Re: No collapsing magnetic field? (was Winding primary)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: "Randy & Lori" <rburney6-at-comcast-dot-net>
 >
 > Antonio,
 >
 > I let this ride because I wanted to see what others had to say about
 > your statement that there is no collapsing magnetic field.  Nobody said
 > anything, which really leaves me curious.  I have been taught, and have
 > taught others, that any wire with current produces a magnetic field; and
 > in the case of AC, the field must collapse and rebuild in the opposite
 > direction for current to be allowed to reverse.

I would say that the magnetic field just follows what the current does.
Currents and the magnetic fields are always associated in direct
proportion. If you revert the current, the magnetic field reverts too.
A tricky part is that the magnetic field stores energy, and to revert
it you must first remove the energy, what takes some power ("reactive"
power) and time.
This is adequately modeled by saying that the wire has some inductance.
If you have a wire with current passing in one direction and wants to
revert the direction of the current, you can apply a voltage in the
opposite direction over the wire. The inductance makes the current
continuously decrease, cross zero, and then revert. The magnetic field
accompains the change in the current. Due to the fact that the magnetic
field stores energy, currents and magnetic fields never "collapse"
instantaneously. They always change continuously (maybe very fast).

 > That collapsing
 > magnetic field will induce a voltage/current in the same direction as
 > the originally applied.  I have lit neon bulbs and drawn arcs with a
 > 1.5V battery and an inductor demonstrating this.  Can you explain what
 > you mean when you say "there is no collapsing magnetic field"?

What happens in this case is that when the circuit is interrupted, the
current continuously decays, while it charges the parasitic capacitances
of the circuit, generating a rising voltage where the circuit was
interrupted, that eventually gets high enough to trigger the lamp.
You can also ignore the capacitance and assume that a high resistance
was suddenly inserted in the circuit. In this case, the high
voltage appears instantaneously (irrealistic, but a common
approximation), but the current still changes continuously.
This happens in microseconds, or less, depending on the element values,
but is never instantaneous.

In a Tesla coil circuit, in the primary circuit the inductance and
the capacitance are quite big, and the magnetic field changes
smoothly. The situation where a high current is interrupted and the
consequent dumping of current over a small capacitance generates
high voltage is absent there. (Unless in the case of a coil with
initial energy stored in an inductor, that would operate as an
induction coil, that corresponds to your example with a transformer
added.)

Antonio Carlos M. de Queiroz