Voltage vs Current Which Occurs first?

Thread Starter

MrAl

Joined Jun 17, 2014
9,633
This is a takeoff from another thread where some people declare voltage to come first followed by current.

Yes - I agree. For switching applications (saturation operation) and for the purpose of finding suitable resistor values it may be easier to view the BJT as a current-controlled device (injecting a base current).
However, each good and experienced engineer knows that the wording "injecting a current" is something like "labor jargon" .
That is because we cannot "inject" a current - a current is always the RESULT of an applied voltage.
In our example: For switching applications, the base current is mainly determined by the large base resistor RB, which forms a voltage divider together with the base-emitter resistance (much samller).
Hello,

As you should know, each application has it's own preferred method or at least each model has it's own requirements. Also, some people are bent on declaring one way of doing it over another but is there really an overall blanket preference? I dont think so.

But i must quote one line from your post in particular:

"That is because we cannot "inject" a current - a current is always the RESULT of an applied voltage"
That's not really true except again in a theory that prefers that definition. We can also say that voltage is the result of a current that flows from one place to another.

I believe the correct way to look at this is that they both occur simultaneously. A way of looking at this is that in order to establish a voltage you must move charge, and charge implies both current and voltage. simultaneously. In other words, energy.

Now it is true that some calculations will prefer one view over the other, but i think it is easy to see how both ideas can play out in real life.
If we "apply" a voltage to an inductor, current flows, but it does not 'start' to flow after the voltage is applied, it starts immediately. But even if we look at it as 'after' then what about the capacitor.
If we "apply" a voltage to a capacitor, we get infinite current flow even with a tiny capacitance. Now we could say that you have to apply a current 'first' in order to charge the capacitor to some voltage, so that the voltage comes 'after' the current. But that' also not right, they both occur simultaneously.

Now we might go as far as to say that the "electric field" is 'first' established in a wire before current can flow. But in that view we are talking about *relative* values of current and electric field. In this case we deem a current as low as 1 femtoampere (for example) to be zero current when really it is not zero. Again it has to be simultaneous. Also we can look at how that electric field got established in the first place.

So the bottom line is that current and voltage are independent only in theory and we decide what theory we want to use.

It's ok to disagree with this but then you should have at least one physical example that demonstrates the contrary.
 

LowQCab

Joined Nov 6, 2012
2,650
Voltage-Potential is created first,
because You can have Voltage with zero-Current.

Current only occurs after a Conductive-Path
is established between one Voltage-Potential
and another Voltage of differing Potential.

That Conductive-Path between 2 differing Potentials,
will always have a certain amount of Resistance,
or there could not possibly have been any Voltage difference in the first place,
therefore, Ohms-Law always applies.

Electricity can not exist without all 3 factors of Ohms-Law being present.
.
.
.
 

LvW

Joined Jun 13, 2013
1,586
Hi MrAl, you wrote:

"As you should know, each application has it's own preferred method or at least each model has it's own requirements. Also, some people are bent on declaring one way of doing it over another but is there really an overall blanket preference? I dont think so."

** My comment: I think, in order to avoid misunderstandings and long dicussions without a result, we should try to clearly distunguish between (a) physical relationships resp. laws and (b) models and/or „preferred methods“ for treating electronic circuits.

"But i must quote one line from your post in particular:
"That is because we cannot "inject" a current - a current is always the RESULT of an applied voltage"
That's not really true except again in a theory that prefers that definition. We can also say that voltage is the result of a current that flows from one place to another."


** My comment: What means “..really true except in theory...“? True or not ?
With respect to my comment above , you are right that „we can say...“ (we can use a kind of model), but my statement you were commenting was just a physical law (cause-and-effect relation). So - let us not mix different things.
I never did deny that it is allowed to calculate a voltage across a resistor using V=I*R (assuming that V is the result from I). But dont you agree that a good engineer should always know if he is using a working model or a correct physical relation?

"I believe the correct way to look at this is that they both occur simultaneously. A way of looking at this is that in order to establish a voltage you must move charge, and charge implies both current and voltage. simultaneously. In other words, energy."

** My comment: No - I cannot agree. I think that the question of „simultaneously yes/no“ has nothing to do with the question „which quantity causes a particular effect“. There are many examples in mechanics where cause and effect occur simultaneously - without us being able to swap cause and effect (gravity, force, pressure,..)

"Now we might go as far as to say that the "electric field" is 'first' established in a wire before current can flow. But in that view we are talking about *relative* values of current and electric field. In this case we deem a current as low as 1 femtoampere (for example) to be zero current when really it is not zero. Again it has to be simultaneous. Also we can look at how that electric field got established in the first place."


