Resistors and voltage

Resistors slow down the speed of the electrons which is a reduction in current. 

So I'm thinking of this from a physics standpoint.  All these electrons are being attracted to a positive charge.  They go in to a resistor which has a bunch of atoms that some of those electrons smash in to (generating heat).  So there are fewer physical electrons coming out the other side.  That is our reduction on current.

But the electrons that do come out are still as equally attracted to the positive voltage, right? 

So I understand that due to Ohm's law there is a voltage reduction after the resistor, but how does the reduction of voltage actually happen?  In other words, I don't understand how the amount of electromagnetic Force has actually reduced after the resistor?  Is it simply that there are less electrons to exert that force on and so therefore that is why it is measured as a lesser amount?

I hope my question makes sense!

EDIT:  Thank you all for correcting many of my mistakes above! 

You must also consider that the resistor is connected to something else that has resistance. If that resistance is near 0 (such as ground or supply), that will define the voltage drop and thus the current. If the something else's resistance is near infinite (such as the gate of a JFET transistor), there is nowhere for the current to go and thus there will be almost no voltage drop at all. So you must consider the 3 dimensions of ohms law simultaneously. If you change one of those dimensions, one or both of the other two must change accordingly.

Think of resistance as friction that hinders (not necessarily slows down) current flow... kind of like the difference between a 1/2" water pipe, and a 6" water main...  The same water pressure will push less water flow (current) through the 1/2" pipe.

JR

PS: This is affectionately known as the "water analogy" because it works. 

Mbira said:

Resistors slow down the speed of the electrons which is a reduction in current. 

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No and no.
Current out of the exit terminal is exact same, (not one electron missing), than current into the input one.

So I'm thinking of this from a physics standpoint.  All these electrons are being attracted to a positive charge.  They go in to a resistor which has a bunch of atoms that some of those electrons smash in to (generating heat).  So there are fewer physical electrons coming out the other side.  That is our reduction on current.

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No.

So I understand that due to Ohm's law there is a voltage reduction after the resistor, but how does the reduction of voltage actually happen?  In other words, I don't understand how the amount of electromagnetic Force has actually reduced after the resistor?

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There is a voltage drop across the resistor, which must be substracted from the source foltage.
Example:
*
You have a 10V supply.
You connect a 1k resistor in series with the positive terminal.
You pass 1mA currentb through that resistor.
It now drops 1V across it.
So at the output terminal you now measure (10-1)=9V

Of course, if you send 1 mA through one resistor leg, you get exact 1 mA from the other, there is no other way.

Is it simply that there are less electrons to exert that force on and so therefore that is why it is measured as a lesser amount?

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No.

Mbira said:

Resistors slow down the speed of the electrons which is a reduction in current. 

So I'm thinking of this from a physics standpoint.  All these electrons are being attracted to a positive charge.  They go in to a resistor which has a bunch of atoms that some of those electrons smash in to (generating heat).  So there are fewer physical electrons coming out the other side.

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That last statement seems misleading. There are the same number of electrons going into one lead of the resistor as there are coming out of the other lead.

That is our reduction on current.

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Again, the current (technically, the number of electrons per unit of time) going in is the same as the current going out.

Perhaps what you mean by 'reduction' is the current would certainly be less than that going through a solid wire across the same voltage.

If you want to know 'where this all comes from,' this article (specifically 'Causes of conductivity') might be what you're looking for:
https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

squarewave said:

You must also consider that the resistor is connected to something else that has resistance. If that resistance is near 0 (such as ground or supply), that will define the voltage drop and thus the current. If the something else's resistance is near infinite (such as the gate of a JFET transistor), there is nowhere for the current to go and thus there will be almost no voltage drop at all. So you must consider the 3 dimensions of ohms law simultaneously. If you change one of those dimensions, one or both of the other two must change accordingly.

