Current direction in a circuit

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Greetings,

I am studying electrical engineering, and one thing puzzles me. I learned that by convention, in solving electrical problems, current is going from plus to minus(direct stable current) as a consequence of electric field lines going from plus to minus charge. Later I learned that electron flow goes from minus to plus, which is reasonable.

Now, example.

(+) ------X------X------ (-)
----------> I

Let X's be 2 light bulbs. Given the situation that we have a metal conductor and the most of the current is carried by electrons, which will light up first? I mean, they cannot light up instantly at the same time, that would imo break the speed of light.

And can you explain to me a bit more, how current "flows" i know that charges are slow and going about a half a centimeter a second, but the current is almost instant!!! How come?

This mainly came from experiment with LED diodes which light up only when a specific current direction is applied, anode goes to plus side and cathode is going on the minus side(if possible, explain how LED diodes work or post a link, thanks)



Thank you
 

Answers and Replies

  • #2
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Water is a pretty standard analogy, so let's use that.

There are two things you could be talking about when asking about the "speed" of electricity: the electron drift velocity, or the signal velocity. As you already mentioned, electron drift velocity is very slow. But signal velocity is different.

As an experiment, imagine a pipe completely full of water some distance long, say a kilometer long or so. Hook a slow pump on one end (to push the water at a rate of about 0.5 m/s), and a water wheel on the other. How long after turning on the pump would it take for the wheel to start spinning?

The answer is not 2,000 seconds. What happens when the pump is turned on is that pump pushes water molecules forward, and those molecules push the ones in front of them forward, and those molecules push the ones in front of them forward, and so on down the line several billion times. This forms a pressure wave that travels down the pipe at the speed of sound (in water), so the wheel ends up spinning very soon after the pump is turned on.

A similar "pressure" wave is formed when a circuit it turned on, only that wave travels at near the speed of light. That is why even though electrons move very slow, the lights go on almost instantly after you "turn the pump on" (ie flip the switch).

Actually, I think there's two signal waves, one going backwards and the other forwards, so which light turns on first is whichever one is closer to the switch (in either direction).
 
  • #3
sophiecentaur
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If you had a lamp at the end of a long (two conductor) cable and another lamp hung across the wires, at half the distance from the source, the nearer one would light up a microsecond or so earlier. This is because it is the electromagnetic wave, guided by the wires, is what causes the electrons to flow actually through each bulb (at very slow speed). It takes time for the wave to propagate (at a bit less than c) along the pair of wires.
This model, with two parallel wires, is easier to understand than considering a huge loop of wire with bulbs connected in series along the wire. The propagation of the electromagnetic wave on and around the loop is much more complicated to consider. Sort out the easier one first???
 
  • #4
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Actually, I think there's two signal waves, one going backwards and the other forwards, so which light turns on first is whichever one is closer to the switch (in either direction).
so, even in direct current,stable, the current goes like from both sides technically meeting at middle at some point? all i got from this that electrons, which are in a wire, going from minus to plus bounce off each other, and this "bouncing" propagates at speeds near speed of light. But, from this you said, does this happen from plus to minus also? what bounces then... I mean electrons are some sort of medium in wires, and a wave propagates through them, thats what puzzles me about these light bulbs... the one closer to minus side should turn first, before next one, imo.
 
  • #5
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If you had a lamp at the end of a long (two conductor) cable and another lamp hung across the wires, at half the distance from the source, the nearer one would light up a microsecond or so earlier. This is because it is the electromagnetic wave, guided by the wires, is what causes the electrons to flow actually through each bulb (at very slow speed). It takes time for the wave to propagate (at a bit less than c) along the pair of wires.
This model, with two parallel wires, is easier to understand than considering a huge loop of wire with bulbs connected in series along the wire. The propagation of the electromagnetic wave on and around the loop is much more complicated to consider. Sort out the easier one first???
That's what i have been talking about, i know that 2 light bulbs in parallel, one closer to source turns before the next one, that is totally understandable. But this in a closed loop is what bothers me, i am kinda guy that tries to understand everything, there must be a explanation. I am probably not the first one who asked this.
 
  • #6
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No JN did not say the current meets in the middle.

He said (correctly) that the light that turns on first will be the one closest to the switch.

Don't forget you haven't shown a switch in your sketch, but a switch is essential to the scenario you describe.

Before the switch is thrown (switched on) charge will accumulate at both contacts to make one + and one -. This is important to realise.

So when the switch is thrown charge passes across the gap in the circuit through the now closed contacts. This leads to two 'pulses or fronts' JN mentioned travelling in both directions away from the switch.

Since both pulses travel at the same speed one will meet the nearest bulb before the other meets its respective bulb.

Hope this helps

go well
 
  • #7
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No JN did not say the current meets in the middle.

He said (correctly) that the light that turns on first will be the one closest to the switch.

