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Short circuits and sources.

by M. next
Tags: circuits, sources
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M. next
#1
May5-12, 09:30 AM
P: 378
Hello Everybody,

My question is: Why when a resistor is in parallel with a "short circuit" the voltage across the resistor is zero, & so will the current passing through it?
But when instead of a resistor we have a voltage source, this is not the case? "I was informed that the voltage source generates current even though it is in parallel with a short circuit..
I dont get it? :/
Another thing, if we have a curent source in parallel with a short circuit.. what will happen exactly? Will the voltage across the current source be zero(like in resistor's case) or not(like in voltage source's case)?

Thanks in advance. Please elaborate :D
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reddvoid
#2
May5-12, 10:04 AM
P: 99
Quote Quote by M. next View Post
My question is: Why when a resistor is in parallel with a "short circuit" the voltage across the resistor is zero, & so will the current passing through it?
current takes the path of least resistance ...if you have 9.9Ω and 0.1Ω in parallel and supply 10A to this combination then 9.9A will go through 0.1Ω and only 0.1A will go through 9.9A
so, shorting=connecting 0Ω so obviously all current goes through the short leaving nothing to resistor
But when instead of a resistor we have a voltage source, this is not the case? "I was informed that the voltage source generates current even though it is in parallel with a short circuit..
I dont get it? :/
First, It's not the good idea to short a voltage source
if you do , i guess current will flow if shorting wire is of bit high resistance if not the wire might burn because the resistance connecting two terminals is zero, maximum current will flow which might over heat the wire
Another thing, if we have a curent source in parallel with a short circuit.. what will happen exactly? Will the voltage across the current source be zero(like in resistor's case) or not(like in voltage source's case)?
:/
Practically, a voltage source is not different from a constant current source. In fact, a source can either work as a current source or as a voltage source. It merely depends upon its working conditions. If load impedance is very large in comparison to internal impedance of the source, it will be advantageous to treat the source as a voltage source. On the other hand, if the load impedance is very small in comparison to internal impedance of the source, it is better to represent the source as a current source. From the circuit point of view, it does not matter at all whether the source is treated as a voltage source or a current source; In fact, it is possible to convert a voltage source into a current source and vice-versa.
skeptic2
#3
May5-12, 10:05 AM
P: 1,810
It is convenient in exercises to use ideal values in order to concentrate on the principle being studied without having to worry about the details. However with your question, you are using ideal components in ways that do not result in realistic answers.

If you put a voltage source across a short circuit, the current produced will be E/0. The answer to your first question is, a voltage source is very different than a resistor. The answer to your second question is yes it will be zero (E = I * R: R =0), however you cannot say that the voltage is not zero when putting a voltage source across a short circuit - that voltage is undefined.

cepheid
#4
May5-12, 10:06 AM
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Short circuits and sources.

To answer your second question regarding the current source: an ideal current source is something that outputs a steady current no matter what, and ideal wire is a perfect conductor that has zero residence. So if you connected these together, you would get a current flowing from the source through the wire with no voltage drop.

However the ideal current source and ideal wire are sort on par with the massless rope and the frictionless plane. They are not realistic by any means. They exist for pedagogical purposes to remove unnecessary complications when learning the fundamentals of circuit theory.

Let's say you implemented a reasonably constant current source in real life (something that would require electronics to actively control and monitor its output). Any real wire that you connected across it would have some non-zero resistance R. If the source outputs a current I, then the voltage across the wire (and hence across the source) would be IR in accordance with Ohm's law. This voltage would be very small due to the small resistance.

Similarly, if you shorted the terminals of a voltage source of voltage V with a wire having a resitance R, then there would be a current of V/R across the source. Since the resistance of a wire is so small, a tremendous amount of current will flow. This is true unless if V is also very very small, which is not usually the case. This could very well be enough current to melt the wire, or, in the case of a battery, to overheat the battery, risking fire and explosion. DO NOT ever short the terminals of a battery!
vk6kro
#5
May5-12, 10:11 AM
Sci Advisor
P: 4,027
These are both extreme examples of following a theoretical definition to its limits.

A voltage generator will deliver any current necessary to maintain a voltage output.

A current generator will generate any voltage necessary to keep its current flowing at the correct value.

They are handy concepts when applied sensibly, but putting a voltage generator across a short circuit is getting ridiculous. It is a bit like an unmovable object meeting an unstoppable force.

Don't take it seriously.

