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So as the question asks, say you have a circuit with a battery connected to a resistor to form a complete circuit in this case does voltage cause current through the resistor? or does the voltage across cause the current?

I feel like there are so many explanations of various electronic circuits and devices in which people say that current cause a voltage to appear and in other cases the other way round which one is it?

As far as I know based on basic physics of voltage and electric fields once a circuit is completed the charge separation between the two ends of a battery cause an electric field through the entire metal, resister/capacitor etc this causes the electron cloud to drift one way or the other am I wrong? how can current cause a voltage drop?

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    \$\begingroup\$ Both, depending on the situation. \$\endgroup\$ – PlasmaHH Nov 18 '15 at 22:47
  • \$\begingroup\$ @PlasmaHH Can you expand on your explanation or maybe link some website or book that will answer my question in more detail? \$\endgroup\$ – hannibal2469 Nov 18 '15 at 23:11
  • \$\begingroup\$ Maybe look at cfa.harvard.edu/smg/Website/UCP/pdfs/SimpleCircuits.pdf although it is written with teachers in mind as the audience. \$\endgroup\$ – Fizz Nov 19 '15 at 1:40
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    \$\begingroup\$ To use the water metaphor, does flow rate cause pressure, or does pressure cause flow rate? \$\endgroup\$ – Nick Johnson Nov 19 '15 at 10:55
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    \$\begingroup\$ Does voltage cause current or does current cause voltage?" Yes. \$\endgroup\$ – Olin Lathrop Nov 21 '15 at 23:23
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One doesn't necessarily cause the other. But you can't (except in special cases such as a superconductor or a perfect insulator, only one of which actually exists) have one without the other.

You can't produce a voltage without supplying charge (or current) to force some place to have that voltage.

You can't force a current to flow through some circuit element without applying a voltage across the element.

In some circuits you might explain the operation by saying a voltage source causes current to flow, or a current source causes a voltage to be produced. But at the deepest level, voltage and current are simply inseperable.

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  • \$\begingroup\$ Not necessarily. Current can flow in a superconductor indefinitely, without any applied voltage. \$\endgroup\$ – Robert Stiffler Nov 19 '15 at 0:07
  • \$\begingroup\$ @RobertStiffler, good point, edited. \$\endgroup\$ – The Photon Nov 19 '15 at 0:18
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    \$\begingroup\$ Thank you, that was very helpful, but what about all these textbooks that explain so many circuit/device operations through a cause-effect relation between current and voltage? wouldn't you agree that especially when they explain diodes, regulators, transistors etc they use voltage and current as one causing the other? I still cant help but think that there must be one or the other that is the cause, can you help me out? could you suggest some book that talks about this? \$\endgroup\$ – hannibal2469 Nov 19 '15 at 1:35
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    \$\begingroup\$ @hannibal2469, if you power your circuit with a voltage source, it's sensible to say that voltage causes whatever current flows to flow in your circuit. If you power your circuit with a current source, the other way around. Like I said in my last sentence, you can explain a circuit in terms of one thing causing the other, but at a deeper level, the two concepts (voltage and current) are inseperable. \$\endgroup\$ – The Photon Nov 19 '15 at 5:23
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    \$\begingroup\$ I mean that (except in certain special circumstances) you can't have one without the other. Every circuit element (or branch of an element) has its I-V curve. If you want to cause it to have a certain voltage you have to be able to supply a certain current; if you want it to have a certain current, you have to be able to supply a certain voltage. \$\endgroup\$ – The Photon Nov 21 '15 at 21:57
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TL;DR: Newton's First Law.

By definition, the movement of charged particles (electrons in most conductive and semiconductor materials, ions in plasma and molten or solution of electrolyte) is electric current, and the difference of strength (potential) of electric field is voltage. However the charged particles and the electric field exert Coulomb forces to each other and that is the link between current and voltage.

If we somehow force charged particles to move (friction, changing magnetic fields, photoelectric effect or chemical reaction) the charged particles that is externally forced to move will create a difference in potential.

