| What happens at the atomic level
| where electrons have an affinity for
| a path of least resistance? Imagine this,
| you have a parallel circuit. And at the node
| where the branching takes place, current can
| flow down one branch(where a resistor exists)
| and it can flow down the second branch(where
| we have a short). Why does the majority of the
| current go down the path where we have a short?
| It isn't like electrons can forecast which path has
| least resistence, right? So how does the principle
| 'follows path of least resistence' hold? Even when
| measurements are taken, it can be observed that
| the majority of current goes down the least resistive
| path. It just doesn't make much sense when you
| know electrons cannot forecast whether a resistor
| or short lies ahead down one of the branches.
The resistance in the wiring (even if it's not much, as copper would be),
is kind of like a self generated bucking force that is proportional to
the current that is flowing. There is a voltage drop across any segment
of wire, and you'll see a voltage that obeys ohms law relative to the
current and the resistance.
The path with less reistance will have less of this bucking force. The
eletrical force is really trying to go in all paths at the same time.
Some paths just don't push back as much.
At the atomic level, I presume the electrons are encountering various
atomic level actions that prevent their most efficient flow, such as the
need to be changing electron orbital levels. The counter reactions could
be thought of as that bucking force.
The electrons cannot forecast which path has least resistance. When the
force (voltage) changes, that's when it can become apparent what is going
on. Suppose you have two very long paths of low resistance, interrupted
by one point of high resistance (e.g. a resistor inserted in the wire).
In one case the resistor is near the voltage source. In the other case
the resistor is nearer the other end (which might be a microsecond away
at near light speed). So what is the electricity doing before it has any
opportunity to "know" the circuit resistance (e.g. in the microsecond
before the leading edge of turned on voltage reaches the far resistor)?
The answer to that is characteristic impedance as seen in transmission
lines. You could, in theory, make 300 ohm twin lead TV wire out of
superconductors that offer no resistance at all (for our convenience in
calculation). There's a resistor at the far end (but it's not 300 ohms).
Or there may be nothing connected (open circuit). Or it may be shorted.
Or the resistor might actually be 300 ohms. But the electricity that
starts to flow can't "know" this for at least a microsecond of time.
What that electricity will do is behave on its leading edge based on
the characteristic impedance of the transmission line, which comes from
the inductance and capacitance of the line itself (real wire will also
add some resistance to that). When the leading edge reaches the far end,
what happens next depends on what is there. If shorted, the leading edge
will return on the other wire (there are 2 wires in the transmission line).
If open, it comes back on the same wire. If 300 ohms is there, it is all
dissipated and nothing comes back as a leading egde (a steady current will
flow). Other resistances will have a lesser effect than shorted or open.
That returning edge can end up reflecting back and forth between both ends
until it reaches a steady state (assuming you switched on DC power). If
you supply AC (changing voltage) these effects just keep on happening.
Radio engineers and ham operators have to deal with this on even very
short transmission lines (from transmitter to antenna) because at radio
frequencies, the voltage is changing up and down in less time that it
takes for the leading edge to reach the antenna. That can get very
convoluted (simplified forms of it get plotted on "Smith Charts"). But
it is fun to study.
In summary, electrons cannot "see" ahead. In fact they don't really even
flow literally very fast or far ("electron drift"). But the "bump effect"
itself moves at near light speed and comes back at near light speed and
that has counter effects that "report the distant resistance" via how much
has come back and when.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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