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Author Topic: Close looped nature of circuits - WHY  (Read 2001 times)
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If the current flow into the load at Vout is small compared to the total current flow, then voltage at Vout is just the current times the resistance  of V2.

If that load is an analog input, very very little current flows, so the load is effectively 0 ohm, and Vout will equal Vin*Z2/(Z1+Z2) as described above.

Really?

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Most of my analog inputs are 50 ohms.
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Ooops - should have said 0 amps there. Good catch.
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Electricity needs a loop because the electrons need to go somewhere after giving up their energy.


The energy is the push-pull called voltage. It is the pressure in the water pipe, not the water except as flow.

You can pour water down a vertical pipe and the weight of the water (mass of the water influenced by gravity) will cause it to flow right up until it runs out. But the water-electric circuit analogy is really for horizontal pipes. In those, without the pump there is no flow and the energy is the pressure and flow.
 
You can take two capacitors joined with one wire. Along that wire have a led, a resistor and a switch. Open the switch so that the two capacitors are unconnected and charge one of the capacitors. When the switch is closed, the led lights up until there is not enough difference between the capacitors to light the led. When the switch is closed, current will flow from the charged capacitor to the other. There is no loop yet **for a while** there is flow.
Even if you did loop this circuit, current does not flow through capacitors (unless there's enough to break them) so no flow that way.

You will understand these things better after you learn the basics of electrons, potential, conductors, insulators, magnetism, fields, etc, including static electricity. That will give you foundation to understand what simple components are to be able to deal with semiconductors.
Without the basics you will have mysterious formulae and cookbook schematics that you understand in ways that might as well be SPAM SPAM SPAM SPAM SPAM, tarot cards and voodoo. Then you can make impossible statements like "if electricity didn't loop back, then we would soon run out".
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You're not include enough in your "water" analogy.  Say on your roof is a water tank that collects rainwater.  The water in there has a certain amount of  potential energy, because it is "up high."   You can turn on a spigot and run water through your set of pipes, possibly doing some work using that potential energy.  In the process the water loses the energy, and spews out the end of the pipe, somewhat lower down than it started.
But that's not the actual end of the circuit!  The water goes down the drain and flows to the bay.  Then the sun evaporates the water (adding energy), and eventually it rains, refilling your water tank.  So the water has gone around a full circuit as well.  See "water cycle" in any "Earth Science" book.

The sun evaporating the water and adding energy is like a solar cell adding energy to electrons in the circuit.
The water tank is like a capacitor (or rechargeable battery), storing that energy so that you can use it.

The water spewing out the end of your pipe is a "low resistance" path back to "ground" (perhaps it even spills on the ground.)
You can make electrical circuits where electrons "spew out" the end of a wire into space (fundamental to the operation of vacuum tubes, and responsible (?) for those lovely arcs that come off of Tesla coils, for instance), but while water finds the open end of a pipe leading to air to be "low resistance", electrons coming to the end of the wire behave differently and find it hard to "jump off."  A bit like holding the end of your water pipe against a block of wood, and still expecting the water to come out because "wood is made up of molecules, just like air" (a block of metal in the path will stop the flow of water, but not the flow of electrons.  An airspace will stop the electrons, but not the water...)
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I don't like to take the water analogy too far. It fails in inconvenient places and time for students to learn ways it does work could be spent on learning real electronics which gets far more fascinating thanks to charge and fields, etc.

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You're not include enough in your "water" analogy.  Say on your roof is a water tank that collects rainwater.  The water in there has a certain amount of  potential energy, because it is "up high."   You can turn on a spigot and run water through your set of pipes, possibly doing some work using that potential energy.  In the process the water loses the energy, and spews out the end of the pipe, somewhat lower down than it started.
But that's not the actual end of the circuit!  The water goes down the drain and flows to the bay.  Then the sun evaporates the water (adding energy), and eventually it rains, refilling your water tank.  So the water has gone around a full circuit as well.  See "water cycle" in any "Earth Science" book.

The sun evaporating the water and adding energy is like a solar cell adding energy to electrons in the circuit.
The water tank is like a capacitor (or rechargeable battery), storing that energy so that you can use it.

