Re: question regarding the boundary behavior of electric current

On Apr 10, 12:12 pm, "laura" <laura.brandu...@xxxxxxxxx> wrote:
Hi,

I have a cable and I send electric current through it. I want to know
the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex
experiment.

I have a very simple network with 2 nodes (A and B). There are 2
cables which are connecting these nodes. The cables have different
lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B--
| |
| |
|______L2_____|

I apply electric power to this device. Because the cables have lengths
greater than zero I assume that it will take a while until the current
traverse the path from A to B.

More than that, in node A, the current is split in 2, because there
are 2 cables linking A with B. Because one of the cables is shorter I
assume that "a part" of the current arrives earlier (denote this by
moment M1) than the other "part" because it has to traverse a shorter
path. Am I correct ?

The other "part" of the current which has traversed cable L2 will
arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope?
Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and
M2 ?

Are there high precision oscilloscopes for this experiment? Or should
I use very very long cables?

I'm not interested what happens after those moments.

Thanks,
Laura


When you're dealing with the transmission time along a cable, it's
good to realize that controlled cables are important. Specifically,
it's very helpful to use transmission lines with controlled impedance.

Yes, you can measure such things with oscilloscopes. It doesn't even
take a particularly fast 'scope. You can use, say, 100 feet of
coaxial cable for your line L1, and 150 feet for your line L2. The
line will be much cheaper than a high speed 'scope, or very much
cheaper than the very high speed 'scope you'd need if you tried to use
lines, say, 1 inch and 1.5 inches long. With 100 feet of common RG-6
coaxial cable like is used to wire houses for television, you'll get
about 125 nanoseconds of delay, and with 150 feet, you'll get 1.5
times as much. Even a 'scope with 5MHz bandwidth should be able to
easily resolve that sort of delay. You also need a signal generator
that can generate a pulse that transitions quickly from one state to
another, but that's pretty easy with modern digital ICs, if you don't
already have an appropriate signal or pulse generator available. Use
shorter cables, maybe 10 feet and 20 feet, if you have a scope that
can resolve shorter times--maybe one with 50MHz or 100MHz bandwidth.

But let's make it even easier. Download LTSpice from the Linear
Technology web site, and you'll have a simulation tool that can easily
show you what happens. Once you learn to run the program (which isn't
difficult), you can set up and simulate just what you've proposed in a
very few minutes. If/when you do, try some different things: Make
the lines both 100 ohms impedance. At B, try it with no load, and try
it with a load of 200 ohms across the line, and 100 ohms across the
line, and 50 ohms across the line.

Cheers,
Tom

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