# What is current in Finite/Infinite Open Line?

#### Hassan Saeed

Joined Feb 1, 2015
2
Dear Members,

While going through a sample test for basic electronics found online, I encountered a question as per the attachment.

What do you think would be the ammeter reading? According to my knowledge, open line means no load, meaning the reading would be 0 Amps?

Thank you.

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#### MikeML

Joined Oct 2, 2009
5,444
It is a voltmeter, not an ammeter: Big difference.

The question is asking about what happens as a function of time after the switch is operated.

What is the velocity of propagation of a wave in free space? On a transmission line?

What happens at the open end of a transmission line?

#### Hassan Saeed

Joined Feb 1, 2015
2
It is a voltmeter, not an ammeter: Big difference.

The question is asking about what happens as a function of time after the switch is operated.

What is the velocity of propagation of a wave in free space? On a transmission line?

What happens at the open end of a transmission line?
Thank you for your response. Please see that there is also an ammeter connected in the circuit, I was referring to that ammeter. The question simply asks "what is the ammeter reading after the switch is closed". Do you think it should be zero, or does it somehow depend on wavelength and line length?

#### MikeML

Joined Oct 2, 2009
5,444
At time=∞, the current will be zero. It is what happens between t=0 and t=∞ that makes the question interesting, and a test of knowledge.

#### crutschow

Joined Mar 14, 2008
26,069
Do you understand what the characteristic impedance of a transmission line is?

#### t_n_k

Joined Mar 6, 2009
5,455
Since this was purportedly a sample question from a basic electronics quiz, I doubt anything other than the most basic response is required. There are probably many levels of subtlety one might address, but they are presumably beyond the scope of the exercise.

#### crutschow

Joined Mar 14, 2008
26,069
The answer depends upon whether the time response is a consideration (e.g. is the AC frequency wavelength comparable to the cable length.

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#### t_n_k

Joined Mar 6, 2009
5,455
One could for instance, propose the (ideal?) line is a quarter wave transformer - hence the input impedance would be zero.
There are many possibilities - none of which are elaborated in the question. Hence my suggestion to assume the answer depends only upon the question as it stands. I'm presuming the 100 ohm transmission line is merely a 'distraction'.
Perhaps there is a remote chance the OP / TS might return with additional information. I'm not holding my breath on the likelihood of that happening.

#### MrAl

Joined Jun 17, 2014
7,891
Hi,

A lot of unknowns here. Any assumptions would have to come from the class instructor or from within the context of how other problems like this were handled in the class, and that is usually commensurate with the academic level of the class. So for example in Circuits 101 we might assume the transmission line isnt any load at all, but in Circuits 502 we'd have to treat it like a true transmission line of some kind.

What else we dont really know for sure is what kind of response we are looking for, or even what kind of ammeter it is. Is it AC or DC?
It could be AC because the source is AC. If it is AC then there will probably be a small AC current even after the transients die down. If it is DC then there will only be DC leakage current, which depends on the equivalent parallel resistance of the transmission line. It could also be AC and DC which would really mean they just want the total time response.
We dont even know the frequency of the AC source though.