Everything that is not superconductor will pose a barrier so some difference of potential is required for the particles to pass through. This is the basis of Ohm's law.

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  • \$\begingroup\$ Thanks for the response, but even in your explanation it seems that a voltage difference would have to exist apriori how can charge flow cause a voltage difference? as per my understanding only a voltage can cause a current and not the other way around \$\endgroup\$ – hannibal2469 Nov 21 '15 at 21:11
  • \$\begingroup\$ @hannibal2469 I actually missed this crucial point: Newton's 1st law. Volts is the strength of electric field while amps is the movement of charge, and since those are the two ends of Columb force amps and volts are just action and reaction. \$\endgroup\$ – Maxthon Chan Nov 21 '15 at 21:22
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In my opinion, the charges will cause voltage i.e also known as electron motive force. suppose assume that there are no charges i.e free electrons than there is no voltage. In another case if we consider, inside a battery, there are electrons due to chemical reaction or due to the liquid, these electrons are ready to find least impedance path to move out. In this case, the accumulation of charges cause some voltage that is the difference in the potential because of this potential, electrons (charges) are driven into circuit.

put it simply, V is proportional to R X I if V = 1.5v is caused due to some charges in the acid/alkaline liquid which offers so resistance because of which the volage is 1.5v if R = 1 (ideally possible) then V is same as I.

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This is one of those topics, like what is better AC or DC, that can have people arguing for ever. However, despite my better judgement, I'm going to weight in.

Voltage Causes Current

Let us consider the two primary methods of creating a voltage.

1. A BATTERY

A battery is one of the most miss-understood devices we use. The classic boilerplate "why is the sky blue?" answer to how a batter works is....

"Batteries have three parts, an anode (-), a cathode (+), and the electrolyte. The cathode and anode (the positive and negative sides at either end of a traditional battery) are hooked up to an electrical circuit. The chemical reactions in the battery causes a build up of electrons at the anode." Soucre

enter image description here

We have passed that at the answer to "How to batteries work?" since the thing was invented and it is generally accepted as a "that makes sense" answer, and people leave it at that. People think of it like a capacitor, that the anode and cathode are like capacitor plates with a charge built up on them.

Unfortunately, it is also totally wrong.

If that were true, two things you know would not be true. First, if the description above were true, a bigger battery would mean more chemical reaction and therefor more voltage. But we know that's not true, a AAA alkaline battery has the same voltage as a D-Cell. Second, if a chemical reaction is causing the voltage, an ideal battery (no current leakage) could not hold it's charge forever if left unconnected. The chemical reaction would, at some point, "burn" itself out.

The truth is a battery generates voltage without a reaction taking place. The reaction happens when you allow current to flow between the terminals.

So what is REALLY going on.

Batteries work because of a lesser known physical effect called Electronegativity.

"Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract electrons (or electron density) towards itself. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it."

What does that mean? It means when you bring two dissimilar elements together in the presence of an electrolyte, a material that facilitates the motion of ions between the metals, there is a force generated that wants to pull electrons from one side to the other. We call that force VOLTAGE.

When you build that you create a Galvanic cell.

"In a Galvanic cell, the metal atoms of one half-cell are able to induce reduction of the metal cations of the other half-cell; conversely stated, the metal cations of one half-cell are able to oxidize the metal atoms of the other half-cell. When metal B has a greater electronegativity than metal A, then metal B tends to steal electrons from metal A (that is, metal B tends to oxidize metal A), thus favoring one direction of the reaction:"

When you connect the terminals together via a conductor, the voltage pulls electrons from one metal to the other through that conductor. This allows ions to move through the electrolyte and the chemical reaction to take place.

But again, to be clear, when disconnected, nothing is moving. There is no current, no chemical reaction and no build up of "charge". But the voltage is still there.

2. ELECTROMAGNETIC INDUCTION

Consider the circuit below.

schematic

simulate this circuit – Schematic created using CircuitLab

OK so if you know your stuff, you know that there is 100V AC appearing at the outputs of that transformer. That's easy enough to understand, right?

But how much current is flowing?

