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Topic: Flow of electricity - Simple Circuits (Read 12189 times) previous topic - next topic

AppleAssassin

Okay, so I have a class for my third year software engineering at university and have to do things with arduino which is fine however:

I took physics in high school and got a B however recently I just can't get my head around circuits   :| The whole flow of current vs the flow of electrons, positive to negative, negative to positive etc. But I just can't get my head around WHY it doesn't matter which side of a simple circuit the resistor is on. I'd like a good analogy, because currently I'm imagining a sort of road with cars as electrons and when they get to the resistor they slow down, but when they come back to the power source they speed up again. I just don't get how they can come up to the LED, without having gone through the resistor, out of the battery and not damage it. I'm so confused.

However upon just typing that paragraph I thought of a new analogy, still with cars though. Is it more like a traffic jam? Like when the circuit is completed the "cars" start to move but are held up by the "cars" in front of them?

polymorph

The much maligned water analogy works, if you don't try to take it too far.

If you put a restriction in a pipe, the water slows down on both sides of the restriction. But the pressure on the downstream side is a lot less.

If you take the restriction out and put it back in the other way, nothing changes. The upstream side has more pressure, but the same number of gallons per minute flows on each side.

The car analogy does not work because cars are a compressible fluid. Water is an incompressible fluid, and if you don't go to extremes, electricity mostly acts like an incompressible fluid. Again, don't try to take the analogy too far.
Steve Greenfield AE7HD
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Grumpy_Mike

Forget trying to work out electron flow it only confuses. Concentrate on current flow from positave to negitave . What is flowing? It is charge. Just think of it as electric charge that flows.
It flows in a circuit, that means it circulates so it is not important what component is encountered first because all components have to be gone through in a compleat lap of a circuit.

cjdelphi

With your car analogy. ..

Think of a resistor as a speed limit, the road (single series circuit anyway) is the wire, the cars all travel at the speed set by the total resistance...

The speed limit affects all cars on that one road before and after (single series circuit) so if we have 2 resistors on the same road combine the value and the resistance equal how fast the cars travel, and they all move at the same speed, decrease the resistance the faster the cars go, on both sides of the resistor.

Now stick an led into the mix, the led will consume (providing threshold voltage met) everything you give it, too many cars/current/electrons will blow it, so you add a speed limit to control the current.. so remember that the resistor slows the electrons down regardless where you place it in a single series circuit.


AiRgeek

I hope this helps to clear up current flow, for reference purpose

Quote

Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source.

Electron Flow is what actually happens and electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source.


Ref: http://www.mi.mun.ca/users/cchaulk/eltk1100/ivse/ivse.htm#

In electrical engineering i.e. for circuit analysis, it is common to adopt  Conventional Current flow.

I am not sure what you mean by "but when they come back to the power source they speed up again", I think looking into The Mesh Current Method of circuit analysis could help to clear things up a bit.
http://www.allaboutcircuits.com/vol_1/chpt_10/3.html

dlloyd

Analogies are great ... the more the better!
Voltage <--> Pressure
Resistance  <--> Pipe Size
Amps <--> flow rate (of charge or force)
Power <--> Speedometer
Energy <--> Odometer
Visual analogy of electricity at work <--> Kinetic Pendulum

I thought check a few videos of kinetic pendulums and after seeing a few, here's a variation that's quite amazing. This (in my opinion) seems to demonstrate the truth about wave forms, where a square wave is actually an infinite series of sine wave harmonics added together. The square wave would be when the pendulums are moving in one line (end to end). Or possibly it demonstrates this: "It has been found that any repeating, non-sinusoidal waveform can be equated to a combination of DC voltage, sine waves, and/or cosine waves at various amplitudes and frequencies".

