Multiple Circuits

Factual Questions
1

Ok, so I’m not an electical engineer. But I do know that electricity has to flow in a circuit to work. I also know that electricity follows the shortest path back to complete that circuit.

What I don’t understand is how electricians power one switch or plug from another. Rather than having individual wires coming from the circuit breakers for each switch, they will power a second switch from the first one.

My question: Why doesn’t the electricity power the first switch (completing the circuit) and bypass the second switch?

2

Pal Joey,

The good news is you don’t have to be an EE to understand the difference between series & parallel wiring, which is (I think) what you are alluding to here.

The bad news is, it is a little heavy to get into here, not being able to draw you some simple diagrams.

As plain as I can make it: electrical outlets are connected in such a way that they can all place equal demand on a circuit for current. That’s why it’s so important that you never overload a circuit.

Electricians will attempt to calculate the approximate load on a circuit when the wiring goes in. House wiring doesn’t require that a circuit be “completed” by anything other than the load itself (whatever you plug in), and each load has free access to the circuit its connected to and can draw whatever current it requires, up to the limit of the breaker.

Pick up a book on basic electricity (not electronics) and read all about the difference between series & parallel wiring, because that will be much more clear than my explanation or any of the dozen that are sure to follow.

Some things that are really pretty basic can become ugly when you try to explain them using only words & limited space.

Recent polls revealed that some people have never been polled, until recently.

3

Basically the amount of current flowing through each device (lamp etc.) is determined by the voltage (115) divided by the resistance of the load. Putting more devices on line causes more total current to be drawn from the source, but the voltage remains the same. As long as the total current available for the circuit (Circuit breaker value) is greater than the sum of the individual loads, all is well. This is a simple case, no reactive/capacitive loads (motors etc.)
Think of it as drawing water off the Colorado river. As long as the river ain’t dry you are OK. Add a big city with thirsty people and you are in trouble.

4

opus sed …

I was thinking the same thing as I tried to spew the above

A point in every direction is like no point at all

5
6

Ain’t it the truth. I get frustrated when I can’t draw a little diagram and I know that’s all that would be needed to make the answer crystal clear. You may have scared him off with your mention of reactive loads, but I liked your thirsty people analogy.

I think Joe is laboring under the false assumption that access points are series-connected and therefore somehow ‘relay’ current from one load to the next. If this were the case (which it’s not), removing a load would kill the whole circuit.

I still can’t think of a better way to word the answer.

Recent polls revealed that some people have never been polled, until recently.

7

The reason is, oversimplifying, that electricity doesn’t really follow the shortest path back.

Electricity is really just the flow of electrons. Imagine if each electron was a person driving in their car trying madly to get from the negatave end of the battery to the positive end through a circuit. In this case the wires are like really fast freeways, allowing the little electron cars to flow very easily. However, somewhere in the middle of the circuit is a load, lets say a light bulb.

The light bulb has a great deal of resistance relative to the wires, or thinking of our electron cars, it is a very narrow congested street. (In fact the reason a bulb gives off light and heat, again oversimplifying, is that it impedes the progress of the electrons, releasing their energy as light and heat.)

Well, the electron cars go rushing madly down the expressway of the wires and turn off to the bulb, and lo and behold, there’s a major traffic jam.

Let’s say that Mr. Electrician then wires runs another set of wires (freeways) from each end of the bulb, and puts a bulb (traffic jam) between the ends of these new wires. Well what happens at the bulb junction?

Some electrons will turn off at the first bulb, and get into the traffic jam there. Others, seeing the traffic jam backing up almost to the freeway say to themselves “I’m going to be a smart electron and avoid the traffic jam,” and turn onto the new freeway that Mr. Electrician just built.

Now, as we know, electrons aren’t really that smart, and the smarty-pants electron will just get himself into another light bulb traffic jam down the road. However, attaching another bulb (or other load) across the terminals of the first bulb (or other load) is clever enough to trick electrons into lighting both bulbs.

Anyway, that’s how the electrician’s union got their motto “Smarter than the average electron.”

