How does a current "know" where to go

[xWaldorf]

Let's say we have a circuit, and in that circuit there's a resistor, and there's a wire that goes around it so that the current can flow freely without getting to the resistor.
my question is this: how does the current "knows" to flow towards the around the resistor? how does it know not to get into the junction that leads to the resistor?

 

[cnh1995]

how does the current "knows" to flow towards the around the resistor?

Ohm's law, V=IR.

 

[xWaldorf]

Ohm's law, V=IR.

Care to elaborate?

 

[MikeeMiracle]

Voltage = Current multiplied by Resistance. It's actually flowing through the both the wire and the resistor in your example. The amount of current flowing through each is dependant on the resistance each offers to the current flow.

 

[cnh1995]

The amount of current flowing through each is dependant on the resistance each offers to the current flow

Yes.
In ideal case, the wire will have zero resistance, so ALL the current will flow through the wire, bypassing the resistance.
Practically, the wire will have some resistance and the current will be divided according to Ohm's law.

 

[xWaldorf]

Yes.
In ideal case, the wire will have zero resistance, so ALL the current will flow through the wire, bypassing the resistance.
Practically, the wire will have some resistance and the current will be divided according to Ohm's law.

That's my question exactly, if we consider an ideal case, what would make the current know that on the one side of a certain junction lies a resistor and on the other is the rest of the ideal wire bypassing the resistor?

 

[DrStupid]

That's my question exactly, if we consider an ideal case, what would make the current know that on the one side of a certain junction lies a resistor and on the other is the rest of the ideal wire bypassing the resistor?

It doesn't know. It just follows the electrical field. If the current would flow into the resitsor it would create a potential difference (according to Ohm's law) that makes it flow back into the wire.

 

[phinds]

That's my question exactly, if we consider an ideal case, what would make the current know that on the one side of a certain junction lies a resistor and on the other is the rest of the ideal wire bypassing the resistor?

Current only flows through a closed loop and it "flows" like a bicycle chain. You don't get one part to move without the rest moving. SO ... the current doesn't flow "towards" the resistor along the path to the resistor it only flows THROUGH the resistor (or not). Thus Ohm's Law is the answer.

 

[DrStupid]

Current only flows through a closed loop and it "flows" like a bicycle chain.

That only works if there are no capacities and it doesn't explain which of two alternative ways is preferred by the "chain".

 

[phinds]

That only works if there are no capacities

A reasonable point. I always think in terms of DC. The curse of a digital logic EE.

and it doesn't explain which of two alternative ways is preferred by the "chain".

Well, stick to DC for the moment and it does. Chains don't have "preferences", they are governed (in the easy to explain DC case) by Ohm's law.

 

[russ_watters]

That's my question exactly, if we consider an ideal case, what would make the current know that on the one side of a certain junction lies a resistor and on the other is the rest of the ideal wire bypassing the resistor?

Electrons don't know anything, they just go where forces on them take them and when there's less in the way, more go in that direction. Say you have a bucket of wa sand, with a big hole and a little hole in the bottom. Do the grains of sand know that more of them should go through the big hole? No, they go through it because there's less in their way to prevent it.

But since your exact question as stated is physically impossible, it may be tripping you up to try to think of it that way.

 

[DrStupid]

they are governed (in the easy to explain DC case) by Ohm's law

And the OP asks how that works. What makes the current go in a way that complies with Ohm's law?

 

[MikeeMiracle]

Not a technical scientific answer but here goes.

For a conductor to be able to pass current, it has to have free electrons inside it to help the current flow, otherwise it would not pass electricity. When you "turn the power on" electrons don't just start flowing from the battery directly, the free electrons in the conductor, the wire in this case, start moving first into the battery at one end and then "pull" electrons out of the battery the other end to continue the electron flow a bit like the bicycle chain mentioned. The eletrons already in the "wire around the resistor" will start moving "quicker" due to the less resistance and so, "pull" more electrons from out of the battery in that direction.

Im not sure how else to put that so that the OP could understand without without them having some understanding of electronics.

 

[Dale]

Let's say we have a circuit, and in that circuit there's a resistor, and there's a wire that goes around it so that the current can flow freely without getting to the resistor.
my question is this: how does the current "knows" to flow towards the around the resistor? how does it know not to get into the junction that leads to the resistor?

