# Current

Discussion in 'General Electronics Chat' started by tahir51214, May 1, 2013.

1. ### tahir51214 Thread Starter New Member

Nov 11, 2011
29
1
I read that there is no voltage when a short circuit occurs. Since current is effect and voltage is cause, How it can be possible to see such a large effect without any cause? Please tell me experimentally and mathematically as well if you know. Im just a beginer.

2. ### #12 Expert

Nov 30, 2010
18,093
9,683
All voltage sources have some amount of resistance in them. If you connect the voltage to 0.00 ohms, all the current will pass through zero ohms and no voltage will be shown there.

Learn Ohm's Law. V = I R
When R = zero, there can be no voltage.

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3. ### crutschow Expert

Mar 14, 2008
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5,820
In theory, using Ohm's law, I = V/R and if there is a V with R=0 then the amount of current becomes infinitely large, as any number divided by 0 is infinite. Since, as #12 noted, all practical voltage sources have some finite internal resistance or current limit, then that is what determines the current if R=0.

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4. ### tahir51214 Thread Starter New Member

Nov 11, 2011
29
1
Thanks
Be like a flower, Which gives present smell to one who cuts it.

5. ### cabraham Active Member

Oct 29, 2011
82
32
Voltage is not the cause of current in general, nor is current the cause of voltage. Which comes first, and which is incidental can vary. If there is one thing I wish to impart it is the following.

In general, voltage and current must co-exist. Neither is the cause of the other, nor the effect. The source powering the circuit transduces energy from chemical to electrical for a battery, mechanical to electrical for a generator, radiation to electrical for a solar cell, etc. Current and voltage are both produced, but one of the 2 can be regulated to a constant vale, while the other is determined by the controlled quantity and the load impedance.

It so happens that power transmission is more efficient using a constant voltage power grid. The power companies could just as well generate a current source but there are problems with that I cannot go into right now. But a current source outputs a constant value of current and the voltage varies w/ load impedance.

We use constant voltage for practical reasons, but Mother Nature did not decree that voltage "causes" current nor vice-versa.

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6. ### crutschow Expert

Mar 14, 2008
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Well, it is somewhat of a chicken and egg question, but in general I think the voltage always comes before current. You can generate a voltage without current but I don't see how you can generate current without a voltage first. Even the current in a superconducting coil, which circulates without voltage drop, was originally started by some type of voltage input or gradient, even if generated in the coil by a magnetic field.

7. ### cabraham Active Member

Oct 29, 2011
82
32
It is indeed a chicken and egg question. No, voltage does not come "before" current. Never did, never will. As far as generating I & V, I don't think you can generate one w/o the other. The current in a superconducting coil is an example of I w/o V, but historically a V was involved when current changed from zero to non-zero.

Likewise a perfect insulator, such as an ideal cap, can have V w/o I, but historically I was non-zero when the V was changing from zero to non-zero. As far as generating a voltage w/o a current, how is that done?

A battery generates voltage by separating ions/charges, transporting them via chemical reaction. In order to get a terminal voltage w/ the battery unloaded, a chemical reaction separated charges and imparted energy to these charges moving them which results in terminal voltage.

In the ac domain, a generator with an open circuit load cannot output a voltage unless a current is generated as well. A time-varying voltage exists when charges are transported through the winding. Because it is open at the ends, the charges build up at the ends. Polarity reversal takes place when the charges reverse direction and move to the other end of the wire.

With a dc battery, or ac generator, it is impossible to generate V w/o I, and vice-versa. In static cases, like a charged cap, or energized inductor, a lossless system can have I w/o V, or the reverse. But energizing an inductor and/or cap requires both I & V.

Your original statement re "chicken and egg" is correct. You then proceeded to state that V is more basic than I but that is not the case. I & V have a relation that is best described as "mutually inclusive", i.e. chicken and egg. Neither one "comes first". Batteries could be built as constant current sources, but the shelf life and performance is not as good as constant voltage.

Likewise, generators could output constant current, but distribution losses are higher, and constant voltage also provides constant frequency, handy for synchronous motor speed consistency, as well as synchronizing multiple generators on the grid. Constant voltage has attractive features so we use it. But constant current power sources can and have been produced.

An example of a device which generates a current w/o a voltage is a photodiode operated in short circuit mode. The PD is placed across the inputs of an op amp, with a feedback resistor across output and negative input. Light incident on the PD generates current. The op amp input pins are at near zero potential difference, the tiny value being equal to output voltage divided by open loop gain.

Just something to ponder. BR.

Claude

8. ### Trimalchio New Member

May 3, 2013
11
0
I came across an interesting comment recently - "Ohm's Law is simply the definition of a linear conductor". No causal hierarchy amongst V, I and R - they are just the parameters we can measure, whose relationship is defined by Ohm's Law. They are all different ways of describing the activity of electrons in a conductor. Is V possible without I or R? I suppose so - in the absence of a linear conductor, when Ohm's Law and I = V/R presumably don't apply. But these are all just my layman's musings!

9. ### #12 Expert

Nov 30, 2010
18,093
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I pretty much never muse with a beginner. It's hard enough to teach them the plain vanilla version.

10. ### Trimalchio New Member

May 3, 2013
11
0
Some of us on this thread had better just shut up then!