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For reference: I was trying to figure out why I can put a resistor before or after an LED for it to work. I found an answer here that says:

The LED only sees the difference in voltage between its two leads, as does the resistor. Since only the difference is important it makes no difference what order the parts are connected in.

So my question is, why doesn't it matter, like physically - if that makes sense? I think I may not be visualizing how power flows through a circuit correctly. I imagine power like being a thing that goes from one thing to the next, as in, it travels like we would. So in series, we go to a place, in order, one at a time. In parallel, we have multiple people who are, at the same time, leaving to go to other places. People being the power and the places being the loads.

So, how is it that an LED not burn out without the resistor protecting it from all the power coming straight out of the battery and into it? and/or how is it that when a resistor is placed after it that it can still affect the LED that the full power has already moved past?

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    \$\begingroup\$ Other duplicates: electronics.stackexchange.com/q/340519/2028 and electronics.stackexchange.com/q/253671/2028 \$\endgroup\$
    – JYelton
    Jan 25 at 21:14
  • \$\begingroup\$ Power is not a thing. Don't try and think about power as a thing that is traveling. Don't even think about voltage. Think about potential differences. Then current forms between the potential differences. And the power comes from the steepness of the difference and the amount of current. \$\endgroup\$ Jan 25 at 21:15
  • \$\begingroup\$ Look up KCL. Current is identical in both cases. \$\endgroup\$
    – winny
    Jan 25 at 21:15
  • \$\begingroup\$ I don't understand what "relative" and "absolute" voltage is. I definitely think I'm misunderstanding how electricity moves in a circuit. It's starting to seem like it doesn't "move" but somehow just exists and I don't understand in what way. I thought maybe the resistor would be in front of behind because of the way they are built, internally. But now I think the problem is just how I'm understanding the flow of electricity. \$\endgroup\$
    – JVDS1069
    Jan 25 at 21:36
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    \$\begingroup\$ @js1069 The difference between absolute and relative is simple. If you have a 1 meter stick, no matter if you throw it into ocean or take it on top of a mountain, the relative difference between stick ends is always 1 meter, even if one end of the stick is at - 11km or +8km in absolute relation of sea level. \$\endgroup\$
    – Justme
    Jan 25 at 21:42

2 Answers 2

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The answer lies in Kirchhoff's Current Law (KCL), from which we can state that within a closed circuit, the current is equal at each point in the circuit. The current that flows in is equal to the current that flows out.

Yes, kind of like water into and out of a garden hose. Push current in, the same current pushes out. What will differ is the pressure at each end. Back in the electrical world, voltage is pressure, while current is flow.

Consider an LED in series with resistor, connected to a battery. That's a closed circuit. KCL tells us that LED and resistor will see the same current, regardless of connection order.

Now, each element has its own voltage drop.

  • LED voltage drop is its forward threshold voltage (Vf)
  • Resistor voltage drop is proportional to current (E = I*R).

The total voltage drop for the two will be Vf(LED) + I*R.

Notice that '+' sign. You may recall that addition is commutative. It doesn't matter what order you do the addition. You get the same total voltage drop.

Further, regardless of connection order, because we know based on KCL that the currents will be the same, so will the terminal-to-terminal voltage drop of each. Vf will not change, nor will E = I*R. The only difference will be the voltage seen between the resistor and LED relative to the power supply.

Here's a quick sim to show what's up (simulate it here):

enter image description here

You'll notice in each case that:

  • LED forward drop is the same
  • Resistor IR drop is the same
  • The currents are equal everywhere

The only difference we see is the voltage between the LED and resistor, since that shifts with the component order.

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Current is the same throughout the circuit.

Here is a way of thinking about the situation that may help it make sense to you:

Imagine a hose filled with marbles being pushed through (the hose is the wire, the marbles are your current). If there is a restriction that slows the progress of the marbles (the resistor), it does not matter if this restriction is near the "beginning" or "end" of the hose...all the marbles will be slowed to the same pace regardless.

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