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### Topic: Close looped nature of circuits - WHY (Read 6621 times)previous topic - next topic

#### mrxyz

##### May 31, 2013, 09:38 pm
I'm having a lot of trouble understanding WHY electricity needs a close looped structure. It explains everywhere that it does but the physical reasons for why are never explained.

Given the analogy often used of water through a pipe. The pipe only has two ends, one filling it and one releasing it as opposed to a O-... structure.

Also, what does it matter the electricity that goes back to GND. Looking at voltage dividers and trying to understand how they work I don't quite get how havingthe second resistor AFTER the point the circuit is tapped make any difference to the electricity there beforehand. I.E in this example image explaining how voltage dividers work http://upload.wikimedia.org/wikipedia/en/f/f4/Impedance_Voltage_divider.png what exactly is Z2 doing and how the hell is it supposed to have an effect on the electricity read out before it at Vout.

I suppose those are two questions but maybe understanding the meaning of the first one will help understand the second one. From the way I've tinkered with circuits so far I've just sent absolutely whatever back to GND thinking it doesn't matter anyway, it just needs to be linked, but since looking at that circuit diagram for voltage dividers I'm starting to get the feeling what goes back to GND somehow has some kind of an impact.

#### larryd

#1
##### May 31, 2013, 09:47 pmLast Edit: May 31, 2013, 09:49 pm by LarryD Reason: 1
Example:
You have a voltage you want to measure.
You have a voltmeter.
Try to measure that voltage with only 1 lead connected to the voltmeter.
You cannot.
You need 2 points to measure a potential difference, one is almost always COMMON or the 0 volt reference point.

A bird can sit on a 500,000 power line and still live. ;-)
No technical PMs.
The last thing you did is where you should start looking.

#### mrxyz

#2
##### May 31, 2013, 10:07 pm
It's most likely due to my level of understanding but I'm not seeing how that explanation relates to my question

#### zoomkat

#3
##### May 31, 2013, 10:10 pm
Quote
I'm having a lot of trouble understanding WHY electricity needs a close looped structure. It explains everywhere that it does but the physical reasons for why are never explained.

The physical reasons are well explained. For this discussion just remember that if electricity didn't loop back, then we would soon run out and be back to warming ourselves around camp fires.
Why I like my 2005 Rio Yellow Honda S2000  https://www.youtube.com/watch?v=pWjMvrkUqX0

#4
##### May 31, 2013, 10:15 pm
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.

In the voltage divider, the current flowing into Z1 then gets split and is shared across Z2 and the load at the output.
If the load current is much less than the current at Z2, it can be approximated as not being there (the beauty of engineering) and
Vout will equal Vin x Z2/(Z1 + Z2). (that's also why voltage dividers are not used as power source)
The current from Vin to Gnd is = Vin/(Z1 +Z2)
With the current known, then the voltage across each Z in known: V=current x resistance
and in total Vin = (current x Z1) + (current x Z2) with Vin = Vz1 + Vz2
With some algebraic substitions, this can be solved to yield Vout (or Vz2) = Vin Z2/(Z1 +Z2)
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### pito

#5
##### May 31, 2013, 10:19 pmLast Edit: May 31, 2013, 10:33 pm by pito Reason: 1
You must understand the whole electronics and the schematics you may see are just a way how the engineers agreed to handle the electrical phenomena - it is just an agreement.
You must not have a ground for electricity to flow - a lightning can strike between two clouds - when one cloud has 100 free electrons inside it, and the second has 50 electrons inside it, the electrical field induced by the electrons will force the electrons  to balance - so 25 electrons will pass via the lightning (the current) to the the other cloud so both will have 75 electrons and the show is over.
For reading the schematics you must accept some rules and you life will be much easier.

When you have a wire hanging from the roof there could be a voltage on it. So you cannot say the circuit must be "closed". The wire has 100 free electrons and your hand has 20 free electrons. So when you are far away from the wire the field is weak, nothing happens. When you get close to the wire the field is much stronger, and at certain distance the field is so strong that the electrons flies out from the wire and and they will try to compensate/balance so the 30 electrons will pass into your hand and show is over.

