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Voltage vs Current. A cause or a consequence?
- Thread starter Raúl Figueirinha
- Start date
MisterBill2
- Joined Jan 23, 2018
- 27,530
Really, it is the voltage that pushes the current, which is why voltage is also called electomotive force (AKA "EMF"). It is that potential difference that pushes the electrons to move.
Then, the current flowing through any resistance causes a voltage drop across that resistance. So there is no voltage drop across a resistance until there is current flow through that resistance. But that voltage is the result of the current, not the cause of it.
Then, the current flowing through any resistance causes a voltage drop across that resistance. So there is no voltage drop across a resistance until there is current flow through that resistance. But that voltage is the result of the current, not the cause of it.
The two are intimately related and whether A caused B or B caused A depends not only on the specifics at hand, but also on the starting point chosen by the observer. For those that claim that no current can flow without a voltage driving it and, hence, voltage causes current you have two problems. First, it is entirely possible to have a current without a voltage driving it, just consider a persistent-mode superconducting magnet. But even more fundamental, where did that voltage come from? A voltage is a measure of the electric field and most electric fields are the result of charge separations and separating the charges is, by definition, an electric current. At least here we can break out and find ways to cause those currents that don't require voltages to drive them -- we have all kinds of ways to use other means to convert energy from other forms to mechanically or chemically move the charges.
To Raul regarding "Is it the potential difference that creates the current or is it the current that creates the potential difference? "
What is current ? one definition reads:
It is an electrical phenomenon caused by flow of free electrons from one atom to another. The characteristics of current electricity are opposite to those of static electricity. Wires are made up of conductors such as copper or aluminum. Glass. nylon or vacuum or air gap to lesser degree are a few of the dielectric materials that can be involved.
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The electron interaction along a gradient of a media with the other subatomic particles gives a longer definition the above definition refers to the cause.
An electron is a fundamental subatomic wave/particle with the ability to convert into different forms of energy as a consequence of charge movement. It has form( to some extent and) is an entity that can be pointed to as a cause unlike an electrical characteristic when boundary conditions are not specified a cause can be misinterpreted. A deduction can be made by proposing a circuit showing how electrons behave when stored. It is in agreement with the problem solving method. Several practical circuits could be constructed to demonstrate flow in both current electricity and static electricity.
A simple plate capacitor can serve as model where a charge is stored, A voltage source can be applied. The fields and characteristics can be observed.
It can be shown using Volts and Amperes however other units will also work. It would need to show that the definition of current above is reasonable.
If the math agrees but the electrons are not directly observed there can be uncertainty so we look toward the math as sufficient.
Current is the flow of free electrons from one atom to another.
What is current ? one definition reads:
It is an electrical phenomenon caused by flow of free electrons from one atom to another. The characteristics of current electricity are opposite to those of static electricity. Wires are made up of conductors such as copper or aluminum. Glass. nylon or vacuum or air gap to lesser degree are a few of the dielectric materials that can be involved.
---------------------------------------------------------------------------------------------------------------------------------------------------
The electron interaction along a gradient of a media with the other subatomic particles gives a longer definition the above definition refers to the cause.
An electron is a fundamental subatomic wave/particle with the ability to convert into different forms of energy as a consequence of charge movement. It has form( to some extent and) is an entity that can be pointed to as a cause unlike an electrical characteristic when boundary conditions are not specified a cause can be misinterpreted. A deduction can be made by proposing a circuit showing how electrons behave when stored. It is in agreement with the problem solving method. Several practical circuits could be constructed to demonstrate flow in both current electricity and static electricity.
A simple plate capacitor can serve as model where a charge is stored, A voltage source can be applied. The fields and characteristics can be observed.
It can be shown using Volts and Amperes however other units will also work. It would need to show that the definition of current above is reasonable.
If the math agrees but the electrons are not directly observed there can be uncertainty so we look toward the math as sufficient.
Current is the flow of free electrons from one atom to another.
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I disagree.
We have well defined equations that precisely describe the relationship between voltage, current, and electric and magnetic fields.
The chicken and egg question of what causes what is of a philosophical nature that is not important to the understanding and use of those equations.
In this case I must ask you: Which effect (force) causes the charges to move inside the resistance?
I disagree, because we should not mix different definitions.
Consider the chemical process inside a battery. If you call this process of charge separation "current" - it is surely another kind of "current" if compared with the quantity we call "current" within an electrical circuit.
Something similar applies to the quantity we call "diffusion current" within a pn junction. Also in this case, there is a certain "force" which causes the charged carriers to move (difference in charged carrier concentration).
This applies also to the photo-electric effect (very often used as a "counter example").
In any case, there must be an external "force" which causes the charges to move.
I am sure that the questioner was referring to the current within an electrical circuitry (transistors, resistors,...) - and of course:
No current without a driving voltage across (E-field within) a conducting device.