** My comment: Yes, of course. The E-field is the result from the voltage applied - and this E-field allows the charges to move (that is current). I think it would be not correct - from the physical point of view - to start with the current. This seems to be obvious because without an E-field there is no force acting on the electrons.

"So the bottom line is that current and voltage are independent only in theory and we decide what theory we want to use."


** My comment: I must admit that I would have problems „to decide what theory we want to use.“ Or do you mean “...to decide which models....“ to be applied for calculation purposes?
Again - we should avoid misunderstandings by mixing physical theory with artificial models.

"It's ok to disagree with this but then you should have at least one physical example that demonstrates the contrary."


** My comment: I think, I have shown that the question of simultaneousness cannot help to answer the question „which quantity comes first ?“.
More than that, remember my comment (in the former thread) to the BJT in saturation mode. Many people say „we inject a base current using a B-value much larger than specified.“
And yes, of course, this works...no doubt about this. In this case, they have a current-controlled current source in their mind. But this is a just a MODEL which does not reflect the physical theory - and it does not consider the DEFINITION for saturation which requires a base voltage Vb that can open the B-C junction.
The only thing I am stressing is that we as engineers should know about this - unless we like to follow established design rules only without knowing something about the physical backgrounnd. Do you now understand my position?
I'm sure you and I are not that far apart in our assessment - but we don't always talk and write about the same things (models as aids for calculation purposes vs. physical theories).
 

LowQCab

Joined Nov 6, 2012
2,650
Then how would you explain photo-current?
.
There are 2 differing materials with an insulator between them,
Electro-Magnetic Waves, called "Light", are directed at the materials,
the materials have differing responses to the Light,
at least one of them starts to develop an Electrical-Charge because of the Energy from the Light,
now there is a different Electrical-Charge on each of the 2-materials, AKA- a Voltage-Potential.
.
.
.
 

Thread Starter

MrAl

Joined Jun 17, 2014
9,633
Voltage-Potential is created first,
because You can have Voltage with zero-Current.

Current only occurs after a Conductive-Path
is established between one Voltage-Potential
and another Voltage of differing Potential.

That Conductive-Path between 2 differing Potentials,
will always have a certain amount of Resistance,
or there could not possibly have been any Voltage difference in the first place,
therefore, Ohms-Law always applies.

Electricity can not exist without all 3 factors of Ohms-Law being present.
.
.
.
What about a supercondcutor that continuously conducts current with no outside influence?

I understand what you are saying about the voltage being present at first like with a battery with no load. That's an interesting view, but i dont think that is the same thing as having a circuit and applying some electrical quantity. Maybe a lighthearted view is, "If a voltage exists and there is no one to measure it, does it really exist?".
But the real idea here is change. A voltage by itself does not change anything unless it is used somehow. I guess you could ask how the voltage got there in the first place also. But for a transistor, we cant excite the base without some current because we have to charge any input capacitance, and in an FET there is still some current and in fact there is a large surge current.
So i think we can say a voltage can exist by itself but not sure if that applies to a transistor circuit or other circuit.
 

Thread Starter

MrAl

Joined Jun 17, 2014
9,633
I think, each solar cell produces a photo voltage which allows a corresponding current. This is the reason we can combine several cells in series to get a larger voltage.
A shorted solar cell still produces current. I actually had to measure this for an application one time out in the sun.
 

LowQCab

Joined Nov 6, 2012
2,650
"If a voltage exists and there is no one to measure it, does it really exist?".
.
This should read .............
If a Voltage exists, and there is nowhere for it to go, does it really exist ?
.
.
.
So i think we can say a voltage can exist by itself but not sure if that applies to a transistor circuit or other circuit.
.
A BJT Transistor turns on because You applied enough Voltage to the Base to cause it to start Conducting.
BJTs are actually controlled by Voltage, it's just that it's convenient to refer to it's Control as being
"Current-Driven" because the difference in Voltage between "Off" and "Smoke" is so small,
and because the Voltage requirements vary substantially with Temperature.
Therefore they are treated as a virtual direct-short, with a Forward-Voltage-Drop thrown into the mix.
.
.
.
 

Thread Starter

MrAl

Joined Jun 17, 2014
9,633
Hi MrAl, you wrote:

"As you should know, each application has it's own preferred method or at least each model has it's own requirements. Also, some people are bent on declaring one way of doing it over another but is there really an overall blanket preference? I dont think so."

** My comment: I think, in order to avoid misunderstandings and long dicussions without a result, we should try to clearly distunguish between (a) physical relationships resp. laws and (b) models and/or „preferred methods“ for treating electronic circuits.