Surface charges produce a local E field and the free charges simply follow the local E field.

 

[vanhees71]

It doesn't know. It just follows the electrical field. If the current would flow into the resitsor it would create a potential difference (according to Ohm's law) that makes it flow back into the wire.

The current is the same along the entire wire (for direct current), which follows from the conservation of electric charge, i.e., there's as much current (charge per unit of time running through an arbitrary cross section of the wire or the resistor) running through the resistor as is running through the wire. The voltage drop is only along the resistor (for ideal conducting wires assumed to have zero resistance).

 

[Merlin3189]

Lets just forget the zero resistance wire. Both paths have some resistance, or if you prefer, conductance. Both must have that.
So some current flows down both paths. That sets up a PD between the start and end of each route $= \frac I S$
If ever that PD is even slightly greater via one path, then the greater PD pushes more current down the other path. Or the electric field at the junction changes direction.

So $\frac{I_1}{S_1} =\frac{ I_2}{S_2} \text{ or } \frac{I_1}{I_2}=\frac{S_1}{S_2}\text{ }$ the current must divide in proportion to the conductances.

Now for Waldorf:
What individual electrons feel (if the physicists will excuse my French) is the potential gradient at the point of decision ( encore je m'excuse.)
(Edit: a further flight of fancy redacted.)
Once they move a little down their chosen path, the potential has fallen and they would have to climb back up to reverse their choice, so they are committed.

Puis alice s'est réveillé et nous retournons dans le monde réel.

 

[vanhees71]

I still don't understand, how you come to the idea that no current can flow in the limit of 0 resistance (or infinite conductivity). The contrary is true, there current flows without resistance, doesn't it?

 

[etotheipi]

I still don't understand, how you come to the idea that no current can flow in the limit of 0 resistance (or infinite conductivity). The contrary is true, there current flows without resistance, doesn't it?

On this point (if this detracts too much from the OP's original question then do please let me know and I'll delete this!), for an ideal wire with $R \rightarrow 0$, the potential is constant across its length $V = I \times 0 = 0$ and consequently there can't be an $\vec{E}$ field. But $\vec{J} = nq\vec{v}_d = \sigma \vec{E}$, so without an $\vec{E}$ field we end up with $\vec{v}_d = \vec{0}$. But that can't be right... shouldn't no $\vec{E}$ field just mean zero acceleration of charges and constant drift velocity, as we'd expect in an ideal wire? A solution would be to say $\vec{J} = \sigma \vec{E}$ doesn't apply to an ideal wire, but this is Ohm's law and the other form $V=IR$ definitely does!

 

[Dale]

But $\vec{J} = nq\vec{v}_d = \sigma \vec{E}$, so without an $\vec{E}$ field we end up with $\vec{v}_d = \vec{0}$.

$\sigma = \infty$ so $\vec{J} = \sigma \vec{E} = \infty \ \vec{0} \ne 0$

 

[etotheipi]

$\sigma = \infty$ so $\sigma \ \vec{E} = \infty \ \vec{0} \ne 0$

That makes sense, silly of me not to factor in the conductivity! And an indeterminate form from Ohm's law usually just indicates the actual value is that of an adjacent part of series circuit for which we can actually calculate the current, due to the continuity condition for current.

 

[klotza]

At equilibrium, the electric field inside a metal is zero. This is because inside a metal the electrons can move freely, and if an electric field is applied to a metal, the electrons will move in the opposite direction leading to a polarization that points in the opposite direction of the external field, cancelling it out. Another related phenomenon is that all the free charges end up on the surface of the metal, not in the interior.

This polarization happens very quickly, much quicker than a circuit typically lasts. If we look at what happens when a wire is in an electric field, the electrons very quickly redistribute to cancel the field, but this leads to a charge on the surface of the wire, going from a positively charged surface on one side of the wire to a negatively charged surface on the other. This surface charge gradient creates a net electric field inside the wire that always points along the axis of the wire.

Now, I said this only lasts a very short time, so how do circuits exist? Basically, they require something to constantly prevent the electric field inside the wire from reaching equilibrium, typically either through a battery or a varying magnetic field via Faraday's law.

