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Re: [Phys-l] cars and physics

On 2011, Feb 22, , at 07:36, John Denker wrote:



You should think twice before letting students observe the system
as a lab exercise, because of the possibility of blowing the front
end off the oscilloscope. There are lots of ways of protecting
against this, but I don't at the moment see how to prevent students
from bypassing the protection





The kinescope analyzer at auto shops (ca. fifties) used the primary as a means of diagnosis -- supposedly the primary mimics what going on w/the secondary.

My ancient story:


VW bugs had the prob. of insufficient EMF to operate the ignition and the starter. So I ran a heavy wire directly from the + (neg. gnd.) to the ignition switch, which cured that prob. My mechanic (German, of course) said many VW owners would install a 12 V batt. and just change the headlamps. They would start immediately! He didn't say anything about adding a ballast or changing the coil.





** Curiously one of the few English words, especially since the invention was by Kettering an American. A suburb of Dayton is named after him.


On 2011, Feb 22, , at 07:36, John Denker wrote:


You can't have it both ways.

If you want to make the magnetic field collapse faster, you don't
want a capacitor in there, since it provides a path for the current
to keep flowing. And making the field collapse faster provably
means having a higher voltage across the open points, which is the
opposite of what you want if you are trying to protect the points.

It would be closer to the truth to say that the capacitor protects
the points by allowing the magnetic field to collapse more slowly
... but that is an overly simplistic view of the situation. It is
better to analyze the circuit as a whole. The capacitor doesn't do
only one thing; there is a choreographed interplay between the R,
the L, the C, the battery, and the switch.

I though the cap. caused the discharge at the points** to end more quickly, and the capacitance so low that it didn't provide much sink for the inductive current.