"But i must quote one line from your post in particular:
"That is because we cannot "inject" a current - a current is always the RESULT of an applied voltage"
That's not really true except again in a theory that prefers that definition. We can also say that voltage is the result of a current that flows from one place to another."


** My comment: What means “..really true except in theory...“? True or not ?
With respect to my comment above , you are right that „we can say...“ (we can use a kind of model), but my statement you were commenting was just a physical law (cause-and-effect relation). So - let us not mix different things.
I never did deny that it is allowed to calculate a voltage across a resistor using V=I*R (assuming that V is the result from I). But dont you agree that a good engineer should always know if he is using a working model or a correct physical relation?

"I believe the correct way to look at this is that they both occur simultaneously. A way of looking at this is that in order to establish a voltage you must move charge, and charge implies both current and voltage. simultaneously. In other words, energy."

** My comment: No - I cannot agree. I think that the question of „simultaneously yes/no“ has nothing to do with the question „which quantity causes a particular effect“. There are many examples in mechanics where cause and effect occur simultaneously - without us being able to swap cause and effect (gravity, force, pressure,..)

"Now we might go as far as to say that the "electric field" is 'first' established in a wire before current can flow. But in that view we are talking about *relative* values of current and electric field. In this case we deem a current as low as 1 femtoampere (for example) to be zero current when really it is not zero. Again it has to be simultaneous. Also we can look at how that electric field got established in the first place."


** My comment: Yes, of course. The E-field is the result from the voltage applied - and this E-field allows the charges to move (that is current). I think it would be not correct - from the physical point of view - to start with the current. This seems to be obvious because without an E-field there is no force acting on the electrons.

"So the bottom line is that current and voltage are independent only in theory and we decide what theory we want to use."


** My comment: I must admit that I would have problems „to decide what theory we want to use.“ Or do you mean “...to decide which models....“ to be applied for calculation purposes?
Again - we should avoid misunderstandings by mixing physical theory with artificial models.

"It's ok to disagree with this but then you should have at least one physical example that demonstrates the contrary."


** My comment: I think, I have shown that the question of simultaneousness cannot help to answer the question „which quantity comes first ?“.
More than that, remember my comment (in the former thread) to the BJT in saturation mode. Many people say „we inject a base current using a B-value much larger than specified.“
And yes, of course, this works...no doubt about this. In this case, they have a current-controlled current source in their mind. But this is a just a MODEL which does not reflect the physical theory - and it does not consider the DEFINITION for saturation which requires a base voltage Vb that can open the B-C junction.
The only thing I am stressing is that we as engineers should know about this - unless we like to follow established design rules only without knowing something about the physical backgrounnd. Do you now understand my position?
I'm sure you and I are not that far apart in our assessment - but we don't always talk and write about the same things (models as aids for calculation purposes vs. physical theories).
Hello again,

Thanks for the interesting reply.

What i am talking about is very simple which i will make more clear now.

First, and less important, when i talked about "theory" i meant in relation to the model being used.

More importantly, in any simpler circuit where we have an excitation and then what we call a response that appears to be delayed (a pulse delay circuit where the leading edge is delayed) we have a little calculation (probably involving an exponential for a capacitor) that reveals to us the exact delay or at least a good approximation. So my next question should be very clear.

If we apply a voltage to an RC circuit with say 1 Ohm and 1 Farad, what is the delay before the current begins to flow?
Now if current somehow comes 'after' voltage, then there should be a delay we can calculate so maybe you can even provide a formula. The point being, if this is true then we need a physical demonstration of something we can use to calculate this delay.
A related question is, if there is a delay, then at what voltage level does current begin to flow? Is it 1mv, 1uv, 1pv, 1fv, etc. It should be obvious that if we apply a 100v pulse it should not take 100v to start the current flow even if there is a delay. But this is a secondary question.

One thing for certain is that Maxwells equations show no delay between the electric and magnetic fields. However, that is within the framework of classical mechanics. Since quantum mechanics is the currently accepted ultimate physical theory i would think if this is possible there would have to be a quantum mechanical explanation, although it would not surprise me if this explanation was again a matter of convenience rather than a physical truth.
 

Thread Starter

MrAl

Joined Jun 17, 2014
9,633
.
This should read .............
If a Voltage exists, and there is nowhere for it to go, does it really exist ?
.
.
.