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To keep things simple, I am just thinking of a battery with a resistor between + and -.

benb said:

Perhaps what you mean by 'reduction' is the current would certainly be less than that going through a solid wire across the same voltage.

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yes-I'm trying to understand the physical difference that happen with and without a resistor.

There IS less current flowing with a resistor vs without a resistor. Right? This is because the resistor is slowing down the speed that the electrons are able to get through the wire. Right?

Don't think of it as slowing down.  Current is the amount of charge that passes through a cross section per second.  It is the amount of charge,  not its velocity.

john12ax7 said:

Current is the amount of charge that passes through a cross section per second.

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This guy is pretty good at explaining this and a few other terms.. he reminds me of Mr. Connery....

https://www.youtube.com/watch?v=AQOM4pkbIcM

john12ax7 said:

Don't think of it as slowing down.  Current is the amount of charge that passes through a cross section per second.  It is the amount of charge,  not its velocity.

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OK, so it doesn't slow down, but is rather the actual physical number of electrons that pass through the point.  Speed doesn't change.  Got it. 

And others have made it clear that you don't lose any electrons when going through a resistor, so I got that too. 

So how about this:

From the negative terminal, the electrons can "see" that there are physical things (atoms and molecules?) that are in the resistor that is up ahead and so because of that resistance, only a certain number of electrons per second will be able to pass through that physical limitation and you can only have one electron leave the negative terminal for every electron that returns to the positive terminal, so that is the reason that there is less current when there is a resistor vs without. 

Is that correct? 

Mbira said:

yes-I'm trying to understand the physical difference that happen with and without a resistor.

There IS less current flowing with a resistor vs without a resistor. Right? This is because the resistor is slowing down the speed that the electrons are able to get through the wire. Right?

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No.

Without a resistor there is no current flowing because the circuit is open.

The positive terminal  of the battery  is at a higher potential than the negative terminal.  When you connect a resistor between the positive and negative terminals,  a  closed circuit is formed and a current starts to flow across the resistor. Naturally the amount of current flows through the resistor is subject to Ohm's law. However, this current also flows through the battery as we now have a closed loop circuit.

Now, the whole battery voltage is across the resistor (this can also be described as dropped across the resistor) and the energy is dissipated as heat across the resistor and also the battery as it also has an internal resistance, until such time that there is no potential difference between the positive and negative terminals of the battery. In other words the whole energy is wasted and there is zero current flow within the circuit.

Mbira said:

From the negative terminal, the electrons can "see" that there are physical things (atoms and molecules?) that are in the resistor that is up ahead and so because of that resistance, only a certain number of electrons per second will be able to pass through that physical limitation and you can only have one electron leave the negative terminal for every electron that returns to the positive terminal, so that is the reason that there is less current when there is a resistor vs without. 

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Yeah, I guess, sort of. The resistive material is filled with electrons. Always. Even when there is no current flowing at all. If the electrons are drawn out of one end (by the positive terminal of a battery for example), then that creates an infinitely small decrease in charge which is, at the speed of light, is reflected on the other end of the resistor which draws in more electrons to balance the charge.

sahib said:

No.

Without a resistor there is no current flowing because the circuit is open.

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Wouldn't it be a short-not an open, and so would send a ton of current from negative to positive (with potentially exciting results)?

PS: Thank you all for taking the time to teach me this stuff.  It's a long and slow road, but very rewarding to try and move past the "paint-by-numbers" that I have been doing for so many years in electronics. 

Mbira said:

Wouldn't it be a short-not an open, and so would send a ton of current from negative to positive (with potentially exciting results)?

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If you remove a resistor and put nothing in it's place then it's open (assuming no parallel path).
But if you mean that you put a wire in it's place (essentially a Zero Ohm 'resistor') then it's a short.