I find that looking at a gif of a Lichtenberg figure is helpful for understanding this. At first, the charge is just moving around everywhere following all these different dead-end paths. But as soon as a connection is made, then the charge suddenly "knows" which way to go, but it doesn't know that instantaneously, it only knew that after there was already the proper charge distribution in the wood:
Just saying "Ohm's law" isn't that helpful.

 

[nrqed]

Let's say we have a circuit, and in that circuit there's a resistor, and there's a wire that goes around it so that the current can flow freely without getting to the resistor.
my question is this: how does the current "knows" to flow towards the around the resistor? how does it know not to get into the junction that leads to the resistor?

I think that it would be important to point out one thing: in a circuit there is a "transient state" and a "steady state". Most people focus on the steady state, where there is no current in the resistor if it is in parallel with an ideal wire. But there is a transient state which is extremely short lived where some electrons will actually go in the branch of the circuit where there is a resistor, but very very quickly there will be a "piling up" of electrons that will create an electric field that will stop the flow of electrons there.

 

[snorkack]

Compare a current of electricity with a switch in one branch and a current of water with a tap in one branch.
Every point of a pipe has pressure, water speed and inertia. Every point of a wire has a potential, current and inductivity.
Close a tap, and water will by inertia accumulate next to the tap, getting slightly compressed and slightly stretching the section of pipe by action of water hammer. The water hammer will travel upstream raising pressure, and cause the water to flow to other branch instead when the water hammer reaches the other branch. Water hammer travels nearly at speed of sound, but is slowed by expansion of the pipe.
Close a switch and electricity will by inductivity charge the switch and the section of wire next to switch. The exact speed of the further parts of wire getting to know that the switch has been closed is necessarily no faster than speed of light in vacuum straight line, but it depends on how the capacitance of the wire is - how the electric field of overcharged lengths of wire interact with materials like insulation in surrounding space, nearby lengths of curved wire et cetera.

 

[robphy]

Possibly useful:
https://matterandinteractions.org/wp-content/uploads/2016/07/SurfaceChargePoster.pdf
https://www.glowscript.org/#/user/m...atterandinteractions/program/18-SurfaceCharge

 

[Tom.G]

Lets try a simpler explanation:

You may have heard that like charges repel each other. Since Electrons all have a negative charge, they repel each other (rather like family members that don't get along).

When we have a spot that has a negative charge, that means it has an excess of Electrons (in relation to some other spot that we are comparing to, or measuring from). For instance the Negative pole of a battery has more Electrons that the Positive pole does.

The Electrons will spread out as far apart as they can. If they are in a conductor, they have relatively free movement and will spread out evenly.

If the conductor is connected to something that has few Electrons (like the Positive pole of a battery), they will spread out to again be evenly spaced. If it is indeed a battery Positive pole they are going to, the chemistry inside the battery keeps moving the Electrons to the Negative pole, sort of like an electro-chemical pump.

So ultimately the Electrons don't 'know' where they want to go, they are just trying to get away from their relatives!

There is the 'simple' version, hope it helps.

Cheers,
Tom

 

[sophiecentaur]

I think that it would be important to point out one thing: in a circuit there is a "transient state" and a "steady state". Most people focus on the steady state, where there is no current in the resistor if it is in parallel with an ideal wire. But there is a transient state which is extremely short lived where some electrons will actually go in the branch of the circuit where there is a resistor, but very very quickly there will be a "piling up" of electrons that will create an electric field that will stop the flow of electrons there.

This is surely the crux of the argument. It takes a finite time for things to settle down and for the simple rules of DC circuits to apply. The steady state equations for many bits of Science are often quite acceptable to use. Electric Circuits can often be analysed satisfactorily without worrying about the fact that it's actually an EM Wave that's being dealt with.
The electrons only behave as if they know what's happening in the rest of the circuit after the initial Switch-on pulse has traveled around and back and forth in the circuit - possibly several times. The very fastest this can happen is at the speed of 1 foot per nanosecond but is can often take a matter of milliseconds.

 

[phinds]

It's why they have resistors. Because current follows the path of least resistance. What they call Ohm's law.

No, current could follow the path of least resistance in some way other than Ohm's Law. Ohm's Law is based on empirical observations of what it does happen to do in our universe but Ohm's law was developed with a voltage across one resistor which has nothing directly to do with path of least resistance.

 

[sophiecentaur]

It's why they have resistors. Because current follows the path of least resistance. What they call Ohm's law.