Putting a short circuit across a current generator is not so bad. It will just generate the right current and there will be no voltage across the short circuit.
M. next
#6
May5-12, 10:20 AM
P: 378
Thank you all, but I kind of realised that you are talking about real-life cases. I am just talking about circuits in books, and only in books.

Just a quick conclusion:
Voltage source parallel with short circuit is undefined and NOT zero.
Current source will keep on producing current no matter what.
Voltage of resistor in parallel with short circuit .. is zero.

Correct me if am wrong.
skeptic2
#7
May5-12, 10:22 AM
P: 1,810
Quote Quote by reddvoid View Post
current takes the path of least resistance ...if you have 9.9Ω and 0.1Ω in parallel and supply 10A to this combination then 9.9A will go through 0.1Ω and only 0.1A will go through 9.9A so, shorting=connecting 0Ω so obviously all current goes through the short leaving nothing to resistor
Current does NOT take the path of the least resistance, if it did, ALL the current the current would go through the 0.1Ω. Current takes the path of the least VOLTAGE. When 9.9A pass through the 0.1Ω resistor, it's voltage is no longer less than the 0.1A flowing through the 9.9Ω resistor. By taking the path of least voltage the current in a parallel circuit proportions itself until the voltages of all branches are equal.

Quote Quote by reddvoid View Post
First, It's not the good idea to short a voltage source
if you do , i guess current will flow if shorting wire is of bit high resistance if not the wire might burn because the resistance connecting two terminals is zero, maximum current will flow which might over heat the wire
A short does not mean "a bit of high resistance". A short does not get hot if it has a resistance of zero ohms. Voltage sources have no maximum current.

Quote Quote by reddvoid View Post
Practically, a voltage source is not different from a constant current source. In fact, a source can either work as a current source or as a voltage source. It merely depends upon its working conditions. If load impedance is very large in comparison to internal impedance of the source, it will be advantageous to treat the source as a voltage source. On the other hand, if the load impedance is very small in comparison to internal impedance of the source, it is better to represent the source as a current source. From the circuit point of view, it does not matter at all whether the source is treated as a voltage source or a current source; In fact, it is possible to convert a voltage source into a current source and vice-versa.
Practically or not, a voltage source is different from a current source and a source cannot work as both at the same time.
M. next
#8
May5-12, 10:35 AM
P: 378
So?
Thanks skeptic2, but what now?
Please summarize an answer to my question.
vk6kro
#9
May5-12, 10:35 AM
Sci Advisor
P: 4,027
Quote Quote by M. next View Post
Thank you all, but I kind of realised that you are talking about real-life cases. I am just talking about circuits in books, and only in books.

Just a quick conclusion:
Voltage source parallel with short circuit is undefined and NOT zero.
Current source will keep on producing current no matter what.
Voltage of resistor in parallel with short circuit .. is zero.

Correct me if am wrong.
Yes, that is OK.

Unfortunately or otherwise, this is a practical Forum and we have to point out that sometimes the proposed circuit is not possible and may even be contradictory.
An impossible situation is still impossible even if the book says it is OK.
M. next
#10
May5-12, 10:47 AM
P: 378
Thanks my friend :)

And concerning the voltage across the IDEAL current source (if it was connected in parallel with short circuit? )
cepheid
#11
May5-12, 10:52 AM
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Quote Quote by M. next View Post
Thanks my friend :)

And concerning the voltage across the IDEAL current source (if it was connected in parallel with short circuit? )
Yeah that would be 0, like I told you in my previous post, since the wire would have no voltage develop across it, and Kirchoff's voltage law would then tell you that the source could not have any voltage across it either.
M. next
#12
May5-12, 10:58 AM
P: 378
Cepheid thanks.
It is just the whole concept that irritated me.
skeptic2
#13
May5-12, 12:05 PM
P: 1,810
Quote Quote by M. next View Post
So?
Thanks skeptic2, but what now?
Please summarize an answer to my question.
To get reasonable answers you need to assign reasonable values to your components.

Why not assign a value of 0.01Ω to the short, 0.1Ω for the internal resistance of your voltage source and a voltage of 10V to your voltage source. Then your current will be 10/(0.11) or 91
A. This means you'll have a voltage of (9.1 * 0.01) or 0.91 volts across the short.

If you have a current source of 100A, then you will have 100A * 0.01Ω or 1V across the short.
M. next
#14
May5-12, 12:08 PM
P: 378
Thanks dear, thanks for bearing with me :D
Thank you all.


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