But as JMFahey indicated your starting points were all wrong.
Need to read some Physics texts but in any case bear in mind that it's not the same electron 'going into' the resistor that comes out the other side. The voltage causes a net drift of electrons that is superimposed on the 'random' (average zero) velocity with no voltage. You may find the drift velocity is surprisingly small. However the speed of electricity itself is that of an electromagnetic wave.
https://en.wikipedia.org/wiki/Drift_velocity
and also look up Kirchhoff's Laws (esp the first wrt current into / out of a circuit node)

In terms of analogies the 'Water' based explanations work well in terms of visualisations and there are mechanical analogies for reactive impedances using springs and stuff (it's been a while

yup  don't get lost in the microscopic details.... Voltage is like pressure, and current is amount of flow...

JR

Hi guys,
Indeed I really DO want to get caught up in the microscopic details right now.  I understand how things work as per the various water analogies and all that. 

What I'm wanting to do now is actually learn more about what is actually going on (or at least what we believe is going on based on Classical Physics). 

Newmarket, I'm glad you brought up Drift Velocity.  That is actually a piece of this that I am just today trying to grasp as it relates to Amps.  I am starting a new thread on that. 

Thanks again all, for taking the time with something that I understand most of you know inside and out!

Mbira said:

What I'm wanting to do now is actually learn more about what is actually going on (or at least what we believe is going on based on Classical Physics). 

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Why? Are you looking to become a physicist?

If you're doing small signal audio + digital like 99% of what you'll find on these boards, understanding the physics of electronics is not particularly useful. I never think about electrons or drift velocity.

What you should be concerned with are the basic principles of impedance, ohms law, how transistors work, negative feedback, transformer ratios vs impedance, etc.

If you really want to understand electronics well, you should download and install LTSpice and simulate circuits. LTSpice will show you voltages, currents, do bode plots (aka spectrogram / frequency response plots), impedance measurements and just about every minutia of how a particular circuit works. It actually simulates the circuit over time so that you can see how voltages and currents and transistors turning on or off and so on.

squarewave said:

Why? Are you looking to become a physicist?

If you're doing small signal audio + digital like 99% of what you'll find on these boards, understanding the physics of electronics is not particularly useful. I never think about electrons or drift velocity.

What you should be concerned with are the basic principles of impedance, ohms law, how transistors work, negative feedback, transformer ratios vs impedance, etc.

If you really want to understand electronics well, you should download and install LTSpice and simulate circuits. LTSpice will show you voltages, currents, do bode plots (aka spectrogram / frequency response plots), impedance measurements and just about every minutia of how a particular circuit works. It actually simulates the circuit over time so that you can see how voltages and currents and transistors turning on or off and so on.

Click to expand...

Why are you wasting your time trying to tell someone what they shouldn't spend their time learning?  How many times do I need to say that I'm interested in learning about this REGARDLESS of whether it applies to circuit design.  Why do you care so much about trying to tell me what I SHOULDN'T learn? 

Please don't tell me what I should and shouldn't be concerned with learning, and then maybe the next guy won't waste his time trying to tell you what you should be spending your time doing. 

If my questions are irrelevant enough to this board, then I assume that John or another mod will tell me to stop.  Until that happens, if what I'm trying to learn about doesn't interest you or seems irrelevant, then why discourage someone else who is interested in it? 

Mbira said:

Why are you wasting your time trying to tell someone what they shouldn't spend their time learning?  How many times do I need to say that I'm interested in learning about this REGARDLESS of whether it applies to circuit design.  Why do you care so much about trying to tell me what I SHOULDN'T learn? 

Please don't tell me what I should and shouldn't be concerned with learning, and then maybe the next guy won't waste his time trying to tell you what you should be spending your time doing. 

If my questions are irrelevant enough to this board, then I assume that John or another mod will tell me to stop.  Until that happens, if what I'm trying to learn about doesn't interest you or seems irrelevant, then why discourage someone else who is interested in it?

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and why listen/argue....

lets keep the stupid arguments in the brewery

JR

JohnRoberts said:

lets keep the stupid arguments in the brewery

JR

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Thumbs up.