You really must not say that. It is not what Ohm’s Law tells you. Look it up. I can’t be bothered to put you straight.
Current will be shared by all the possible paths - according to their Conductance. Knowing that could save your life one day so don't forget it.

 

[thetrellan]

No, current could follow the path of least resistance in some way other than Ohm's Law. Ohm's Law is based on empirical observations of what it does happen to do in our universe but Ohm's law was developed with a voltage across one resistor which has nothing directly to do with path of least resistance.

I stand corrected. Actually Ohm's law helps us measure resistance. If I understand right, current flows everywhere, given equal resistance down all paths.

 

[phinds]

I stand corrected. Actually Ohm's law helps us measure resistance. If I understand right, current flows everywhere, given equal resistance down all paths.

No, that's not a good way to state it. It's sort of a chicken and egg situation but experimental evidence on resistive values came first and THEN Ohm's Law, so NOW we use Ohm's Law to calibrate resistance measurements but originally there WAS no law to help measure resistors. SO ... orginally Ohm's Law didn't measure resistance, it was a result of measuring resistance and figuring out the relationship with voltage and current.

 

[DaveC426913]

Safer to say 'current flows down all paths as the inverse proportion of the resistance'.

i.e. Double the resistance, halve the current.
Increase the resistance without limit, current drops to approaching zero.

There are proper technical ways to say this, I'm just reframing the erroneous 'path of least resistance' statement.

 

[snorkack]

Ohm´ s law is in no way necessary for the current to find the way of least resistance. You don´ t need to have a definable resistance.
What you need is that

1. there is a potential/pressure definable at any point and time
2. the current will not persist in flowing from lower to higher potential - it will at least keep decelerating if it encounters a higher potential

Given those two requirements, current still finds path of least resistance even if the circuit is higly nonlinear... or does it?
Hm. What are the conditions for a circuit to spontaneously produce unsteady, oscillating output for a steady input?

 

[DaveC426913]

Given those two requirements, current still finds path of least resistance even if the circuit is higly nonlinear... or does it?

No.

It finds all paths - it just follows them to varying degrees.

A circuit with a single 1 Ohm resistor and 49 x 1000 Ohm resistors (all in parallel) will still see the current follow fifty paths...

... only one of which is "the path of least resistance".

 

[etotheipi]

The phrase 'current follows the path of least resistance' should be wiped from the internet and sent to Physics room 101 along with 'there's no gravity in space' and the Aristotelian '$f = mv$'. It is only true in the limit that one branch has zero resistance and the others non-zero resistance. Instead, it is true to say current splits in proportion to the conductance of the branch like others have mentioned.

It's things like that phrase which produce a big hurdle to students learning electronics, since you start becoming inclined to analyse circuits based on "what electrons know" and "decisions electrons make" and other gobbledygook.

Like many others have said above, the only way to do proper circuit analysis is to do the maths and write down the equations using the constraints provided by what we know about electric potential and conservation of charge.

 

[tech99]

I usually think in terms of the initial surge at switch-on. The circuit behaves as a transmission line system with a characteristic impedance which will dictate the initial conditions. The pair of wires could be viewed as a balanced line, which would be easier to explain, but as the two sides of the circuit may be unbalanced, it is more exact to consider each wire from the switch as a separate transmission line.
Consider the moment of switch-on. A unidirectional EM wave travels from each side of the switch along the two wires. These waves are in anti phase and travel away from the switch at nearly at the speed of light . For each of them the ratio of voltage to current is dictated by the characteristic impedance of the wire. The load on the circuit does not initially dictate V/I. In an ordinary circuit the two waves arrive at a resistor from opposite ends, and being in anti phase they add. If the resistor is equal to the characteristic impedance, they deposit all their energy into it. If it is not, the residual wave continues towards the switch and carries on round the circuit a few times, gradually depositing the energy of the initial wave. It is like a step function with an initial ripple. Steady state conditions are gradually created and we then see Ohm's Law at work.
We can say that if the initial wave tries to go down a branch that is open circuit, a wave is reflected from the open end and comes back to cancel the current flow in that branch. If it goes down a branch that is short circuit, the reflected wave cancels the voltage but enhances the current.
Without this initial wave happening, current will never start in the circuit, so the initial conditions dictate the subsequent steady state conditions.