On Dec 16, 9:06 am, Cecil Moore wrote:
Keith Dysart wrote:
Why would anyone refuse to calculate the length of
600 ohm line needed to produce 0 ohms? I think I
was the first to calculate it at 43.4 degrees.

So the next question is: What is the phase change
at the terminals of the black box?

1) -93 degrees? (previous answer when it was a capacitor)

I might be wrong about that one. It might instead be
180 - 93, but that would just be a stupid math mistake.
The main thing is that it is different from the other two.

2) 36.6 degrees? (previous answer when it was 10 degrees of 100 ohm
line)
3) 0 degrees? (previous answer when it was 46.6 degrees of 600 ohm
line)

There's nothing wrong with those answers except maybe
a stupid math error. Each condition indeed does have a
different phase shift that can be measured one inch on
the other side of the terminals if one is simply allowed
to make those measurements.

But the rules for black boxes do not allow measurements
on the inside. This is how they help clarify the thinking.

This is an example of how models can get you into trouble.
Not allowing us to look inside the black box doesn't change
the laws of physics and make all the phase shifts the same.

Nor has that been claimed.

It just means that the phase shifts are unknown and need
to be measured.

As you say, the phase shifts are unknown so from
my list of possible answers, I would have recommended
selecting

4) undecidable

but this leads to it also being

6) irrelevant

since the problem can be solved without the information.

....Keith