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Circuit Theory Q:What actually provides energy to a device?

#1
12-28-08, 03:44 PM
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Circuit Theory Q:What actually provides energy to a device?

While learning that voltage is lost to resistance and length of wire, I came up with a question. I noticed that a length of 50' 12AWG wire could cause a 2% voltage drop.
Does this voltage drop start from the beginning of the wire or at the end of the wire?
Also, does the current that passes through a wire and a device like a lamp on the wire still have the same amount of amps from beginning to end? Is the amps used by the device? What actually powers the device if no electrons are lost?

#2
12-28-08, 04:59 PM
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The voltage drop occurs throughout a conductor's length, just as the total resistance is distributed along the wire.

A 2-conductor circuit will have the same current (amps) throughout the circuit.

No electrons are lost, but power and voltage to the load are diminished, due to resistive heating in the conductors.
Doug

#3
12-28-08, 05:58 PM
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Ok, so we're talking about Joules. We lose Joules as they are converted from electrical energy to heat energy through a lamp. How is a Joule different from current and voltage?

We use watts of power to get the loads to do work. Isn't there anything lost when the load does it's work?
If a wire's purpose is to transfer current across a load so that it can do work, what is used up? I assumed it was electrons were wasted, but now I am discovering that that isn't so. We will have the exact same amount of amps from one end to the other end, no matter how many loads it goes through.

But how is this so when there is conversion of electrical energy to thermal energy in a lamp? Something has to be lost as something is changed and not reverted back. What gives?

#4
12-29-08, 09:50 AM
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How is a Joule different from current and voltage?
Voltage (volts) is a measure of potential energy, the ability to do work. It represents how "charged up" the electrons are. You can equate this to physical objects by picturing a bowling ball suspended from a tower. The bowling ball is "charged" against the force of gravity and therefore has the potential to do work by falling to the earth. Much in the way that a some apparatus must have expended fuel to hoist the bowling ball up the tower against gravity, a generator must be used to increase the charge on electrons. When the electrons do work, their charge is reduced.

Current (amps) is how many electrons you have.

Power (watts) is the rate at which work is done. Electrically speaking, power is voltage times current (how charged up your electrons are times how many electrons you have).

Energy is work done over time. The standard unit of energy is the Joule which is equal to one watt per second; meaning that if you had a 1 watt light bulb switched on for one second you would have expended 1 Joule of energy; a 100W light bulb switched on for 10 seconds would be 1000 Joules. Another common unit of energy is the kWH (kilowatt-hour) which is the unit the electric company uses to bill its customers. One kWH equals 3,600,000 Joules.

When electrons flow through any wire, some of their charge is lost and converted to other types of energy. Some will become heat, some will become sound, some will become mechanical (movement), some will become chemical.

When we design circuits we try to optimize for a specific type of energy such as a high resistance filament which glows to produce light, or a magnetic coil which spins in a motor. An efficient circuit is one in which the highest percentage of energy is converted into a desirable type.

#5
12-29-08, 05:25 PM
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Originally Posted by ibpooks

Voltage (volts) is a measure of potential energy, the ability to do work. It represents how "charged up" the electrons are.

When electrons flow through any wire, some of their charge is lost and converted to other types of energy.

Ok, from what I think I am gathering here is that you are saying that the load/lamp is getting it's energy from voltage, that voltage(charge) is what does the actual work in a load. Am I getting this right?

#6
12-29-08, 07:16 PM
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When electrons flow through any wire, some of their charge is lost
Some of their "energy" is lost, but none of their "charge" is lost.

One way to think of this (not exactly correct, but there's really no "correct" way to think of this) is that before going through the load, the electrons are very "excited", or vibrating wildly. After passing through the load, the electrons are much less excited (perhaps they took a tranquilizer).

There are still the same number of electrons on both sides of the load and they are still the same number of amps. But the electrons are really "tired". Once they get back to the power company, they are again "excited" by an infusion of energy.

#7
12-29-08, 07:36 PM
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Wow, that's pretty wild stuff. It intrigues me very much so that I would love to understand that better. What you wrote is starting to paint a picture. Thanks for getting the ball rolling on that for me.

About the part where you wrote, "Once they get back to the power company, they are again "excited" by an infusion of energy. "

I'm not sure of the accuracy in that statement but I am guessing you are just using it further my understanding of this charged state. Thanks for that.

This does raise another question though. AC current is bi-directional, with hertz measuring how quickly the current changes direction within a second.
How is this possible? Does the current actually go back into where is was generated from?

#8
12-29-08, 07:52 PM
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The electron really doesn't need to go all the way back to the power company, but it probably does. Current flow is not really so much the movement of electrons, but the transfer of energy from one electron to its neighbor. Electrons actually move rather slowly through wire, but the energy transfer is very fast.

Although the direction of movement and energy transfer reverses 120 times a second, and this seems very fast to us, it is an agonizingly slow to an electron. If you think of a car driving back and forth across the U.S., and reversing direction once a year, that's a better image.

#9
12-30-08, 09:23 AM
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I like John's description that "excited" electrons is a better term to use than "charged"; "charge" means something else more specific.

The transfer of energy between electrons in a wire is like one of these:

You put energy into one side and it quickly comes out the other side, but the balls themselves don't really move very much except from some vibration back-and-forth.

#10
12-30-08, 02:07 PM
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That is absolutely amazing stuff. Thanks for the explanations.

