Current Flow

[jason619]

Hello everyone

I am really having hard time when it comes to the flow of current in electrical wiring diagrams. I cant seem to grasp this current flow. Would highly appreciate if one could assist me in this regard.
May be my basics are not good but any help would be welcome.

Regards

 

[Jraef]

You have not described what it is you do not understand. The Force is not strong enough in you for most of us to be able to read your mind.

Current flows where a conductor goes. Does that help?

 

[Smart $]

DC = direct current. Current flows in one direction.

AC = alternating current. Current flows back and forth.

Insulation is like the banks of a river. An insulation fault is analogous to a flood.

 

[GoldDigger]

An insulation fault is analogous to a flood.

Or if the voltage is high, a burst radiator hose.

 

[ActionDave]

I don't think the OP is asking about "how current flows?". I think he is looking for some tips about how to read a diagram or schematic. If that he is not what he means I don't care, since I suck at reading diagrams, I want to see what tips the smart members here who can read diagrams have to offer.

 

[rcwilson]

If you are asking about control wiring diagrams try this:

1. Find the power source and identify the hot and neutral leads/terminals.
2. Imagine current flowing from the "hot" power terminal to the neutral terminal. (L1 to N, H1 to N, X1 to X2,....).
3. Make a copy or several copies of the schematic or wiring diagram.
4. Highlight all energized parts of the diagram, starting at the 'hot" terminal. Do not go across open contacts or open switches.
5. Imagine closing the first switch or contact. (Maybe it is the thermostat contact on your furnace control calling for heat).
6. Highlight the path of current across that contact. Note if any control relays or motor starters or motors (like a furnace purge fan) are now energized.
7. Find the contacts that get closed by those relays or motors. (Example: the furnace air pressure switch actuates when the purge air fan runs. Control relay contact CR-1 actuates when the relay coil CR1 is energized).
8. Highlight the paths across those contacts.
9. Note if any of the previous contacts open and de-energize their circuits. Mark that circuit as off.
10. Continue until you are done.


Note that this method probably won't help much if you are looking at current flow in three phase wiring diagrams where currents add vectorially.

 

[Jraef]

I don't think the OP is asking about "how current flows?". I think he is looking for some tips about how to read a diagram or schematic. If that he is not what he means I don't care, since I suck at reading diagrams, I want to see what tips the smart members here who can read diagrams have to offer.

So my rule still stands: Current flows where the conductor goes. If you have a switch, it is a conductor that changes from conducting, to not conducting. When it is a state of not conducting, it no longer follows the above rule. If at the end of the conductor, you have a load, look inside. Somewhere there is a conductor inside of that load, and when current flows through that conductor, it performs whatever task it was supposed to do.

 

[jason619]

It seems like people have different answers for my question. my question is about basics

say we have a dc source, a lamp and a switch. when the switch is turned on, current flows
through +ve terminal of battery through switch, lamp to -ve terminal. what happens after that?
i mean does the same current then goes from -ve to +ve terminal of battery or is tht
current has completed its path and it will never start again from +ve of battery...if it is so
then i believe that DC source will die after sometime.?...please reply.
what if we replace dc source with AC? how will current flow during +ve and -ve cycles?

 

[GoldDigger]

A DC source might be a battery. In that case, the electrons that make up the current are moved from one battery terminal to the other by a chemical reaction which supplies the energy which is going into the load. Eventually the chemicals are depleted and the battery, depending on the type, will be recharged or discarded.
If the source is a generator, either AC or DC, there are are wires inside it which complete the circuit. A moving magnetic field, powered by an engine, forces the electrons back "uphill" and supplies the power which goes to the load.

 

[mivey]

Jason,

Current is charge flow. You may think of the source as acting like a charge pump. Think of it like a pump circulating water in a closed loop.

Current and voltage are mechanisms we use to deliver power through the fields they create. The current is not delivering power like a bucket full of water. We use current to create magnetic fields and voltage to create electric fields. We use these fields to exchange energy. Since current is a flow of charges, it is not "used up". Rather, it is the energy used to pump the charges that is used up.

With DC, the electron charge carriers flow in one direction. They can accumulate or "pile up" like in the case of a capacitor (like pumping up a bladder full of water) but otherwise the charges flow in a loop as they create the needed magnetic field.

With AC, the charges just oscillate back and forth as they create the magnetic field we need but for the most part don't really travel much from their original position. In fact, the same electrons that were in your house wiring at install are probably still there.

PS: Strictly speaking, it is not "current flow". Current is charge flow so "charge flow flow" would be redundant.

 

[mivey]

If you want the gory engineering details, I suggest the IEEE publication "Power Definitions and the Physical Mechanism of Power Flow" by Alexander Eigeles Emanuel. It is the best I've seen on the topic.

If you want the abbreviated version, just ask some more questions and I'll answer.

 

[Little Bill]

PS: Strictly speaking, it is not "current flow". Current is charge flow so "charge flow flow" would be redundant.

Charge flow flow....
Say it ain't so so!

 

[mivey]

Charge flow flow....
Say it ain't so so!

Let get out my harmonica and we be jammin'.