Repeating Waveforms <-->
http://www.youtube.com/watch?v=yVkdfJ9PkRQ






JimboZA

And whichever way it really flows, the key principles are those of Kirchoff:


  • The sum of the current at a node is zero: what comes in must go out, no "nett" current

  • The sum of voltages round a loop is zero: if you have a nothing but a battery and a resistor, the addition to the circuit by the battery must be subtracted by the resistor so there is no "nett" voltage

Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

dave-in-nj

I think one part that might be missing is relativity.

the electron entering the LED can only move through the LED as fast as the flow allows.  the resistor limits the flow.

for the classic resistor and led, one way to look at it is a pipe,  the resistor limits the speed (voltage) from the point the individual pipe is connected to power all the way to the point it is connected to something on the other side.

if they were beans, then you could count how many went into the pipe, or in the middle or out the other end.   it would not matter.

back to relativity.   if you need 20 beans, you calculate the resistor.  you get 20 beans.

no where does the amount of beans available ever come into play.  it could be a cup you are pouring in, or it could be a 5 pound bag, or a cargo ship at the dock.    if your LED wants 20 beans, your resistor is selected, you only ever get 20 beans in that pipe.

so, if your power is 9 volts or 8,000 volts, the resistor only allows 20 beans to pass. 

Many people try to think the available quantity has any effect.   

another idea might be if you take a cup to the water fountain and try to fill it.  all you care about is how full the cup is.
next use a garden house.  were you able to fill the cup any more ?
now, dip it in the swimming pool.  could you fill it more ?
now go to the ocean, could you fill it any faster or and fuller ?

what is available has no bearing on what you use.

JimboZA

Quote
the resistor only allows 20 beans to pass.


.... per unit time.

Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

polymorph

OK, this is getting confusing even to me.

Voltage is not power. Current flow is not speed. Given a specific flow of current (1A = 6.24x10^18 electrons/sec past a point), the speed of the electrons in a thin wire are much faster than the speed of the same current flow in a thick wire. Just as the water shooting out the nozzle on the end of a garden hose is moving very quickly, but the exact same amount of gallons per minute in the big pipe feeding your block is very slow.

Imagine a big loop of pipe connected to a pump we'll call V1, with two valves inline, we'll call the valves R1 and R2. The system does not leak.

Open R1 partway, and R2 partway. With the pump running and creating a constant pressure of water between its inlet and outlet, can you see that both valves inline reduce water flow to less than what it would be if there were only one valve? Can you see any reason it would matter which valve is "first"?
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
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mirith

Physicists use Electron Flow
Real people use Current/Traditional/Ben Franklin Flow
I would try and ignore Electron Flow unless you are specifically wanting to get deep into the physics aspect of it.  Or you are trying to do Semi-conductor/material science.

Grumpy_Mike

Quote
Physicists use Electron Flow

No they don't.
They use charge flow depending upon what is the majority carriers in the substance they are studying.

Quote
the speed of the electrons in a thin wire are much faster than the speed of the same current flow in a thick wire.

No the speed of electrons is always about the same irrespective of the wire. That speed is only about 30 miles per hour so it is not very fast.

Quote
so, if your power is 9 volts or 8,000 volts, the resistor only allows 20 beans to pass. 

Total rubbish.

Quote
back to relativity.   if you need.......

Relativity concerns traveling at close to the speed of light, nothing to do with this argument.


Teslaspigeons


Physicists use Electron Flow


Physicists use the majority charge carrier which depends on the material.  It can be electrons or it can be holes moving through a lattice. 

retrolefty

#13
Feb 06, 2014, 02:08 pm Last Edit: Feb 06, 2014, 02:15 pm by retrolefty Reason: 1

Physicists use Electron Flow
Real people use Current/Traditional/Ben Franklin Flow
I would try and ignore Electron Flow unless you are specifically wanting to get deep into the physics aspect of it.  Or you are trying to do Semi-conductor/material science.


From my experience that is exactly backwards. I was taught in military electronic training that EE physics teaches and uses the concept of 'conventional' current flowing being positive to negative while we were taught to use that electron/current flows negative to positive. It's an old argument/preference dating back forever to the early discovers of electricity. The physicists, once aware of their basic error invented 'hole flow' so they could continue to believe (in error) that current flows from positive to negative.

Take your side of the 'argument', but just stay consistent and your circuits will still function. It wouldn't be a problem except the physicists got to define which way the arrow points in semiconductors symbols so as to maintain their original error in defining current flow direction, so all our schematic drawings are wrong.  ;)

polymorph

Quote
The physicists, once aware of their basic error invented 'hole flow' so they could continue to believe (in error) that current flows from positive to negative.


I don't even know where to start with how wrong this is.

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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