Electrical Engineering - question regarding the boundary behavior of electric current

Author

question regarding the boundary behavior of electric current

laura

2007-04-10, 5:25 pm

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

TimPerry

2007-04-10, 9:25 pm

laura 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 ?
>

a measurement device called a time domain reflectometer (TDR) can be used to
measure the length and character of a cable, usually a coaxial cable.

a more elaborate device is called a network analyzer (nothing to do with
computer networks).

a rudimentary TDR can be created with a fast O'scope and a pulse generator.

the propigation velocity of a cable varies depending on dielectric type. see
chart for examples

http://www.rfcafe.com/references/el.../coax_chart.htm
special types of cable called "radar delay lines" are designed to resolve
timing issues in microwave circuits.

the following is a referance to a pioneer in measuring the speed of
electricity

http://en.wikipedia.org/wiki/Ren%C3%A9-Prosper_Blondlot

Salmon Egg

2007-04-11, 3:25 am

On 4/10/07 12:12 PM, in article
1176232364.596289.216770@y80g2000hsf.googlegroups.com, "laura"
<laura.brandusan@gmail.com> 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
>
It seems obvious that you are in over your head. You really need to learn
something about transmission lines.

It is true that if you have two cables of different lengths excited from a
common point, you can measure the time delays using a suitable oscilloscope.
A wave travels on a line at approximately 1ft/ns. Thus, if you coil up one
of the cables to get there outputs close to a dual trace oscilloscope, there
should be no problem. But to carry out such experiments meaningfully, you
have to know about how waves travel on transmission line. You have to know
about terminations, mismatch, and reflections.

Bill
-- Fermez le Bush--about two years to go.

LinkBot