Series circuit

Discussion in 'General Electronics Chat' started by sonofabit, Jul 21, 2009.

1. sonofabit Thread Starter New Member

Jul 21, 2009
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hello! new to the board as I'm trying to help my stepdaughter learn about electricity and circuits. well ok, I'm learning too

i'm confused as to how it doesn't matter what side you put the resistor on. While I understand that in series, the current is the same throughout the circuit, how is this possible?

see, I'm visualizing the electrons flowing out of the negative terminal, and if they get to the LED before they get to the resistor, wouldn't that burn out the LED, because they didn't get a chance to be slowed by the resistor?

I'm sure that I'm wrong, but why doesn't this happen?

2. beenthere Retired Moderator

Apr 20, 2004
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The nicely predictable thing about a series circuit is that all the elements get exactly the same current. It is the statistical motion of electrons that matters, and Ohm's law gives it every time in a DC circuit - E = IR. For a series circuit, we rearrange the equation: I (current) = E (voltage)/R (resistance). Think of it as traffic. Some cars go faster, some slower, but the average flow is due to conditions.

Have you had a look into our Ebook yet? The basics are well explained - http://www.allaboutcircuits.com/vol_1/chpt_1/1.htm

Last edited: Jul 21, 2009
3. GetDeviceInfo Senior Member

Jun 7, 2009
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think of it as a string of marbles inside a tube. when you push a marble into one end, all the marbles move along and one comes out the other end.

4. steveb Senior Member

Jul 3, 2008
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You can also take a math approach to understand it. Voltages add in series, and since current is the same in a series circuit, the voltages on each component don't change when you swap positions.

So, saying that the order of components in a series circuit is unimportant is similar to saying that the order that you add a bunch of numbers together doesn't matter.

That is, 2+3=3+2 for example.

Also, the equal current in a series circuit comes from a basic physics law: conservation of charge. The charges will flow equally unless they are diverted away by another current path, in which case it is no longer a series circuit. If they don't flow equally, the charges would need to build up somewhere. This build up of charge can't happen in a good conductor.

I like the marble idea. It's also similar to water flowing. Conservation of mass says that the water in a pipe can't disappear. And the same water flow rate is in the pipe unless the water flows out through a leak.

Last edited: Jul 21, 2009
5. Vaughanabe13 Active Member

May 4, 2009
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The reason it is confusing to think about is because when we draw a circuit, we look at it like a road map. Logic tells us that when you are trying to get from point A to point D on a map you have to go in order from A to B to C to D. It seems like that is how you should treat a circuit, i.e., the current will go into a wire, go through the LED, go through the resistor, and then go to ground. That is not true in circuits. You have to think about your LED circuit as one whole unit. In this 'unit' the order of the LED and resistor doesn't matter because ultimately you are starting and ending at the same place.

Another way to think about it is if you were driving on a closed roundabout with several other cars (essentially a circular road with no entrance or exit). Say all of the cars are evenly spaced out and driving at 100 MPH - that is like a battery with the positive terminal directly connected to the negative terminal. It can't exist that way because there is nothing to slow the cars down and control them, so eventually they will crash (battery will heat up and could explode).

In order to control our circuit, we have to slow the speed of the moving cars. We do this with a resistor, which in this situation would be a car braking. If one person happened to slam on their brakes and drive slower (resisting the flow of traffic), the cars behind them would be forced to slow down, which would cause a ripple effect and eventually all of the cars in the loop would be forced to drive that same slow speed, even if they are at a different position on the roundabout. It is the same with your LED circuit, since you can think of power and ground as your 'closed loop' of road. The only difference is this "slow down" effect of the resistor will take effect at an extremely fast speed, since we are in fact dealing with electrons. The end result is that it doesn't matter where the "resist" is in the circuit, the LED will still see the same amount of current, or "speed of the cars."

Make sense? I'm actually quite proud of that analogy, haha.

6. russ_hensel Distinguished Member

Jan 11, 2009
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I believe the speed of the electrons ( net ) is about .5 cm per second ( the drift velocity ). Inside the LED they may be holes ( depends on the LED ). Generally the speed of the electrons means little, current is the flow of electrons. In the hall effect and an electron beam, the speed of the electrons does matter.

7. sonofabit Thread Starter New Member

Jul 21, 2009
3
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well first of all, thank you for all the replies!

unfortunately, I'm still not understanding it. I mean I understand and believe that it "happens" but I just can't physically understand why...

I like the marble analogy, but the way I picture it, to match the analogy the electrons or "marbles" would have to be spread across evenly on the whole circuit, then start moving. I was always under the assumption that the electrons wait at the negative end of the battery, spread evenly up to the switch.

when the switch is closed, the electrons flow through it and into the rest of the circuit, meaning they would hit the led first, then get slowed by the resistor.

the same goes for the traffic analogy. sure the resistance will slow the traffic, but the people at the end of the line won't slow right away; there is a ripple effect!

so why doesn't the LED burn out if the circuit is on the other side?

sorry if this seems so obvious. like I said, I believe you! I just don't understand how it's possible!

many thanks again!

8. steveb Senior Member

Jul 3, 2008
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The electrons are "waiting" at every point in the conduction path. The battery provides the voltage potential to drive the electrons into motion. The effect of the voltage moves at the speed of light causing electrons at every point in the circuit to start moving at approximately the same time, hence there is little ripple effect in a circuit that is relatively small.

I don't know if you noticed, but your question spawned another (more advanced) thread about antennas. Some of your intuition is actually correct in the general case where the dimensions of the circuit become larger. But, for all practical purposes with small circuits operating at low frequency, the effects you are thinking of are not significant because the speed of light is very fast. The main thing to point out is that the ripple effect you are thinking of applies to the voltage which can not instantaneously act at a distance, not to the electrons which are free to move at all points in a circuit and are just looking for any excuse to get going.

9. sonofabit Thread Starter New Member

Jul 21, 2009
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so if i'm understanding you correctly, electrons are distributed evenly across a circuit, and closing it puts them all into motion at the same time because of the force applied by the voltage difference between + and - on the battery.... is that right?

so, voltage isn't created by cramming a bunch of electrons at the negative end of a battery? how do they create that potential power then?

Apr 20, 2004
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11. steveb Senior Member

Jul 3, 2008
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That's basically correct, although there is no requirement that the electrons be distributed evenly. For example, a charged capacitor would have extra electrons on one plate and a lack of electrons on the other. However, the rate of electron flow (current) will be driven by the voltage which acts almost instantaneously (speed of light) across the circuit. Granted, the voltage can be created by a charge imbalance, and if the current flow ends up eliminating the imbalance, then the voltage and current will decay to zero.

You can create voltage by cramming charge, but a battery is more complex than that. There are many ways to generate voltage such as, "cramming charge" for electrostatic potential as in a capacitor, chemical reactions as in a battery and magnetic induction as in a generator. These are the most common. There are also other effects such as thermoelectric, photovoltaic and piezoelectric.