The answer is...... NONE.

You know by conservation of energy, that since the outputs are disconnected no power is being removed from the output of the transformer and therefore no power is being inserted into the input either. Again, as a consequence, no power is being removed from the generator either. Current in the above circuit is zero on both sides. There is no motion of charges.

So how on earth is there a voltage on the right side?

The truth is the rotating magnets induce an electric field in the coils of the generator which is propagated through the conductors at the speed of light. That moving electric field generates a magnetic field in the transformer which in turn generates the opposite electric field in the secondary.

That all happens without any charge movement.

In the primary the transformer effectively produces a back-emf equal to the emf of the generator. Since the Voltages are the same no current can flow through the wires.

When you attach a load to the secondary, the voltage generates a current which causes it to collapse some of the electric field. The back emf on the primary collapses in sympathy. That unbalances the voltages in the primary and current flows from the generator.

A FINAL DEMONSTRAION

Consider a DC motor. OK we all know you attach a voltage to the motor the motor gets all "excited" and starts turning. Current flows through the motor and creates torque.

But then that odd thing happens called "Back EMF".

Here is your final proof that voltage is not caused by current.

You KNOW your current is "going in" the RED LUG of the motor and yet miraculously this voltage is appearing in the wrong direction. Charge can't be building up on the coil because you know the electrons are going the other way.

Voltage is, a thing to itself. It needs no circuit current.

BUT....

The truth of the "matter" is it's all theory and conjecture to try to build a workable model to describe an observational effect.

As with all theories, the more answers you give the more questions get asked. Further each theory has it's own set of believers and disbelievers. Theories and models also change with time. That's why we call them theories.

It's like peeling an onion. The deeper we go, the weirder it gets.

However, at the macroscopic level we work at, you open the switch, the current disappears but the voltage remains. At the end of the day, that's all that really matters in this forum.

How it physically gets there is not really of any great consequence unless perhaps you are building a quantum computer on your breadboard.

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    \$\begingroup\$ I would like to add to the stuff about the motor. The back-emf is there because of Maxwell's equations about the effect of changing magnetic field with respect to the coil. There was no initial charge movement within the coil to begin with. If the speed is maintained, that emf (which is basically voltage) is going to be there, regardless of the current existence. Which direction the current flows will tell whether the machine is running as a motor or generator. \$\endgroup\$ – Transistor Overlord Apr 10 '17 at 15:57
  • \$\begingroup\$ For the battery: the fact that you have reached equilibrium for the special value of I=0 does not mean that voltage is independent of current. That equilibrium is there only if you draw no current. As soon as you put a load and currents 'flow', causing the ions inside the battery to be disturbed, the only way to re-estabilish the equilibrium is to bring charge back to the battery (closing the circuit). So, you need that current to maintain that voltage. For a nice (nearly) one-page intro on how a battery works you can see Kip, "Fundamentals of Electricity and Magnetism" 2nd ed., p. 229 \$\endgroup\$ – Sredni Vashtar Apr 10 '17 at 17:04
  • \$\begingroup\$ For the generator: deep down it all boils down to the fact that magnetic fields are electric fields in a different frame of reference. And what produce electric fields? Charge distributions. I can see another problem with your view: you seem to be implying that the fact that we can readily have approximately ideal voltage generators (because air or vacuum is basically a good electrical insulator) means that in general voltage has to be independent of current. But that is just the particular shape of the V-I relationship of an ideal voltage generator. \$\endgroup\$ – Sredni Vashtar Apr 10 '17 at 17:08
  • \$\begingroup\$ @SredniVashtar and again.. none of that happens without the base level physical effect Electronegativity. But believe what you will. \$\endgroup\$ – Trevor_G Apr 10 '17 at 17:10
  • \$\begingroup\$ Electronegativity does not come out of a dictionary. It is the result of unbalanced charge distribution at an atomic level. en.wikipedia.org/wiki/Electronegativity \$\endgroup\$ – Sredni Vashtar Apr 10 '17 at 17:14
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What happens is what happens. The causes are what you choose to explain it.

