Why is all voltage lost at the last component?

Bruce_Wiseman

Joined Feb 18, 2016
5
In my physics class we have learnt that all voltage is lost at the last component. If there is one resistor then all the voltage will be lost at that component if there are more than one then all voltage will be lost at the last component. When I asked how do the electrons "know" that all charge must be lost there, I was told to not worry about it because I don't need to know it for the course.

So why if there was circuit (C1) with a 5v battery connected to a 100 Ohms resistor (R1) would all the voltage be lost at that resistor, and if there was another circuit (C2) with a 5v battery connected to a 100 Ohms resistor (R1) then to a 50 Ohms resistor (R1) why would all the voltage at the 50 Ohms one? Why wouldn't it all be lost at R1 in the second circuit? How do the electrons know?

MrChips

Joined Oct 2, 2009
22,505
Your concept of what happens in a circuit is flawed. No voltage is lost anywhere.
A better way to understand what happens to current and voltage in a circuit is to apply Ohm's law.

Assume that we have a voltage source of V-volts connected to three resistors R1, R2 and R3 in series.
No voltage is lost anywhere.

First we calculate the current in the circuit using Ohm's Law.
The current is
I = V/(R1 + R2 + R3)

Now we calculate the voltage across each resistor, again using another form of Ohm's Law:

V1 = I x R1
V2 = I x R2
V3 = I x R3

Now, you will find that V1 + V2 + V3 = V.

No voltage is lost anywhere.

There is no first and last component in a series circuit.
Neither R1 nor R3 is the first or last component. We can rearrange the order of the resistors such as R2 + R3 + R1. The voltages as calculated above remain the same.

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AnalogKid

Joined Aug 1, 2013
8,806
I completely understand reducing an advanced concept down to a more basic level so that people with less of a background in the topic can come away with a little understanding, but you physics teacher went way too far, and by the way is wrong. Your intuition is correct.

Sticking with DC batteries and resistors, all of the current in a loop flows through all elements in the loop. As simple as that sounds, it is critical to everything electrical. So from the battery positive terminal the current goes through one, two, or 99 resistors connected end-to-end in series, then back to the battery negative terminal. All of the current goes through all of the parts, basically because there is nowhere else for it to go. If there are three identical resistors in series, then 1/3 of the battery voltage appears across each one. If one resistor is larger than the others, a greater voltage is developed across it. The math is called Ohm's Law, as above.

And just to be clear, voltage never is "lost". This might be just a language translation thing, but lost is not only not the right term, it denotes something that is not correct. In electronics we say that a voltage is developed across a resistance.

ak

Bruce_Wiseman

Joined Feb 18, 2016
5
I am beginning to have a better understanding. She said that the 'model' we use of electrons having a charge is too simple to explain what I asked. Is the electrons charge lost by the end of the circuit?

ErnieM

Joined Apr 24, 2011
8,078
No, the electrons never loose charge. They are fundamental particles.

When you complete the circuit the voltage from the battery forms a field thru the wiring across the components like resistors that drive the electrons to move.

Actually, unless you have superconducting wires there is also a very small voltage across the ends of the wire too, though for most practical purposes you may ignore that.

Your instructor may be thinking of one point, say the negative battery terminal as a reference. If you connect a meter lead to that and probe around the circuit you will see various voltages until you come back to that same point and both meter leads are at the same point. That may be what is meant by the voltage being lost.

dl324

Joined Mar 30, 2015
12,237
Is the electrons charge lost by the end of the circuit?
No, they go back to the voltage source to complete the circuit.

MrChips

Joined Oct 2, 2009
22,505
A typical electrical circuit creates a complete loop. No electrons are gained or lost.

Think of a circuit as a tube or hose with a single row of marbles tightly packed in the tube.
The interesting thing about electron flow is how slowly they actually move.
When the electrons are set in motion to create a current, the velocity of the electrons is of the order of cm per hour.
When you set one marble moving, all of the marbles have to move together around the loop, that is, a wave is set in motion, like a wave of the spectators in the stands of a ball-game.

How fast does this wave move?

About the speed of light.

The speed of light is 300,000 km/s.

The wave of the electrons will move at about 60% of the speed of light, i.e. about 200,000 km/s while the electrons themselves only move a fraction of a millimetre per second.

Like the spectators in the stand, they don't move seats. What you observe is the wave motion across the stadium.

alfacliff

Joined Dec 13, 2013
2,458
it sounds more like what is lost is your money paying for a class with a poor instructor.

atferrari

Joined Jan 6, 2004
4,169
And just to be clear, voltage never is "lost". This might be just a language translation thing, but lost is not only not the right term, it denotes something that is not correct. In electronics we say that a voltage is developed across a resistance.
ak
That is what I thought when I read the OP. I would like to know what was the word given in the original language.

Not a native myself, I realized that I never heard that expression related to the subject. Yes suitable instead, to those 400 Deutsche marks I lost when visiting Frankfurt many years ago. That convinced me to use a wallet all the time.