Hope this helps.

8

Here’s one site that has some pretty good basic information, just click on “install it”. There are many others. “ehow” works too.
http://www.homedepot.com/cgi-bin/prel80/index.jsp

Peace,
mangeorge

I only know two things;
I know what I need to know
And
I know what I want to know
Mangeorge, 2000

9

If I understand the question correctly, you’re asking “since electricity follows the shortest path, why does it bother venturing beyond the first complete circuit?”

Well, if the first circuit happens to be a ‘short-circuit’ (or a 100% shunt) then your intuition would be correct. The thing is, circuits aren’t all that usefull, unless they’re doing something. When you put a load on a circuit, then it offers some resistance to the flow of electricity. That saying you are thinking of, is actually “Electricity always follows the path of least resistance, to the great exclusion of all others.”

Let’s say you have two parallel circuits…

[A1]//switch\//load\[A2]
.…
.…
.…
.…
[B1]//switch\//load\[B2]
.…
.…
.…
.…
[C1]//////source\\\[C2]

Ignore the periods, they’re just space holders. The brackets are the wire. [X#] represents a connector.

Now, the first thing to consider is that since there is (virtually) no resistance between [A1], [B1] and [C1], they are electrically the same point. The same is true of [A2], [B2] and [C2]. Now, if you close the switch between [B1] and [B2] current flows through the load and back to the source. The load at [B1]-[B2] offers some resistance to the flow of electricity.

Now, open the [A1]-[A2] switch and the surplus current that was resisted at [B1]-[B2] makes its way through the [A1]-[A2] load, which also offers resistance.

You can imagine that the source is a water pump. If you open the gate at [B1], the water flows through an impeller (load) and has to work to get to the [B2] side. The resistance of the impeller means there is enough pressure on the inlet side, so that when you open the [A1] gate, there is enough left to drive the second impeller. If the pump is powerful enough, and/or the impellers offer enough resistance, you can keep adding circuits.

If you remove the load and simply put a big pipe between [B1] and [B2] then you have a short circuit and no work will be done at [A1]-[A2].

Clear as mud…

Stephen
Stephen’s Website
Satellite Hunting 2.0.1 (Y2K compliant!) visible satellite pass prediction
shareware available for download at
Satellite Hunting

10

Hopefully, this diagram will look more like I’d planned…

[A1]//switch\//load\[A2]
.…
.…
.…
.…
[B1]//switch\//load\[B2]
.…
.…
.…
.…
[C1]//////source\\\[C2]

Stephen
Stephen’s Website
Satellite Hunting 2.0.1 (Y2K compliant!) visible satellite pass prediction
shareware available for download at
Satellite Hunting

11

Ok, I now know in principle why parallel circuits don’t short circuit through the first load. But since wire does have a finite resistance, isn’t there a limit on how long the wire can be before a load on the far end has significantly more resistance than an identical load nearer the power source?

Soon afterwards, Deimos simply vanished from the sky.

12

As I recall, the phrase was electricity follows the path of least resistance. And in the simple case that means that the amount of electricity (current) is inversely proportional to the resistance to the flow. It does not mean that the eletricity follows the shortest physical path. Perhaps that is where joemill was mislead.

Example:

a circuit with a 50 & 100 ohm loads in parallel

I = E/R (Ohms Law)

  • ------|-----|
    | |
    100v 50 100
    | |
  • ------|-----|

I = current flow in each branch (amps)
E = 100 v supply across each branch (voltage)
R = 50 ohm and 100 ohm load (resistance)

50 ohm load = 100/50 = 2 amps current
100 ohm load = 100/100 = 1 amp current

total current = 3 amps

More current flows through the load with less resistance.

The total 3 amps of current flows into the circuit, 2 amps flows through the 50 ohm load, leaving the remaining 1 amp to flow through the 100 ohm load, for a total of 3 amps. You can add more loads as long as the total current does not exceed the supply current.