Below the simple physics that you mention, voltage difference, electric field, charge separation, lies a more fundamental level of energy, quantum mechanics, photons mediating forces between charged particles, that you could regard as causing it.

Below that level, the causes of that level, is perplexing the leading edge physicists of the world, there's no agreement between multiple universes, hidden variables, and what have you.

Chasing causes will do your head in.

Probably the most appropriate model to use, the one I find has most explaining power at my level of comprehension, is a transmission line. Track wavefronts and their reflections up and down the connecting wires between ideal batteries, switches and loads.

A wavefront comprises a voltage wave and a current wave, their ratio being given by, or defining if you will, the impedance of the transmission medium.

But in using this model, don't worry about what causes the wavefronts, the impedance ratio, the resistive behaviour, the voltage source behaviour, these just have to be taken a priori. At whatever level you choose to operate, you will have to accept some givens.

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This is my understanding.

Every charged particle or a charge exerts an electrical force that causes movement of charges.

So charge causes voltage that causes flow of charges.

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    \$\begingroup\$ So X causes Y which then causes X. \$\endgroup\$ – pipe Sep 24 '16 at 2:40
  • \$\begingroup\$ The block of steel on my desk is full of charged particles. I can't measure any voltage across it and no current out of the ends. You might wish to upgrade your understanding and revise your answer. \$\endgroup\$ – Transistor Sep 24 '16 at 10:10
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Voltage causes Current . There will not be an electron movement (current) if there is no potential difference ( voltage ).

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  • \$\begingroup\$ What about a current transformer where secondary current produces the voltage? \$\endgroup\$ – Transistor Nov 19 '15 at 7:21
  • \$\begingroup\$ @transistor It's not the current that produces a voltage. It's a collapsing magnetic field that creates an electric field which causes the voltage which then causes a current. \$\endgroup\$ – horta Nov 28 '15 at 2:38
  • \$\begingroup\$ Let's say I drive by a stationary charge. From my view, it's a moving charge, and therefore a current. What voltage caused this current? \$\endgroup\$ – neonzeon Jan 27 '17 at 18:36
  • \$\begingroup\$ Superconductors? \$\endgroup\$ – Oskar Skog Apr 10 '17 at 18:45
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Voltage causes current. If you short out a 1.2v AA battery, it should give you about 2A. Using ohm's law A=V/R, and let's assume that R being shorted out is a very low value, such as 0.01ohm; then V/R=1.2V/0.01ohm=120A, but then why I said a 1.2v AA battery will give you only 2A when shorted out while this calculation is so different? It's the internal resistance of the battery. If you connect 100x 1.2V AA battery in parallel, it will give you 1.2V, but if you short them out, it should able to give you more than 200A current, so Voltage causes current!!

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    \$\begingroup\$ And if you apply a constant current source to a fixed resistor? \$\endgroup\$ – The Photon Nov 18 '15 at 23:15
  • \$\begingroup\$ @ThePhoton could you show me a constant current source outside of circuit theory? All of them that I know are controlled voltage sources masquerading as current sources. \$\endgroup\$ – horta Nov 28 '15 at 2:39
  • \$\begingroup\$ @horta, a current mirror is a very common example. \$\endgroup\$ – The Photon Nov 28 '15 at 3:47
  • \$\begingroup\$ @ThePhoton A current mirror regulates a voltage source to act like a constant current source, but in no way does it source current. If anything, it'd be be more accurate, but less simple to think of mirrors as continuously varying resistor to approximate a current source, but I don't see any way it can actually source current. \$\endgroup\$ – horta Nov 28 '15 at 18:07
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    \$\begingroup\$ @ThePhoton Please expand or your chemical cell example? Are you talking about charge or discharge? In the charging case, you require a voltage source to move the electrons. In the discharging case, you have a voltage source moving electrons from the negative terminal to the positive one, where the voltage source is caused by the charges already being separated. And if you look into the Ebers-moll model, you'll see that a bjt is actually a voltage controlled device. \$\endgroup\$ – horta Nov 28 '15 at 18:40

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