Magnet Myths and Misconceptions

[MileHigh]

So are you up to the question or not?

[EMJunkie]

So are you up to the question or not?

Ask away, My electronics is OK, my Magnetics is much better.

[EMJunkie]

Copper cables are not coaxial cables.  Every setup will have it's characteristic impedance and the formula for the speed is something like the square root of the permittivity divided by the permeability.   That's probably wrong but it is in the ballpark.  I would have to go look it up but I am not going to bother.  It's all part of transmission line theory.

Ok, so here you prove my point again. Look we cant all know everything all the time. I am not going to nail you to the wall on this! Because really I don't care if you don't know the real reason why.

Still, your demand is a ridiculous one and proves nothing! It is not constructive and helps no-one!

[MarkE]

Copper cables are not coaxial cables.  Every setup will have it's characteristic impedance and the formula for the speed is something like the square root of the permittivity divided by the permeability.   That's probably wrong but it is in the ballpark.  I would have to go look it up but I am not going to bother.  It's all part of transmission line theory.

In a lossless transmission line the propagation velocity is:  c/(uR*eR)^0.5.  Most of the time in cables and etch inside printed wiring boards, uR is close to 1.0 and it is the eR that sets the velocity.  Typical printed circuit boards have eR values of 4-5 so the velocity is half or less of c inside the board.  The speed actually varies depending on how much of the fiberglass is glass and how much is resin.  At the ferocious data rates that we have today that can be a big problem because how traces line up with the glass bundles changes their timing.  In a coaxial cable that uses very low density PTFE foam the average eR is only about  1.5 and the velocity is about 0.8 c.  Higher density of PTFE increases the eR and further reduces the propagation velocity.  Traces that run on the top side of the circuit board send some of the energy through air above the trace and other energy through the board material underneath the trace.  These components run at different speeds and that distorts the signals.  But what can really drive fast signals batty is the protective nickel plating that is often applied between outside traces and gold top finish.  Nickel has a high uR. 

[MileHigh]

Okay, here we go.

The circuit is just a variable power supply connected to a coil - that's it, nothing else.   The coil is 2 Henries.   The coil is an ideal coil with zero wire resistance.  The power supply is an ideal power supply with zero output impedance.

At the start of the test (time zero), the power supply outputs 7 volts.   After five seconds, the power supply outputs 5 volts.

Please describe what happens with this circuit starting from time zero.