I really wish I could post a picture …

13

It’s just like your plumbing - your bathroom sink and shower and toilet can all come off the same pipe. Depending on your water pressure, you may be able to run two or three of these things at the same time without affecting the others. If you have lower pressure, though, turning on one will affect the others - if you’ve ever been scalded by hot water when someone flushes the toilet, this is you.

So, the bigger the circuit breaker, the more ‘electric pressure’ (I know this isn’t exactly right, but it fits the plumbing metaphor) there is to be had on the circuit without affecting the other loads. If you turn on one light it’s like running one faucet, if you turn on another it’s like running two. If you turn on too many, you can either a) dim the lights as the circuit struggles to keep up (just like flushing the toilet and getting scalded in the shower) or b) blow the circuit (not a situation you have in plumbing - imagine your water meter turns off your water if you use too much.)

So, you can add as many plugs/switches to the circuit as you want, as long as you don’t exceed the rating of the circuit breaker or the wiring you are using, just as you can add as many faucets to your house as you want. If you want more current/water, you need to add more lines from the main supply.

Was that at all helpful?

14

Well, for a veeerrrrry long run, sure. But if you need to construct such a circuit, you use high quality, low resistance, wiring and if you’re really stretching things, add more power to the mix and use step-down transformers at your loads.

Stephen
Stephen’s Website
Satellite Hunting 2.0.1 (Y2K compliant!) visible satellite pass prediction
shareware available for download at
Satellite Hunting

15

Stephen, I’m glad you finally brought up the path of least resistance thing. Actually, in that way among many others, the flow of electrons that makes up what we call ‘electricity’ obeys pretty much the same rules that water does. The ‘plumbing’ examples are remarkably good!

How many folks have noticed a momentary dimming of the kitchen lights when the fridge kicks on? The ‘flushing toilet’ example parallels this situation almost exactly! Refrigerator compressor motors typically feature starter capacitors (sometimes called ‘coils’) that hold a charge to ‘kick’ the motors into motion. When the fridge thermostat activates the compressor motor, the capacitor ‘dumps’ its charge – almost instantaneously – to the motor and begins recharging immediately. The recharging process is a low-resistance affair that diverts current (electrons) from other, higher-resistance loads, such as light bulbs. In concept, the big difference between the electric circuit and the plumbing system (when a toilet flushes) is the time scale. Discharging and recharging the refrigerator capacitor usually happens in something less than a second; refilling a toilet tank can take minutes. But the flow principles are the same.

16

I didn’t realize that starter capacitors were called coils, in fact a coil (inductor) is the opposite of a capacitor. Could just be antiquated motor jargon though.

Some basic (non-calc) info.

Impedence http://whatis.com/impedanc.htm

Capacitive reactance http://www.sweethaven.com/acee/forms/frm1104.htm

Inductive Reactance http://www.sweethaven.com/acee/forms/frm0604.htm

17

Yep, ‘antiquated’ is prob’ly a good word. ‘Incorrect’ is also correct. :smiley:

My grandpa also called a paper bag a “poke,” and cottage cheese was “smearcase” to him.

Actually, I’ve heard a number of folks refer to a starter capacitor as a ‘coil.’ Maybe because of a passing similarity to the ignition coil on an automobile engine?

I don’t know why fortune smiles on some and lets the rest go free…

T

18

I hope these folks weren’t schooled in electronics!

Capacitors & coils have nothing in common (even physically), and I doubt that even an amateur technician would confuse the two. They behave in precisely the opposite way in electronic circuits. Capacitors used to be called condensers and coils were called chokes.

Take it easy on the Ohm’s law & the diagrams, kids. Remember we’re trying to explain electricity to somebody who knows little or nothing about it.

Recent polls revealed that some people have never been polled, until recently.

19

Must have been that hump on the motor. Hump … What hump … ::looking around::

What were those equations ?.. V = L di/dt ; I = C dv/dt

20

JOmill- another name for voltageis potential. the car idea is the easiest to comprehend but he went too far in his explanation.Simply take the first idea and double it. Also electricity is like people .It is lazy. it will take the easiest path to ground.