Don't forget you haven't shown a switch in your sketch, but a switch is essential to the scenario you describe.

Before the switch is thrown (switched on) charge will accumulate at both contacts to make one + and one -. This is important to realise.

So when the switch is thrown charge passes across the gap in the circuit through the now closed contacts. This leads to two 'pulses or fronts' JN mentioned travelling in both directions away from the switch.

Since both pulses travel at the same speed one will meet the nearest bulb before the other meets its respective bulb.

Hope this helps

go well
Can u help me out with a picture or something, words confuse directions you are speaking, we might not think the same. Thanks
 
  • #8
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Can u help me out with a picture or something, words confuse directions you are speaking, we might not think the same. Thanks
When the first electron moves, it pushes the one in front of it forward and also leaves an empty space the pulls the one behind it forward as well. The front electron pushes the one in front of it forward, and the rear electron, when it moves forward leaves another void that pulls the electron behind it forward. The electrons themselves are only moving in one direction, but they are forming two different waves going in opposite directions, both traveling at about the speed of light. Even though the waves are traveling in two directions, the electrons are only going in one.

Here is a physics simulator made by University of Colorado that should explain how the signal wave works. Watch closely and you can see the two waves go forwards and backwards when you flip the switch.

http://phet.colorado.edu/en/simulation/signal-circuit
 
  • #9
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OK

I have drawn a simple picture. To emphasise a point I have placed the lamps at exactly the same distance from the battery, but at obviously very different distances from the switch.

This is the situation when the switch is open. The switch contacts are shown in blue.

When the apparatus is first connected a very small current will flow for a short time until

all the wiring and lamps connected to the positive terminal (red) are at the same voltage as the +ve terminal.

&

all the wiring and lamps etc connected to the negative terminal (black) are at the same voltage as the -ve terminal.

I have drawn little + and - to show that this goes all the way round the circuit.
It also shows that the switch contacts are at different voltages.

When the switch is closed the contacts come together and cannot be at different voltages.
The equalisation of the voltages causes pulses or fronts or switching transients to move as I have shown.
The posh term is switching transient so I have labelled them T1 and T2.

As you can see, T2 has a lot further to go to reach lamp L2 than T1 has to reach lamp L1.

So L1 will light first.
 

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  • #10
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OK

I have drawn a simple picture. To emphasise a point I have placed the lamps at exactly the same distance from the battery, but at obviously very different distances from the switch.

This is the situation when the switch is open. The switch contacts are shown in blue.

When the apparatus is first connected a very small current will flow for a short time until

all the wiring and lamps connected to the positive terminal (red) are at the same voltage as the +ve terminal.

&

all the wiring and lamps etc connected to the negative terminal (black) are at the same voltage as the -ve terminal.

I have drawn little + and - to show that this goes all the way round the circuit.
It also shows that the switch contacts are at different voltages.

When the switch is closed the contacts come together and cannot be at different voltages.
The equalisation of the voltages causes pulses or fronts or switching transients to move as I have shown.
The posh term is switching transient so I have labelled them T1 and T2.

As you can see, T2 has a lot further to go to reach lamp L2 than T1 has to reach lamp L1.

So L1 will light first.
I think i get it now, you have been very helpful, do have any book recommendations where these pulses are worked in detail? So i can put a nail in this (: thanks
 
  • #11
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When the first electron moves, it pushes the one in front of it forward and also leaves an empty space the pulls the one behind it forward as well. The front electron pushes the one in front of it forward, and the rear electron, when it moves forward leaves another void that pulls the electron behind it forward. The electrons themselves are only moving in one direction, but they are forming two different waves going in opposite directions, both traveling at about the speed of light. Even though the waves are traveling in two directions, the electrons are only going in one.

Here is a physics simulator made by University of Colorado that should explain how the signal wave works. Watch closely and you can see the two waves go forwards and backwards when you flip the switch.

http://phet.colorado.edu/en/simulation/signal-circuit
I swear of God, I had like 2 intense "ahaaaaaaa" moments!!! THANK YOU

And those waves or signals are both considered as current?
 
Last edited:
  • #12
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So, basically if have conductor and i if connect it to one of the poles of the battery, after some very short time, that conductor will also get electric potential as that pole?
 
  • #13
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I should mention that these transients are absorbed in the battery and do not appear again while the switch is closed.
After this a steady state appears, with current flowing one way only round the loop.

I'm sorry to tell you that transients are not dealt with further in any elementary textbook that I know.
Such books treat the situation where the switch is open
Jump past the transient condition
And proceed directly to the steady state condition with statements like

after a long time or after a steady state has been reached etc.

Edit

So, basically if have conductor and i if connect it to one of the poles of the battery, after some very short time, that conductor will also get electric potential as that pole?
Yes exactly so
 
  • #14
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thanks
 

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