The water spewing out the end of your pipe is a "low resistance" path back to "ground" (perhaps it even spills on the ground.)
You can make electrical circuits where electrons "spew out" the end of a wire into space (fundamental to the operation of vacuum tubes, and responsible (?) for those lovely arcs that come off of Tesla coils, for instance), but while water finds the open end of a pipe leading to air to be "low resistance", electrons coming to the end of the wire behave differently and find it hard to "jump off."  A bit like holding the end of your water pipe against a block of wood, and still expecting the water to come out because "wood is made up of molecules, just like air" (a block of metal in the path will stop the flow of water, but not the flow of electrons.  An airspace will stop the electrons, but not the water...)


So the electrons reach GND to be recharged so they can repeat the cycle again? A bit like a recycling system for electrons. Can someone confirm if I've understood this bit correctly.
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Raised to a higher potential at least.

So if I heat a wire and that drives electrons up my non-circuit circuit should work?
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Hmm. Some of the explanations went over my head but this seems to make sense:
Electricity needs a loop because the electrons need to go somewhere after giving up their energy.
Just your water pipe idea, where a pump returns the water to the top flow down again, the electrons return to a battery, say, to be re-charged flow again.

So the electrons aren't exiting the system, they're just all losing their charge and returning back to Ground to get recharged and go for round 2.

You don't mean charge in the second paragraph, all electrons have the same charge all the time.

The energy is a function of the whole system, not just the electron, in fact the energy resides in the electric field as
a whole.

In all everyday situations the positive charges and negative charges balance out to dozens of orders of magnitude,
as the energy to separate significant amounts of charge is astronomical.

In a metal wire the positive charges are fixed, they are the metal atoms (whose electrons have wandered off),
and the electrons are mobile, but they stay with the metal to balance out the charge.

When they move en-masse in the same direction you have a macroscopic observable current - the negative
charges move, the positive ones stay still, net flow of charge (but stays balanced).   You cannot have all
the electrons marching out of the metal into the air and building up an imbalance, except in miniscule amounts
(a millionth of percent imbalance would generate a potential in the millions to billions of volts, something
absolutely vast).   

So the circuit has to be closed, the mobile charges stay in close association with an equal number of fixed
charges (in semiconductors and ionic solutions the charge carriers can be of either charge polarity).
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Most of my analog inputs are 50 ohms.

At the Arduino pins AFAIK all inputs have megohms impedance. But don't give them 12V.

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Well, you can give them 12V, but it must  be at very low current, and then you rely on the ESD protection diodes to control the voltage.

This Atmel application note discusses it.
I'd rather use a voltage divider to keep it <= 5V or 3.3V if so powered.

* Atmel AVR182 Zero Cross Detector.pdf (95.14 KB - downloaded 13 times.)
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Water can fall out of an open pipe onto the floor.

Electrons prefer to be attached to atoms.
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Quote
Electrons prefer to be attached to atoms.

Except when they aren't like in a TV screen tube, most any electron tube and sparks, electron tunneling, plasma, etc.

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You have had a bunch of good answers already.

If you want you electrons to perform a decent piece of work, you need lots of them. That is why you "recycle" them in a closed loop system.
You use chemical or mechanical energy to pump them around the circuit.

But if you did not have a loop, but jut terminated the "pipe" (your electrical conductor) after the light-bulb, then it would not be an open pipe. It would be a capped pipe! Because you have nothing to conduct the electrons from there.
That means the flow will stop, and there will be no light, because it is electrons moving through that does the work.

You can build up pressure (but that is the field of static electricity we're moving into. As long as you do not understand the regular electric games, stay out of electrostatics!) but it will not give you any work.

Please read:
http://science.howstuffworks.com/environmental/energy/circuit1.htm






Apart from that, I agree that the water analogy is very illustrative, but quite bad.
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If instead of capacitors at the ends of a wire with components on it there was a piezo disc at each end and they were driven by out of phase vibrations then there could be an AC flow without a closed loop. I could see such a setup used in an alternate power generator.

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Sometimes an example says more than many times as many words.

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