 

[JoeStillman]

The single most important fact to remember when reading ladder diagrams is this;

Normal means "not energized". All contacts are shown in the normal (i.e. coil not energized) position.

This may seem counter-intuitive at first because for many devices, the ordinary condition is to be energized. However, once you get your head around this concept, it all falls into place.

A well-drawn diagram will also give you little clues about the nature of electro-mechanical devices like float or pressure switches. You can show whether or not they make or break on rise or fall. Same thing with timer contacts. An up arrow means time delay on energize. Down means TD on de-energized.

 

[Besoeker]

Hello everyone

I am really having hard time when it comes to the flow of current in electrical wiring diagrams. I cant seem to grasp this current flow. Would highly appreciate if one could assist me in this regard.
May be my basics are not good but any help would be welcome.

Regards

The best non-electrical analogy is the pump and pipes.
The source of pressure (analogous to voltage) pushes water down the pipes. The flow is current.
More pressure from the pump results in more flow, all other things being equal.

 

[Smart $]

...
More pressure from the pump results in more flow, all other things being equal.

Or less restriction (resistance, impedance).

I'd say bigger pipes, but the the analogy fails in that larger wires will not give an equivalent increase in flow to the analogy of larger pipes... unless you include some type of utilization equipment which provides essentially the same analogous restriction to flow.

 

[GoldDigger]

Or less restriction (resistance, impedance).

I'd say bigger pipes, but the the analogy fails in that larger wires will not give an equivalent increase in flow to the analogy of larger pipes... unless you include some type of utilization equipment which provides essentially the same analogous restriction to flow.

And, very nicely, for a fixed restriction load (spray nozzle for instance) there will be an undesirable pressure drop in a length of narrow pipe just as there will be a voltage drop in a small wire. But the overall effect (in a well designed setup) of that pressure drop will be small compared to the pressure drop at the nozzle.

 

[charlie b]

The best non-electrical analogy is the pump and pipes. The source of pressure (analogous to voltage) pushes water down the pipes. The flow is current.
More pressure from the pump results in more flow, all other things being equal.

One weakness of this analogy is that I can take water from a lake and push in into a tank, and the water never has to return to the lake. Electrical current cannot flow from one place to another, without a return path.

To address at least part of the OP's question, once current has made its way from the +ve pole of the battery, through the switch, along the wires, through the load, and returns to the -ve pole of the battery, it travels through the battery, picking up the energy that it had lost in its first run through the circuit, and then starts the trip all over again.

NOTE TO THE PHYSICS MAJORS: Let's please not confuse the OP with any true stories of drift current and the way current really behaves. Let's keep things simple, until the OP is ready to go deeper into the theories of our industry. It is not a bad start to think of current as though it were cars going around a race track. We know that that is not the truth, but it works as a first lesson in electrical theory.

 

[mirawho]

It seems like people have different answers for my question. my question is about basics

say we have a dc source, a lamp and a switch. when the switch is turned on, current flows
through +ve terminal of battery through switch, lamp to -ve terminal. what happens after that?
i mean does the same current then goes from -ve to +ve terminal of battery or is tht
current has completed its path and it will never start again from +ve of battery...if it is so
then i believe that DC source will die after sometime.?...please reply.
what if we replace dc source with AC? how will current flow during +ve and -ve cycles?

Hmmm...... you are talking about flashlight basics. First off, in a DC circuit, electrons flow from negative to positive. Next, if you are just using a battery as a source of energy, since the lamp you are using is a load, the load consumes power and this eventually will drain a battery. I am not sure why this seems to be a mystery to you. If you do not understand this, I suggest you read up on basic electrical theory. A battery is a temporary source of energy and that source will be expended after a while. You can replace the battery with a DC producing source and this will run until someone unplugs it or something breaks mechanically. Power flows as long as there is a load consuming the power. If there is no load, the power does not complete a circuit path. You seem to somehow have guessed that the power will eventually be expended which makes it even harder for me to understand exactly what you don't understand. Get a glass of water and close it. You can sit there and look at it for a year and all the water will still be in the container. Now drink it. What happens? It is gone. Same with a temporary power source.

 

[Smart $]

One weakness of this analogy is that I can take water from a lake and push in into a tank, and the water never has to return to the lake. Electrical current cannot flow from one place to another, without a return path.

Actually there is a return path, in a way. The water taken from the lake is replaced with air, just as the air in the tank is displaced by the water taken from the lake. Roughly same volume of air vs water.

To address at least part of the OP's question, once current has made its way from the +ve pole of the battery, through the switch, along the wires, through the load, and returns to the -ve pole of the battery, it travels through the battery, picking up the energy that it had lost in its first run through the circuit, and then starts the trip all over again.

NOTE TO THE PHYSICS MAJORS: Let's please not confuse the OP with any true stories of drift current and the way current really behaves. Let's keep things simple, until the OP is ready to go deeper into the theories of our industry. It is not a bad start to think of current as though it were cars going around a race track. We know that that is not the truth, but it works as a first lesson in electrical theory.

I'll honor the request (for now ), but would like to add that the cars around a race track are bumper to bumper... in the analogy, of course.