So it is basically ruled by law of equilibrium - the concentration of electrons must be same in the whole universe at the end of the day. Because it is would be crazy to discuss number of electrons in cloud A and B, the engineers agree the cloud A with 100 electrons will be Minus (because a lot of free electrons there) and cloud B with 50 electrons will be Plus (because it is lacking 50 electrons to be in equilibrium with cloud A). Therefore the voltage difference (potential) will be 50. When the cloud A discharges into cloud B the 25 electrons flow from clod A to B - that is the current and comes to an equilibrium. You may see we do not speak about GND.

Long time back people agreed the potential difference in number of electrons will be called Volt in honor of Mr. Volta, and the stream of free electrons flowing (to create the equilibrium between two different electron concetrations) will be called current named after Mr. Amper.

GND - that is an agreemnt to have something like GND which is the common reference point for measurement - imagine you will not have a GND - how you will reference voltages in such schematics as of your TV set? Against what?

#### mrxyz

#6
##### May 31, 2013, 10:26 pmLast Edit: May 31, 2013, 10:32 pm by mrxyz Reason: 1
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.

If that is the case then I think I get it. It still wouldn't explain that diagram though and how having a resistor after the point the electricity is taken affects the electricity before it. Thing is I can use the equations and work it all out but that kind of learning isn't what I'm into. I need to understand what is going on before I'll feel comfortable using the equations.

#7
##### May 31, 2013, 10:36 pm
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.

Say Z1 was 5K and Z2 was 10K and  Vin was 15V.
Total current flow thru both resistors is then 15V/(5K+10K) amps or .001A (1mA) because they are in series - the current must go thru both.
So the voltage from Vin to Gnd is then split: across Z1 is then 5K x 1mA = 5V and across Z2 = 10K x 1mA = 10V.
So if minimal current is then flowing into a load connected to Vout, Vout would = 10V.
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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### pito

#8
##### May 31, 2013, 10:40 pm
..do not use Z - that is not resistance.. Stay rather with R for educational purposes

#### mrxyz

#9
##### May 31, 2013, 11:07 pm
Still not following.

Here's what I think is happening...

The electricity runs through vin and remains approx the same until it enters the first resistor. At the point it leaves the first resistor the voltage and current should now be decreased based on however many ohms that resistor holds. Let's say it decreases from 5v to 4v and 3amps to 2. Half of this then go into each of the two routes so Vout gets 2v and 1amp, the other route gets the same. Then Z2 based on how many ohms it is does something to it and sends that value back to ground (which i see as a dustbin).

Where is/are the problem(s) in my description?

#### Grumpy_Mike

#10
##### May 31, 2013, 11:41 pm
Quote
Where is/are the problem(s) in my description?

It is totally wrong.

I have told you many times a resistor does not of and in itself reduce a voltage. It is only the current flowing through it that causes a voltage drop across it. Until you can get that fact in your head you will continue to go round in the same sort of circles that current does.

Quote
Let's say it decreases from 5v to 4v and 3amps to 2.

A series circuit has the same current flowing through all components current is not reduced by a resistor.

#### mrxyz

#11
##### Jun 01, 2013, 12:02 am

Quote
Where is/are the problem(s) in my description?

It is totally wrong.

It would've helped if you followed that up with a thorough explanation of why I'm wrong in a way someone who's just learning could understand that fault in their thinking.....

#### pito

#12
##### Jun 01, 2013, 12:18 am
You have to understand this forum cannot provide you with such service as teaching you very basics of electronics. I would recommend you to spend ie. a few months reading the books about electronics basics. Then, when you are prepared somehow, we may help you with concrete issues..

#### Grumpy_Mike

#13
##### Jun 01, 2013, 12:34 am
Quote
It would've helped if you followed that up with a thorough explanation of why I'm wrong in a way someone who's just learning could understand that fault in their thinking.....

I thought that is what I did.