# Spreading of electric current -- start signals in a circuit

• sweet springs
In summary, the surface charge feedback mechanism causes the current to start spreading from the switch location quickly and with a slight decrease in speed of light.
sweet springs
Hi
I have a question about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html.
Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from the location of switch and spread by exactly or slightly less than speed of light c. Could you teach me about the mechanism of this spreading, e.g. EM waves generation ?
Best.

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sweet springs said:
Hi
I have a question about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html.
Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from the location of switch and spread by exactly or slightly less than speed of light c. Could you teach me about the mechanism of this spreading, e.g. EM waves generation ?
Best.
Look up "surface charge feedback mechanism" in electrical circuits. This discussion might help.

Thanks cnh1995. Now I share difficulties with the authors of that link :
How the state transfer, e.g. starting from where, spreading with what speed and amplitude, how much relaxations and transient time, through
1 turn-off steady state, 2 transient state and 3 Turn-on steady state ?
It'a kind of progress anyway. Or Did I fail to find fine thoughts in that thread ?

Hi. I now assume as follows.
First electron motion at the switch contact generates the magnetic field in the vacuum with delay of light speed and dissipation by distance.
Thus generated magnetic field is further transmitted to the position of wire nearby and make electrons there move.
Such a feedback consequence takes place in the transient state. The first weak interaction, from the position of switch to the wire position concerned, comes with light speed. It is followed by stronger interactions that is retarded by further distance of vacuum-wire transmissions but enhanced by accumulation from nearer and multiple current sources.
Does this make sense? Best

sweet springs said:
First electron motion at the switch contact generates the magnetic field in the vacuum with delay of light speed and dissipation by distance.
I am not sure about that since magnetic field requires a steady current. But as the switch is closed, there is a change in electric field, so there can be a magnetic field.
But as far as I know, the interactions inside the wire are purely governed by the electrons (as far as surface charge feedback is concerned). When you close the switch, electrons near the +ve terminal are attracted towards it and those near the -ve terminal are repelled. This creates a difference in charge densities and this difference propagates along the wire through the surface charge feedback. Excess charge in a particular section in the wire ends up on the surface of the wire, making "surface charge rings". The surface charge gradient is responsible for the electric field inside the wire. This surface charge formation continues until there is a steady current in the circuit, meaning no excess charge in any part of the circuit (KCL). If there is no gradient, there is no field inside the wire. Hence, for conductors, the surface charge gradient is very small compared to that for the resistive elements and we approximate the voltage across the conductors to zero.

These videos might help.
This one describes the surface charge feedback mechanism...
and this one is the experimental verification of the existence of surface charges in a circuit.

cnh1995 said:
This creates a difference in charge densities and this difference propagates along the wire through the surface charge feedback.

I have no doubt about charged wire. I wonder whether the charge distribution difference can describe the propagation speed that is as fast as light.

The propagation is not necessarily along the line in case of redundant wiring. I drew some figures
#1 http://fphys.4rm.jp/modules/xelfinder/index.php/view/149/スライド1.jpg
#2 http://fphys.4rm.jp/modules/xelfinder/index.php/view/150/スライド2.jpg
#3 http://fphys.4rm.jp/modules/xelfinder/index.php/view/151/スライド3.jpg
where yellow circle is the light speed sphere of current starting signal.
Best

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Thanks cnh1995. I did not find a specific description of the transient state of my concern in your recommended page. Please tell me the exact lines if I failed.

When current starts at the switch point, charged state there changes. In order to keep the charged state current should start at neighbors thus expands.
So we might say the same thing in different ways.
I attributed the speed of c to electromagnetic information speed as is in Lienart Wiechert Potential. That's enough or should you explain a great speed of charge density change propagation in wire?

## What is the definition of electric current?

Electric current is the flow of electric charge through a conductive material, such as a wire. It is measured in amperes (A) and is caused by the movement of electrons.

## What is a circuit?

A circuit is a closed path through which electric current can flow. It typically consists of a source of electricity, such as a battery, wires to carry the current, and a load, which is the device that uses the electricity.

## How does electric current spread in a circuit?

Electric current spreads in a circuit through a process called electron flow. The source of electricity, such as a battery, provides a potential difference, or voltage, which causes electrons to flow from the negative terminal to the positive terminal, completing the circuit.

## What is a start signal in a circuit?

A start signal in a circuit is a signal that triggers the flow of electric current. This can be in the form of a switch being turned on, a sensor being activated, or a timer reaching a certain time. It is the initial signal that starts the flow of electricity through the circuit.

## What factors affect the speed of electric current in a circuit?

The speed of electric current in a circuit is affected by several factors, including the resistance of the circuit, the voltage supplied by the source, and the type of material the current is flowing through. Additionally, the length and thickness of the wire can also impact the speed of the electric current.

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