Last time I checked: Electric current is defined as the rate at which charge flows through a surface. As a rate there is no distinction about how, where or what means causes the flow. The energy in the em field is from the work (charge separation force) during charge separation (an EMF) commonly measured as a voltage potential that also have the ability to do work but we need to be careful about what is what.
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I'm not aware of different definitions of electrical current -- I thought it was defined as a flow of net charge past through an area.
If there can be no current without a driving voltage across a conducting device, then how is it we can take a superconducting magnet and short it to itself and remove it from any power source and it will sit there happily for months before the current dies off even a few percent (due to parasitics)?
I was elated this morning to see so many great post, excellent! Great videos (CBU physics department a college in Nova Scotia)
The battery, the Van der Graaf, the presentation, key points, emphasis on the concepts that are important as basic foundation.
The battery, the Van der Graaf, the presentation, key points, emphasis on the concepts that are important as basic foundation.
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mirror_pole
- Joined Nov 14, 2020
- 10
depends what u are looking at i would say.
Everything has its "origin" in the maxwell equations: If you think about a static problem (rot E=0) then u need a voltage (static electric field), hence change in potential (E=- grad phi). Here a voltage is a cause and current a consequence.
On the other hand u can also say that rot E = -dB/dt so a change in the magnetic field over time creates rot E. B is dependent on the current.
Here the changing current, hence B produced rot E and therefore a voltage.
This question becomes interesting from a circuit level perspective: Think about a simple current mirror. The cause of the Ugs voltage of the diode connected transistor lies in IREF (the "golden" current source). Thats why u need this diode connection in first place, to copy all the important properties of IREF.
Everything has its "origin" in the maxwell equations: If you think about a static problem (rot E=0) then u need a voltage (static electric field), hence change in potential (E=- grad phi). Here a voltage is a cause and current a consequence.
On the other hand u can also say that rot E = -dB/dt so a change in the magnetic field over time creates rot E. B is dependent on the current.
Here the changing current, hence B produced rot E and therefore a voltage.
This question becomes interesting from a circuit level perspective: Think about a simple current mirror. The cause of the Ugs voltage of the diode connected transistor lies in IREF (the "golden" current source). Thats why u need this diode connection in first place, to copy all the important properties of IREF.
At this moment, I like to repeat the original question:
Is it the potential difference that creates the current or is it the current that creates the potential difference?
I agree that we have one single definition for an electrical current - BUT: We can have different kinds of "forces" which act as a cause for movement of charges.
In this context, I have mentioned three examples: (a) chemical processes, (b) diffusion across a pn junction, (c) photo-electrical effect.
I am not sure if a "superconducting" device can be used as an example for answering the original question.
Therefore, to be as precise as possible:
Within an electronic circuitry we have (and can measure) several different currents and voltages. All these currents and voltages fulfill the laws as formulated by Kirchhoff.
And - as an answer to the question: All currents within the circuit are driven by corresponding voltages (potential differences). Such a current through a part cannot produce a voltage across this part because this voltage (resp. the corresponding E-field within the part) is necessary to allow this current.
However, in this context we should realize that the term "current source" is something like "labor jargon". In fact, there is no current source at all. In electronics, we use a voltage source with a very large source resistance (very often not a static but a differential/dynamic resistance) for allowing a current that is - more or less - constant and does not depend on the connected (varying) load.
If you want to create a special case circuit theory sandlot where Kirchhoff is always true (no non-electrostatic EMF, the net charge in different parts of the conductor does not changes with time) then maybe but in the general case of actual energy moving charge it's not true as a root cause and effect.
https://www.av8n.com/physics/kirchhoff-circuit-laws.htm
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As I have mentioned - I have tried to answer the question which clearly was related to our classical electronic circuits (amplifiers, filters, ...). Where am I wrong?
Therefore, I think this is not the right place (occasion) to discuss the limitations of Kirchhoffs laws.
The question was
"Is it the potential difference that creates the current or is it the current that creates the potential difference?"
This is a question that can't be answered in the context of classical electronic circuits (amplifiers, filters, ...)
this means your answer is Not even wrong.
May I kindly ask you to explain to me why this question "can't be answered in the context of classical electronic circuits"?
If THIS is your answer to the questioner - he certainly will be confused...I am afraid.
Because classical electronic circuits are engineering solutions. They make the proper simplifying assumptions of quasi-static conditions because it works if you follow the engineering rules. If you don't follow those rules, things out of the realm of quasi-static conditions can start to happen. A single ended audio signal circuit is receiving noise from a motor not in the amplifier circuit loop. Something is moving charge, causing currents and voltages in the audio circuit. The engineering solution is to shield wires/enclosures , twist wires, etc ... problem fixed using the assumptions and approximations of things like transmission line theory. The question of why charge is moving, causing currents and voltages as a cause and effect is a different realm.
