How does a current "know" where to go

[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.

 

[sophiecentaur]

Actually Ohm's law helps us measure resistance.

? It relates to Resistance. Your statement is the equivalent of saying Speed helps us measure Speed.
Resistance is just a Ratio which may or may not remain constant, depending on the conductor involved. Ohm's Law is only followed by metals at constant temperature. Non-Ohmic components exist and you can still measure V and I.

 

[rephael]

all the material have a different current conductivity. so the current will go to material that have the haigh current conductivity. example : 2 material /// man and steal . the current will go to steal

 

[DaveC426913]

all the material have a different current conductivity. so the current will go to material that have the haigh current conductivity. example : 2 material /// man and steal . the current will go to steal

Again: false.

Current will go through both. to varying degrees, depending on their resistance.

 

[sophiecentaur]

I usually think in terms of the initial surge at switch-on.

I don't think you meant that . You would only need to think of that when looking at a change. We launch into the simple basic DC calculations first and often we think no further except when there's a PF post, a smartypants Exam question to answer or when we're dealing with very minority situations where the warning light in our brain has been lit.
I could be wrong about this and you may be a designer of Time Domain Reflectometers or a power systems analyst.

 

[jbriggs444]

I don't think you meant that . You would only need to think of that when looking at a change. We launch into the simple basic DC calculations first and often we think no further except when there's a PF post, a smartypants Exam question to answer or when we're dealing with very minority situations where the warning light in our brain has been lit.
I could be wrong about this and you may be a designer of Time Domain Reflectometers or a power systems analyst.

I like to think of it in terms of an equilibrium and relaxation toward a stable equilibrium.

As long as one is dealing with typical passive components, the possibility of more than a single equilibrium state does not arise. If there is an equilibrium state, it will be approached.

 

[tech99]

I don't think you meant that . You would only need to think of that when looking at a change. We launch into the simple basic DC calculations first and often we think no further except when there's a PF post, a smartypants Exam question to answer or when we're dealing with very minority situations where the warning light in our brain has been lit.
I could be wrong about this and you may be a designer of Time Domain Reflectometers or a power systems analyst.

Exactly; the question of how does the current know where to go comes up here perennially for some reason.

 

[sophiecentaur]

for some reason.

The reason is that Electricity is Hard. I never know why people try to tell us otherwise.

 

[Klystron]

Exactly; the question of how does the current know where to go comes up here perennially for some reason.

Humans tend to anthropomorphize electrons because we appear to control, generate and manipulate electrons and EMF. Ascribing human attributes to things aids acceptance, reduces fear and eventually promotes rational understanding.

Electrons become the 'familiars' of our technology 'wizards'.

We name large storm systems. We describe pandemics with emotional terms.

 

[Stephen Tashi]

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

The usual model of current is that it is an aggregation of electrons. So you need to ask how an individual electron "knows" which way to go. If you are in a stream of people leaving a building that has 2 exit doors, the exit you take is probably influenced by the behavior of people near you. I don't know anyone has modeled current flow to a level of detail that shows the behavior of individual electrons moving as a crowd. Your question seems to ask for that level of detail.

 

[tech99]

The usual model of current is that it is an aggregation of electrons. So you need to ask how an individual electron "knows" which way to go. If you are in a stream of people leaving a building that has 2 exit doors, the exit you take is probably influenced by the behavior of people near you. I don't know anyone has modeled current flow to a level of detail that shows the behavior of individual electrons moving as a crowd. Your question seems to ask for that level of detail.

I think that is an example of partition noise.

 

[russ_watters]

The usual model of current is that it is an aggregation of electrons. So you need to ask how an individual electron "knows" which way to go. If you are in a stream of people leaving a building that has 2 exit doors, the exit you take is probably influenced by the behavior of people near you. I don't know anyone has modeled current flow to a level of detail that shows the behavior of individual electrons moving as a crowd. Your question seems to ask for that level of detail.

I think this is valid/true. The wording of the OP implies to me that he doesn't recognize that the wires are already filled with electrons prior to turning on the circuit. It's as if he thinks the electrons would wander down the wire until they get to to the resistor, then turn around and go back the other way after finding out it is in the way. It's equivalent to standing by yourself in a lobby with two doors and not knowing which one is locked and which isn't. If instead you have a line of people that is moving fast through one door while a bunch of people are pushing in vain on a locked door next to them, it becomes obvious why you go through the open door. You don't even have to "know" anything; you could close your eyes and let the crowd push you through the open door.

 

[gmax137]

It's equivalent to standing by yourself in a lobby with two doors

... and then Monty Hall says, "wait! what if I open this door..."

Sorry , now back to the thread...