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I've noticed something strange in the circuits schemes I saw in a book.

Let's say that there is an LED and it's anode is connected to a 5V power source, and it's cathode is connected to ground.

Obviously there must be a resistor between the LED's anode and the power source, but in some circuits schemes the resistor is between the LED's cathode and ground, and strangely for me, it works!

How?

I mean in the first example the resistor is before the LED so it can decrease the current so the LED doesn't burn or explode, but in the second example how can the resistor affect the delivered current to the LED and the resistor is after the LED?

The electrons will reach the LED before they reach the resistor so how can the resistor affect components behind it?

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With components in series the same current flows through each.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Alternate series connection of resistor and LED. The result is the same.

In both circuits of Figure 1 we can see that the voltage drop across the LED is 2 V and that 7 V is dropped across the resistor. We can calculate the current from Ohm's Law, \$ I = \frac {V}{R} = \frac {7}{1k} = 7 \ \text {mA} \$. The individual components have no way of knowing their order. The current flow is determined by the total resistance of each leg of the circuit and in this example they're both the same.

The electrons will reach the LED before they reach the resistor so how can the resistor affect components behind it?

Think of "current" rather than electron flow. The current flows in all parts of the circuit simultaneously in much the same way as the links of a bicycle chain all move simultaneously.

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Think of electricity as it was water in a pipe, it works the same way.

  • The current is the same as the current of water, how many liter goes through a minute.
  • The voltage represent the pressure of the water, how many "PSI" you have on the pipe.

As you increase pressure, the current will increase.

A resistor will basically create a tight spot on the pipe (a resistance to the current flow) that will slow down the current, reducing the current flow.

If you push water in a pipe, the current flow will be the same at the beggining and at the end. You cannot add water in the middle as the pipe is closed.

if you squeeze the pipe and check the current, it will be slowed down, either you squeeze the pipe at the beginning or at the end.

So it does not matter where is the resistor placed, as long as it's in "serie" or on the "same line" as the LED as the current on the resistor and the current on the LED will always be the same.

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  • \$\begingroup\$ Well explained my electronics teacher in school used the same Water explanation with a resistor being a reducer in the pipe and variable resistor being a adjustable valve in the pipe he had something about capacitors too but cannot really remember that part \$\endgroup\$ – Tjaart van aswegen Jan 7 '19 at 19:44
  • \$\begingroup\$ Maybe a water tank \$\endgroup\$ – Damien Jan 8 '19 at 5:09
  • \$\begingroup\$ The tank was the Battery \$\endgroup\$ – Tjaart van aswegen Jan 8 '19 at 17:41
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Correct me if I am wrong, but what I think you are really trying to ask is: “how does the circuit magically know what components are down stream in the circuit?” Information can only travel as fast as light, so there must be some amount of time where the electrons leaving the voltage source are unaware of the resistor on the other side of the diode.

The short answer is the electrons don’t know. Research transmission lines to understand this.

The long answer is the electrons don’t know what is down stream, and they begin to travel down the wire as if it is a short circuit to ground. The wire is conducting a very high current at this point. We model this as a capacitance in parallel with the component (resistor, diode, the wire itself).

This current continues until the electrons reach something that impedes their flow, like a diode, at which point some of the current is reflected due to the impedance of the diode literally impeding the flow of electrons. The electrons leaving the voltage source are still unaware of what is downstream even though some previous electrons already reached the diode and reflected back. This leads to a buildup of electrons at the diode and creates a rise in voltage, even higher than the voltage source, and sends a current into the voltage source, which has an impedance and causes another reflection which travels back down the wire. This “ringing” eventually settles to a steady current flow into the diode. This is modeled as an inductor in series with the component (resistor, diode, the wire itself).

Keep in mind that all of this occurs very quickly in a circuit, on the order of picosecconds or less, depending on the length of the circuit. These effects are mostly ignored unless you’re dealing with very fast and/or sensitive electronics, or long traces/wires/cables. With long cables, you can actually see the reflections using an oscilloscope because the reflections occur on the order of microseconds.

This happens for every component in the circuit and the back flow of electrons (the ringing) settling to steady-state is really how the electrons coming out of the voltage source know how to behave (i.e. how much current should be flowing).

This is just a basic explanation. To really understand transmission lines, there has got to be a video or opencourseware lesson that adequately explains it.

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    \$\begingroup\$ Actually, yeah. I've always wondered about this. Thank you. \$\endgroup\$ – Azzam Alsharafi Jan 6 '19 at 3:21
  • \$\begingroup\$ That makes sense to me too \$\endgroup\$ – Tjaart van aswegen Jan 7 '19 at 19:41

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