Which direction electricity follows in a breadboard?

Other Hardware General Electronics
1

Hi all

I'm a complete beginner who is waiting his first arduino kit to arrive via mail. Meanwhile, I have started learning and experimenting with how circuits work. I 've no prior experience in electronics whatsoever, so please excuse my probably stupid question and potentially wrong terminology.

So, here is what I am struggling to understand:

I am a bit confused on which direction the electricity flows in a circuit which is powered by a 9V battery. I have created a simulated circuit on a breadboard, to light up a led. You can find my breadboard here: http://123d.circuits.io/circuits/622863-the-unnamed-circuit and run a simulation.

Here is a screenshot as well:

So, from what I understand, the electricity flows from the Battery's positive terminal to the negative. This means that at the top breadboard, the electricity passes from the resistor at first, then from the led, and then goes to the ground. So far so good.

My question arises at the second breadboard, at the bottom. Here the electricity passes first from the led, and then from the resistor, and finally to the ground. So, why does the led lights up normally and does not explode, since the resistor is placed AFTER it and not BEFORE it?

I am missing something quite fundamental here, and so far, the online reading I've done does not seem to help. Any help would be quite appreciated!

Cheers!
John

2

The resistor and LED are in series in both examples, so the same current flows no matter how the components are connected.
The current will be (9V - Vf)/resistor = current

If there are say 2.5V across the LED when it turns on (Vf, or forward voltage), the other 6.5V is across the resistor. If you know the voltage and the resistor value, you can determine the current.
Using Ohms low, V=IR, or V/R=I, you see that you can change the resistor and this change the series current flow.
So for the above, if you wanted 20mA to flow thru the LED and the resistor:
(9V - 2.5V)/0.02A = 325 ohm resistor needed.
330 is a standard value and result in a little less than 20mA
(9V - 2.5V)/330 ohm = 19.7mA

3

The direction of current flow is an arbitrary convention and is stated as being from positive to negative. However it doesn't start at one point like water running from a tap. Everything already contains electricity and it all starts moving at the same time in all parts of the circuit as soon as a voltage is applied. So it doesn't matter in which order the resistor and LED are connected.

Hope that helps.

Russell.

1 Like
4

Hey CrossRoads

Thank you very much for taking the time to explain this.
So, if I get it right, it doesn't matter where the current starts from and where it goes to, right? This means that the "order" of the components does not matter, and what matters is the effect that each component (eg a resistor) has in the circuit as a whole, in which the current runs in a sort of... continuation.

Thanks again man!

John

5

russellz:
The direction of current flow is an arbitrary convention and is stated as being from positive to negative. However it doesn't start at one point like water running from a tap. Everything already contains electricity and it all starts moving at the same time in all parts of the circuit as soon as a voltage is applied. So it doesn't matter in which order the resistor and LED are connected.

Hope that helps.

Russell.

Thanks Russel, helps indeed. This sounds... fascinating :slight_smile:
I'm excited now.

Cheers!

1 Like
6

Yes. Think of circuits as being a loop. Treat the current in the loop as flowing from high to low.
Simple circuits like yours, the loop is easy to see.
More complex circuits will have many loops in parallel.
The loops may be interrupted with switches - actual mechanical switches, or electronic switches, i.e. transistors.
Further, the transistors may be used as a switch, in which case they act the same as a mechanical switch with a resistor in series (MOSFET resistor), or they may have a fixed voltage drop (BJT transistor) similar to an LED or other diodes.
Or, the transistor may be controlled in a small region where the current that is allowed to flow is limited by the current into the control pin (BJT), or by the voltage at the control pin (MOSFET).
With digital circuits, its usually the on/off usage. Other components in series with the switch determine the amount of current flow.
With analog circuits, the small range is usually, such as for audio - a small control signal determines how much of a larger current can flow, so the output is an amplification of the small signal.

7

"Conventional current" flows positive to negative. Electron flow is in the opposite direction.

