MH's ideal coil and voltage question

[minnie]

   What does old Lewin think?
   YT  Kirchoffs rule is for the birds. Stripey shirt 16.01.

[poynt99]

Perhaps the sim skids out at these lower R settings, and defaults to 0 rather than crashing as it does with no resistor at all.

It does seem to settle at the 2.4A point when R is relatively small though. So, yeah it must be right.

It doesn't makes sense to me from the infinite tau perspective though.

[MileHigh]

The Tau business is fairly simple to explain.

When you transition from a finite Tau to an infinite Tau the current waveform goes from an inverse exponential curve to a straight line.  Note that it is a straight line with a constant slope of V/L.

So Tau at infinity just means the current trace is a perfectly straight line.  Since it is a straight line the concept of "reaching 63% of the maximum value" does not apply anymore because that concept does not exist when the current waveform is a perfectly straight line.  i.e.; "There is no time constant."

So Tau being infinity does not mean stopping current flow, it means linearly increasing current flow.

Since we are discussing limits, the only possible way for the current to flat-line at zero "forever" would be for the inductance to be infinity.  Then you have a "more real" Tau = infinity because this time L/R becomes infinity/R.

So when Tau = infinity/R that gives you the horizontal current trace stuck at zero with a slope of zero (V/infinity), whereas when Tau = L/0, you get a current trace that is a straight line with with a slope of V/L.

[poynt99]

Thanks for the explanation MH.

[minnie]

   My idea was to stick in a series resistor of 1-66666666 Ohms.
           John.