Hello there,

This question came up in a previous thread and is interesting to think about not only from the standpoint of understanding it, but for people just getting into circuit analysis it seems to be mysterious idea.

That question seems to imply a cause-and-effect rule of some kind, but the best view for a person just getting into this I think is to look at Ohm's Law, which does not provide a cause-and-effect rule it just provides a rule about the relationship between the current and the voltage in a resistance. I believe that is best because the one-or-the-other is either simultaneous or it happens so fast that we probably can't measure it ourselves without a good lab with special measuring equipment, and even then maybe not.

It all boils down to the initial derivatives. If at t=0 the first derivative starts out at zero and ramps up, there can be no cause-and-effect relationship that we could ascertain because that would mean that they both happen at the same time. That would mean of course that we could never say that the current 'causes' a voltage, and we cannot say that the voltage 'causes' a current, because when a current appears so does a voltage, and when a voltage appears it's always accompanied by a current.

Another view is the electric field as it establishes along say a length of wire. The field cannot run along the length of the wire in zero time, it takes some finite amount of time. That seems to imply that the field comes first. But is that really true? If the field starts out at one end of the wire, it must affect something in the wire before it is completely established. We have to be careful here though because it may not be a complete current it may just be surface charges, but it still seems like there is no REAL cause and effect if they both start at the same time. In reading about this we also have to be careful because often the text will assume it is a 'meaningful' current, and that would be different because that could be similar to a static current which probably does establish start right away. However, the electric field has not fully established itself yet either. This would mean we have to look at some incredibly small increment of wire to see what is going on, and that might be very difficult. If along (say) 1 nanometer of wire the electric field has established itself and no charge whatsoever has moved even a tiny amount, then we might be able to say that the electric field causes the charge to start to move, but if the charge starts to move immediately then we are back to no cause-and-effect result.

The idea of a 'meaningful' current is useful here anyway i think, because in the context of someone just entering the field they need a working knowledge of the typical current and voltage relationships in a simple component like a resistor. The best bet I think is just to hand them Ohm's Law and explain that you can look at it either way in practice. We can think of the voltage establishing a current or the current establishing a voltage, keeping in mind that in the bottom-line physics they are probably starting at the same time.

We can look at this more if anyone would like to.

 

Voltages and currents are derived from the aggregate behavior of an arrangement of charges. They are not physical things that cause anything. All cause and effect is due to the interaction of the individual charges. IMHO it is wrong to attribute cause to either of them.

 

OK - after reading the first two posts, I have the following question:

Without taking into account the timely behaviour of the individual effects (field build-up, voltage function, current function) - is the following simple statement incorrect?

A necessary precondition for the movement of charges within a conductor (definition of current) is the existence of an electric field in the conductor.
Thus, the voltage across a resistor is clearly the cause of the corresponding current.
Without this voltage (and the associated electric field), there is no force that can cause the movement of charges.

Wrong ? Or to simple ?

* Added later: I think, it this context also the following question arises:
From the physical point of view - does something like a "current source" really exist? Or is a device we call "current source" nothing else than a voltage source with a large source resistance (static or dynamic).

 

Nice reasoning but then what is a solar panel? Voltage or current source..

 

Yes.

 

Voltage causes current vs current causes voltage. It's the chicken and egg problem.
Ohm's Law was based on observation:
I = V / R
The corollary is:
V = I x R

But as @MrAl says, Ohm's Law does not provide cause-and-effect. It states the mathematical relationship between current and voltage.

Rather than debating cause-and-effect, you have to go to the fundamental principles of EM theory where both the electric field and magnetic field coexist.

 

The voltage is an abstraction. It is the forces between charges that cause the current to flow. The field is also an abstraction.

Wrong ? Or to simple ?

 

If you consider the tiny bits of wire, you'll end up with electromagnetic field, which is something that the Maxwell equations tell us that are simultaneous. And we can understand why, the magnetic field is the rotation of the electric field, and vice-versa. This question would be "which derivative comes first?" when dealing with Maxwell equations, which makes no sense.
Also, as we can interpret magnetic fields (generated by electric charges) as charges having length contraction or time dilation, we see that it's just only one thing.

 

Thus, the voltage across a resistor is clearly the cause of the corresponding current.

Unless the current is induced in the conductor by a varying external magnetic field. Ultra-simplistically and guaranteed to piss off somebody - *In this case* the field creates the current, and the current "creates" the voltage.

or something like that.

Note that the logo for the IEEE is the right-hand rule: wire, field, and current.

ak

 

To properly answer this question, you must start at the beginning.

All electrical generation starts with emf, the first form of electrical energy is always current, from current comes voltage. (potential difference)

 

To properly answer this question, you must start at the beginning.

All electrical generation starts with emf, the first form of electrical energy is always current, from current comes voltage. (potential difference)

Not so fast. What is emf? Electro-motive force. Is force a current or voltage? I would say it is neither.

 

Keep in mind that a field differential can cause two points to have a voltage potential across them, even without current flowing, e.g. the plates of a capacitor.

Current flow through a conductor will also cause a voltage difference. This being due to resistance in the conductor.

 

Semantics and shrodingerss cat.
Cant measure one without the other .

 

Not so fast. What is emf? Electro-motive force. Is force a current or voltage? I would say it is neither.

Exactly...the point is you can create current without voltage.

 

Exactly...the point is you can create current without voltage.

A corresponding example would be nice....

By the way - when I speak about "current" in this thread, I am referring to a "classical" current within an electrical circuit and NOT about carrier separation/movement within a voltage source.
(In an answer above I could read "...what is a solar panel? Voltage or current source.."
Answer: Of course, it is a voltage - as a consequence of charge separaton !)

Let me state the following:
The statement "current through a resistor R causes a voltage across R" raises a logical problem.

(1) Lets take the following example:
A (nearly) ideal voltage source V0 is connected to two resistors in series (R1, R2).
So we can say that
V2=I*R2 and V1=I*R1.
In words (assuming I causes V) we could say that both voltages V1 and V2 are caused by the current I - and, as a consequence, also the sum of both: V0=V1+V2.
Final result: The voltage V0 is caused by the currentI I. Really?
Remember - the example has started with Vo being the source of everything that has happened.

Here is another example:
A voltage source drives a current through a potentiometer I=V0/Rtotal
Does it make sense to say that the voltage in part A (R1) or part B (R2) of this device would be caused by this current?

 

Angels, pins.

 

Btw, there is no such thing as a voltage or current source, both are conventional terms relating to regulation.

Current is a process that occurs in a circuit, and voltage must always be "developed" where it exists.

 

To properly answer this question, you must start at the beginning.

All electrical generation starts with emf, the first form of electrical energy is always current, from current comes voltage. (potential difference)

Please, could you explain/verify this claim?
When "current" (movement of charges) is the "first form of electrical energy" - where is the force to enable such a movement?
(By the way - within the contents of this thread I refer to current through conducting material only). If other forms of moving charges can be called "current" is another question. )

 

Nice reasoning but then what is a solar panel? Voltage or current source..

Either , depending on where you are on the curve.

 

For some reason they decided to change “potential difference” to voltage thereby hiding its true meaning. Typically we still say current instead of amps. Cassels Popular Science, a book published in the early 1900s explains the analogy between a head of water and a potential (voltage) difference and current and the flow of water. At an early age this helped me have a “light bulb moment” when I realized how to make a low voltage bulb turn on with a battery. The head of water makes the current flow, not the other way around.