Guitar signal

[Active8]

On Thu, 17 Mar 2005 09:32:43 GMT, Kevin Aylward wrote:

Roger Johansson wrote:
Active8 <reply2group@ndbbm.net> wrote:

Yes. The vibrating string in a magnetic field changes the induced
current in a coil.

At the risk of sounding pedantic, it's a *voltage* that is induced
by a time varying mag field.

At the risk of sounding even more pedantic..

The electromagnetic influence causes an electromagnetic force which is
both a voltage and a current in a conductor. And magnetism around it.

It is scientifically incorrect to see voltage as a cause of current or
vise versa,

Not really, but the point is a bit subtle. The ultimate cause of *all*
of E&M is "charge", sort of. "Charge" directly produces the electric
field, to which voltage is associated with. Current is the flow of
charge. It is the electric field by (static) charge that actually causes
the flow of this charge. That is, the *accelerating* electric field
produces a force that accelerates charges, i.e. Electric field (voltage)
is causing current. The *motion* of charges is really secondary.

I like it, but when you say "accelerating E field", what do you
mean? IOW, if a stationary or static charge causes another charge
(charge 2) to accelerate toward it, I can understand that the field
associated with charge 2 as accelerating, but not the field of the
original static charge. Maybe you're referring to the net E field?

"Charge" is in quotes because, charge itself is nothing more than a
number describing the exchange of photon momentum between "charges".
Ultimately, *any* motion can only be instigated by other motion, i.e.
exchange of motion from one entity to another, so in this sense it is
always motion causing other motion, i.e. Newton's conservation of
momentum. However, the motion that is being exchanged in E&M to do this,
is not the motion associated with the "current" itself, so in this sense
current itself doesn't cause anything.
--

Best Regards,
Mike

 

[Fred Abse]

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

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)