Engineers & technicians use conventional current and it's really just a tradition. If you take an electronics class, current flows form positive to negative. The "arrows" that are part of diode & transistor schematic symbols show the direction of conventional current.

If you take a physics class, current (electrons) will flow from negative to positive. :fearful:

I don't remember the details, but I was told that one of the early scientists got it wrong, maybe Alessandro Volta or Andre Ampere. By the time someone discovered electrons and figured-out which way they were flowing it was too late to change the "convention".

The water-in-a-pipe analogy is pretty good but it's not perfect. Voltage is pressure and current is flow. A small pipe is like a resistor and it reduces water flow. You can get more water flow (current flow) with more pressure (more voltage) or by using a bigger pipe (less resistance).

If you haven't studied it yet, [u]Ohm's Law[/u] describes the relationship between voltage, resistance, and current.

The BIG difference is - If you cut the pipe, water flows out. If you cut the wire you get infinite resistance and no current flows. Also... Nothing "bad" happens with water and zero-resistance. Bad things can happen with zero resistance (i.e. a short circuit) across a power supply or across an Arduino output. Zero-resistance is not always a bad thing... Wires have (nearly) zero resistance and a switch has (nearly) zero resistance when turned-on.... But, there has to be something to limit current flow....

[u]Kirchhoff's Laws[/u] describe how voltage is divided (or summed) in series circuits and how currents divide (or sum) in parallel circuits.

For example, in your LED circuit the 5V is divided with about 2V across the LED* and 3V across the resistor. Since they are in series, the same current flows through both. If you know the voltage across the resistor and the resistance, you can apply Ohm's Law to calculate the current.

If you've got 10mA through the resistor & LED and you add another resistor & LED to the same Arduino output (in parallel), you'll still get 5V out of the Arduino and you'll also get 10mA through that LED & resistor for a total of 20mA.

That concept will hold until you try to get too much current from the Arduino... If you try to get too much current out of an Arduino pin, at some point the voltage will drop (because Ohm's law is always true and something has to "give") and you might damage the Arduino.

  • LEDs are non-linear (like all diodes) and the resistance changes when the voltages change. So, we use Ohm's law with the resistor instead of the LED.
8

It gets even more confusing when you factor the positively charged holes attracted toward the ground while the negatively charged electrons are attracted by the positive terminal of the battery. There is no point in considering either of these. All you need to think about is the magnitude of the values, (voltage, current etc) and as Crossroads pointed out, whether components are in series or parallel.

9

If you take a physics class, current (electrons) will flow from negative to positive.

No that is not what happens in a physics class. I know because I have taken them and given even more.
Convention current is taught. Electron flow is used when needed, it is just that in electronics it is never needed.

1 Like
10

Indeed the charge on an electron is negative, and thus current flows in the opposite direction
to electrons.

In physics there's not just electrons to talk about though, you get ions, positive and negative,
positrons, holes in a semiconductor, some charge carriers are positive and some negative.

If anyone wants a physics class I'd recommend watching Walter Lewin's lectures on the
MIT website if you want to study electromagnetism - great fun at times but rigorous.

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Grumpy_Mike:
Electron flow is used when needed, it is just that in electronics it is never needed.

Never say never, what about thermionic valves, and crts. They are still occasionally used!

Russell.

12

russellz:
Never say never, what about thermionic valves, and crts. They are still occasionally used!

Different matter, not relevant to technical discussions. Selling things to cashed-up "Audiophiles". :grinning:

We should be so lucky!

13

russellz:
Never say never, what about thermionic valves, and crts. They are still occasionally used!

Russell.

It is still not neded in dealing with thermionic valves conventional current theory works fine.

The point is if you think current direction matters then you are missunderstanding electric theory like the OP is. It does not matter.

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Thanks for taking the time to reply guys! I'll go through each reply and see if I can get it to work in my mind. The whole thing is fascinating. Can't wait to get my hands on the breadboard and experiment!

Thanks again!