Bruce_Wiseman

Joined Feb 18, 2016
5
The language it was given in was english. I was told that if a battery has lets say 5 V then by the time it reaches the end all the voltage will have been 'used up' (I can't remember what the exact word she used was), no matter what. So by the end the voltage at the end of the circuit will be 0. No matter what even if there is just a single resistor on the circuit then the voltage will all be 'used up' in that one resistor.

I am trying my best to explain it. Tomorrow I will ask if she actually knows why it does that instead of accepting 'You don't need to know it.' as an answer. But all that I have said is to the best of my knowledge from class, if she doesn't know why I will see if she can give me more information just to help the search for an answer.

dl324

Joined Mar 30, 2015
12,237
The language it was given in was english. I was told that if a battery has lets say 5 V then by the time it reaches the end all the voltage will have been 'used up' (I can't remember what the exact word she used was), no matter what. So by the end the voltage at the end of the circuit will be 0. No matter what even if there is just a single resistor on the circuit then the voltage will all be 'used up' in that one resistor.
She probably said something like there is a voltage drop across each element. KVL says the sum of the voltages around a loop is zero, so it would make sense to say the voltage drop across the last component would "use up" any remaining voltage.

Bruce_Wiseman

Joined Feb 18, 2016
5
It may have been that what she was talking about. I thought of a way to ask the same question but changing the way of asking it.

In a circuit if 3 resistors are in parallel then unequal amount of current will go to each one correct? The same amount of voltage will be supplied to all of them but the current will be different. How does the circuit or electrons 'know' how much current to send to each resistor?

Now I realise I changed it to current but I just want to know how it 'knows' how much to send to each resistor. Same for the question I originally asked.

The voltage must have dropped by the end of the circuit to 0V, no matter what, for some reason. My teacher said that the remaining voltage will be dropped at the last component, how does it know that the component it is at is the last one?

dl324

Joined Mar 30, 2015
12,237
It may have been that what she was talking about. I thought of a way to ask the same question but changing the way of asking it.
That's what made us think something was lost in translation...
In a circuit if 3 resistors are in parallel then unequal amount of current will go to each one correct? The same amount of voltage will be supplied to all of them but the current will be different. How does the circuit or electrons 'know' how much current to send to each resistor?
Depends on the resistance values.
The voltage must have dropped by the end of the circuit to 0V, no matter what, for some reason. My teacher said that the remaining voltage will be dropped at the last component, how does it know that the component it is at is the last one?
"It" (whatever that means) doesn't know. In a single loop, the current in all components will be the same and each will have a voltage drop determined by it's resistance. The last element will always appear to "use up" the remaining voltage.

Bruce_Wiseman

Joined Feb 18, 2016
5
The last element will always appear to "use up" the remaining voltage.
That's what I want to know: how does it 'know' that the last component is the last one? Because it does not think and the electrons cannot go forwards and then backwards to drop the remaining volts at the last component how does it know to drop it all there?

dl324

Joined Mar 30, 2015
12,237
That's what I want to know: how does it 'know' that the last component is the last one? Because it does not think and the electrons cannot go forwards and then backwards to drop the remaining volts at the last component how does it know to drop it all there?
The total resistance and voltage in the loop will determine current. Each resistance will have a voltage drop determined by Ohm's Law. No matter where you start in the loop, or which direction you go, the last element will always drop the remaining voltage because that's what KVL requires.

alfacliff

Joined Dec 13, 2013
2,458
not a very good explanation of ohms and kirchoff's laws. this has actually been hashed out a lot of years ago. trying to change the terminology form voltage drop to voltage lost isnt a good idea, just confuses everything.

MrChips

Joined Oct 2, 2009
22,505
"The last component is the last one."

When you have a bag of jelly beans, Smarties, Thorntons toffees, or whatever, the last one you eat is always the last one.

That does not apply to current and voltage in an electrical circuit.

So stop using that language.

You have 10 pounds in your pocket. You give 5 to Mary. Two to John. Two to Paul and the last one to Nigel. How come the last person gets the last pound?

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WBahn

Joined Mar 31, 2012
26,398
That's what I want to know: how does it 'know' that the last component is the last one? Because it does not think and the electrons cannot go forwards and then backwards to drop the remaining volts at the last component how does it know to drop it all there?
It doesn't know. You could just as easily say that, in a circuit with a battery and three resistors, that some voltage is "lost" ("dropped" or "developed" is a better term) in the first one and the last one and that "all of the remaining" voltage is dropped across the middle one. Or do the same thing but rearrange things so that all of the remaining voltage is dropped by the first one.

A resistor is a device that requires that a voltage be applied across it in order for current to flow through it (and, equivalently, you could say that it is a device that develops a voltage across it whenever a current is flowing in it). These are proportional -- twice the voltage means twice the current (and vice-versa).

When you apply a voltage across a string of resistors, then current starts to flow in all of the resistors and that current is the same in all of the resistors. Each resistor develops a voltage across it in proportion to the current and its resistance (Ohm's Law). If the sum of the voltage drops is less than the battery voltage, the amount of current will increase and the voltage across each resistor will increase. If the sum of the voltage drops is more than the battery voltage, the amount of current will decrease and the voltage across each resistor will decrease. This will settle out (almost instantaneously) to the current at which the sum of the voltage drops exactly equals the battery voltage.