.
A BJT Transistor turns on because You applied enough Voltage to the Base to cause it to start Conducting.
BJTs are actually controlled by Voltage, it's just that it's convenient to refer to it's Control as being
"Current-Driven" because the difference in Voltage between "Off" and "Smoke" is so small,
and because the Voltage requirements vary substantially with Temperature.
Therefore they are treated as a virtual direct-short, with a Forward-Voltage-Drop thrown into the mix.
.
.
.
Just how does a voltage "go" anywhere?

Ok so if i apply enough voltage, how long does it take until 1 trillionth of a trillionth of a trillionth of an amp flows?
 

Ya’akov

Joined Jan 27, 2019
6,856
Since voltage is a measure of potential difference caused by an imbalance of electrons, and therefore negative charge, between to bodies, it seems to be the case that the voltage is present so long as the imbalance is present. On the other hand, current is a measure of the movement of charge in response to the imbalance, so it is not present until there is a path for the flow.

So it seems to follow that voltage as a measurement expressed in Volts, which cannot be made without current flow, is not what is being discussed rather it is the existence of the potential difference, whose magnitude we can’t determine absent the presence of current, is the voltage that is applied as a precursor to current.

The fact that practical measurement of voltage must always involve current seems to be a source of confusion. It we substitute the word “potential“ in. the original case, would you still say “potential and current occur simultaneously?”.

It seems much clearer that potential is something that has independent existence. The word “voltage” is too overloaded for it to be clear.
 

LowQCab

Joined Nov 6, 2012
2,650
If we apply a voltage to an RC circuit with say 1 Ohm and 1 Farad, what is the delay before the current begins to flow?
.
There is virtually "zero" delay before Current starts to increase,
but it may not increase at a rate that You would like.

You seem to be wanting an explanation of how Electricity works,
but the fact of the matter is that no one "actually" knows,
there are simply rules that, when applied in a prescribed manner,
cause a desired result very consistently,
and, of course, the flavor of the day speculations, which are very likely completely wrong.
.
.
.
 

LvW

Joined Jun 13, 2013
1,586
A shorted solar cell still produces current. I actually had to measure this for an application one time out in the sun.
Yes, and a battery also "produces " a current - provided there is closed current loop. However, it both cases, it is the VOLTAGE which drives the current. No current without a driving voltage.
(I am not sure if superconductivity would be a good counter example - the only one?)
 

LvW

Joined Jun 13, 2013
1,586
More importantly, in any simpler circuit where we have an excitation and then what we call a response that appears to be delayed (a pulse delay circuit where the leading edge is delayed) we have a little calculation (probably involving an exponential for a capacitor) that reveals to us the exact delay or at least a good approximation. So my next question should be very clear.
..............................
If we apply a voltage to an RC circuit with say 1 Ohm and 1 Farad, what is the delay before the current begins to flow?
When I bump a ball (mechanical impulse), is there any delay between impulse and movement of the ball?
I don't know - it is even unimportant for our question, because it is clear:
The mechanical impulse is the cause for the movement of the ball - independently of whether there is a delay or not.
 

LowQCab

Joined Nov 6, 2012
2,650
A battery only "retains" a Voltage-Potential-Difference between it's Terminals, sort of like a spring, it doesn't "do" anything else.
A Battery can not "Produce" Current,
Current "may" "occur" if a Conductive-Path is provided between it's Terminals.

Current is a measurement of how fast a "Voltage-Difference" between 2 points
is trying to equalize it's self back to zero difference.
.
.
.
 

ElectricSpidey

Joined Dec 2, 2017
2,183
This question cannot be answered without a definitive definition of "current".

Voltage is a separation of charge...but you need current to separate that charge. (movement of charge)

But many will argue that the movement of electrons that develop a voltage does not always constitute a current. (example: in the case of photovoltaic)

So, if "current" is only the movement of charge caused by voltage...then voltage has to come first, but if current can be the movement of charge by any means, then current can come first.

If anyone has seen the lightning in a volcano plume, those charges were separated by friction, so the question...does separation of charge by friction constitute "current"?
 

LvW

Joined Jun 13, 2013
1,586
So, if "current" is only the movement of charge caused by voltage...then voltage has to come first, but if current can be the movement of charge by any means, then current can come first.
Yes - that is also my opinion.
As far as the last part of your sentence is concerned: I think, here we are speaking of a quantity called "current" which can be measured in electronic circuits (driven by a voltage).
More than that, I think it would not be meaningful if all movements of charges caused by other phenomena (e.g. as a result of a chemical process) would also be called "current".

Additional remark: I think, the title of this thread is somewhat misleading (...which occurs first?):
According to my interpretation, the word "first" does not apply to a timely sequence (delay between some occurences) but to the "cause-and-effect" problem. Am I right?
 
Top