Guitar signal |
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On Fri, 18 Mar 2005 20:18:30 +0000, Fred Abse wrote:
| Quote: | On Thu, 17 Mar 2005 07:20:51 +0100, Roger Johansson wrote:
Think of the ignition in a car. A voltage from the battery "causes" a
current in the ignition coil. The power supply is suddenly cut off, the
current continues to flow through the coil,
No, it doesn't, the current stops (more or less) instantly. In this
example, there's no capacitor across the contact breaker, to simplify
things.
that is the nature of coils.
This current "causes" a very high voltage to appear at the ignition plug
and results in a spark.
No, it works like this, in inductors, the relationship between voltage and
current is:
V = -L di/dt
|
Sorry, V = L di/dt di/dt is negative, decreasing and sets up a
voltage opposite to that which was present before.
You can also look at it as a collapsing mag field (changing flux)
which induces an EMF. But is d_phi/dt dependant on di/dt or is there
really some inertial thing going on with one or the other or both?
Either one is just a buncha photon spin action, eh?
Kevin!!!
And the current very badly wants to *not* turn off instantly. It's
one of the premises used to determine initial conditions for a
transient analysis. And a cap doesn't want to change voltage
instantly.
| Quote: |
If current is cut off suddenly, di/dt is very high. in a perfect world,
infinite, but we're not in a perfect world, so there is finite decay time.
L is fixed, hence V is also very high.
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Hey this is cool! We're OT and still talking electronics.
--
Best Regards,
Mike |
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Kevin Aylward
Guest
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| Posted:
Sat Mar 19, 2005 1:53 pm Post subject:
Re: Guitar signal |
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Active8 wrote:
| Quote: | On Fri, 18 Mar 2005 20:18:30 +0000, Fred Abse wrote:
On Thu, 17 Mar 2005 07:20:51 +0100, Roger Johansson wrote:
Think of the ignition in a car. A voltage from the battery "causes"
a current in the ignition coil. The power supply is suddenly cut
off, the current continues to flow through the coil,
No, it doesn't, the current stops (more or less) instantly. In this
example, there's no capacitor across the contact breaker, to simplify
things.
that is the nature of coils.
This current "causes" a very high voltage to appear at the ignition
plug and results in a spark.
No, it works like this, in inductors, the relationship between
voltage and current is:
V = -L di/dt
Sorry, V = L di/dt di/dt is negative, decreasing and sets up a
voltage opposite to that which was present before.
You can also look at it as a collapsing mag field (changing flux)
which induces an EMF. But is d_phi/dt dependant on di/dt or is there
really some inertial thing going on with one or the other or both?
Either one is just a buncha photon spin action, eh?
Kevin!!!
|
Our general terminology in E&M is very useful but not how things
"really" "are". Of course we all use concepts such as, changing magnetic
fields "causing" electric fields an other such heuristic descriptions,
but one should always keep in mind that it is just macro descriptions of
things we don't even know the "true" operation of.
Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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Fred Abse
Guest
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| Posted:
Sat Mar 19, 2005 6:17 pm Post subject:
Re: Guitar signal |
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On Sat, 19 Mar 2005 01:02:37 -0500, Active8 wrote:
| Quote: | Sorry, V = L di/dt di/dt is negative, decreasing and sets up a
voltage opposite to that which was present before.
|
No, the direction of the induced EMF is such as to oppose the change of
current that produced it, hence:
V = -L di/dt
As you say, di/dt is negative, hence the induced voltage is positive,
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock) |
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Fred Abse
Guest
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| Posted:
Sat Mar 19, 2005 6:17 pm Post subject:
Re: Guitar signal |
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On Sat, 19 Mar 2005 07:53:17 +0000, Kevin Aylward wrote:
| Quote: | Our general terminology in E&M is very useful but not how things
"really" "are". Of course we all use concepts such as, changing magnetic
fields "causing" electric fields an other such heuristic descriptions,
but one should always keep in mind that it is just macro descriptions of
things we don't even know the "true" operation of.
|
Does it matter?
If it matters, does it matter that it matters?
:-)
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock) |
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Active8
Guest
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| Posted:
Sat Mar 19, 2005 10:44 pm Post subject:
Re: Guitar signal |
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On Sat, 19 Mar 2005 12:17:04 +0000, Fred Abse wrote:
| Quote: | On Sat, 19 Mar 2005 01:02:37 -0500, Active8 wrote:
Sorry, V = L di/dt di/dt is negative, decreasing and sets up a
voltage opposite to that which was present before.
No, the direction of the induced EMF is such as to oppose the change of
current that produced it, hence:
V = -L di/dt
As you say, di/dt is negative, hence the induced voltage is positive,
|
Hmmm. Difference between voltage drop and counter EMF. Let's see how
many screw ups I can find.
I see Haliday and Resnick managed to foul things up by first writing
lineIntegral [ E.dl ] = -d_phi/dt
and then in the chapter on Maxwell they drop the negative sign and
reference the above eq as if they didn't change it. That's Table
37-1 with no mention of why it's dropped. In Table 37-2 the put it
back and never mention why but they discuss adding a missing term to
Ampere's Law from a prior chapter.
No where else to look for screw ups. Oh well, at least I remember
which way the protection diode goes.
--
Best Regards,
Mike |
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Rich Grise
Guest
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| Posted:
Wed Mar 23, 2005 3:14 am Post subject:
Re: Guitar signal |
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On Sat, 19 Mar 2005 07:53:17 +0000, Kevin Aylward wrote:
| Quote: | Active8 wrote:
You can also look at it as a collapsing mag field (changing flux)
which induces an EMF. But is d_phi/dt dependant on di/dt or is there
really some inertial thing going on with one or the other or both?
Either one is just a buncha photon spin action, eh?
Kevin!!!
Our general terminology in E&M is very useful but not how things
"really" "are". Of course we all use concepts such as, changing magnetic
fields "causing" electric fields an other such heuristic descriptions,
but one should always keep in mind that it is just macro descriptions of
things we don't even know the "true" operation of.
|
When you learn to see the higher dimensionality of Reality, it becomes
crystal clear, and impossible to describe or explain. ;-)
Cheers!
Rich |
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