Originally Posted by John Nelson
The electron really doesn't need to go all the way back to the power company, but it probably does.
This is a big wonder for me, where the heck does the reversed electron flow go? I am having a hard time understanding it because I see how a DC current will want to reach the other end. There is a difference in electrons and the circuit wants to help balance that difference. The negatively laden end of the circuit desires to balance the positive side that is actually lacking electrons.
So, this confuses me when considering AC and finding out that current will go back the other way in a split second. Why the heck would it do that? It's coming from an overabundance of electrons, the negative side isn't it? How can this happen with AC current?

Thanks again for the explanations, they are driving me to understand more. I am really appreciating these viewpoints.

Last edited by foolios; 12-30-08 at 02:34 PM.
#11
12-30-08, 06:10 PM
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As I am not a physicist, these questions are difficult (for me) to answer. Even harder is translating my thoughts into text.

I tried to answer the original questions, but in doing so I have only created more questions for myself.

Anyway, here is an interesting website I just found: The Speed Of Electricity that says while electrons do not move at the speed of light through a wire, effectively they do because one electron exits the wire at the same time another one enters (much like the swinging wonders pictured earlier).

As for the alternating current, if you were to graph the voltage of your 120v household current (120v is the RMS voltage) then you would get a sine wave (similar shape to sound waves, radio waves, etc.). The voltage starts at zero, rises and peaks at about 170v, then falls back to zero, then falls(?) to -170v then returns to zero. This cycle happens 60 times per second, which seems like a lot, but compare that to electronics and radio waves which have cycles per second (Hz) in the millions (MHz). Basically, the power turns off and on so maybe the electrons complete their travel when the circuit is off?

Now, unrelated, but I have always wondered what happens to the electrons that a capacitor "stores". The idea that a capacitor would 'steal' electrons from a battery and save them for later didn't seem right--the battery would be short electrons. This site: Howstuffworks "How Capacitors Work" shows that capacitors charge by losing electrons on one side and gaining them on the other.

There's plenty of theories to look at on the net...but they can make your head hurt.

#12
12-30-08, 06:26 PM
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Voltage is a force, just like gravity is a force. An electron is compelled to go the way it does just like a ball is compelled to drop when you let it go. The difference with voltage is that you can reverse it. Imagine a world in which gravity reversed fairly frequently. That ball you dropped would then yoyo, similar to what electrons do in a wire.

#13
12-30-08, 07:09 PM
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Try study OHM's Law.Once you understand that you got it licked.

#14
12-31-08, 08:33 AM
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Originally Posted by John Nelson
The difference with voltage is that you can reverse it.
Is the reversal intentional? Or is it a by-product of using it to transfer electrons?

So, what's happening is that voltage will go from a positive range to a negative range. This actually causes current to switch directions. Wouldn't this mean that when an electron goes into the wire causing an electron to be pushed out of the wire; when the reversal occurs, that an electron will now be pushed into the opposite end of the wire causing an electron to be pushed back into the power source?

Will a load on the circuit get power when the current is coming back to it from the opposite direction? Or will there be a slight pause in power until the voltage goes back into the positive range and causes the current to flow back into the original direction?

Thanks a mill for the explanations.

#15
12-31-08, 10:27 AM
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The reversing is intentional. The story of the battle between George Westinghouse (who favored alternating current and who won the battle) and Thomas Edison (who favored direct current) is very interesting. Read more about it here:

War of Currents - Wikipedia, the free encyclopedia

Yes, the power is not perfectly uniform. Light bulbs do flicker 60 times a second, but most people's eyes are not sensitifve enough to notice.

Anything with a motor (like a hair dryer) usually relies on the alternation, and needs it to function. Also, clocks use the alternation to keep time.

#16
12-31-08, 04:19 PM
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The positive and negative part of the cycle really depend on how you look at the circuit. Say you have a hot wire and a neutral wire. While one wire is positive, the other wire is negative; then they switch. The plus and minus just indicated which way the electricity is charged relative to the wire you are looking at. Power is delivered both on the positive and negative side. No power is delivered at zero, but the time it is actually at zero is very brief.

For electrons to flow in a circuit, you must have a complete circuit. The power source, wires, switches and load are all connected in a loop. Perhaps saying that electrons "enter and exit" a wire was misleading. What about a visualization of electrons around a wheel, going back and forth?

#17
12-31-08, 04:55 PM
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Originally Posted by foolios
Is the reversal intentional? Or is it a by-product of using it to transfer electrons?

So, what's happening is that voltage will go from a positive range to a negative range. This actually causes current to switch directions. Wouldn't this mean that when an electron goes into the wire causing an electron to be pushed out of the wire; when the reversal occurs, that an electron will now be pushed into the opposite end of the wire causing an electron to be pushed back into the power source?

Will a load on the circuit get power when the current is coming back to it from the opposite direction? Or will there be a slight pause in power until the voltage goes back into the positive range and causes the current to flow back into the original direction?

Thanks a mill for the explanations.
You really should get a book on basic electrical theory. There is no "pause" in power. The instantanous voltage between the two leads of a 120 volt single phase power source alternated in a sine wave pattern. In fact the difference between peaks is about 339 volts. 120 is the RMS ( something like an average, but derived as the integral of the area under the curve).

#18
12-31-08, 07:26 PM
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Originally Posted by Strategery
While one wire is positive, the other wire is negative; then they switch. The plus and minus just indicated which way the electricity is charged relative to the wire you are looking at. Power is delivered both on the positive and negative side.
I didn't realize this. The negative is actually a hot as well, not just the black positive wires? You could get zapped just by touching the negative wires, the grey(white) wires? We don't even need to touch the black and white wires together to make a complete circuit and short/get shocked?

Originally Posted by Strategery
What about a visualization of electrons around a wheel, going back and forth?
The negative thing is hard for me to get around. It would seem that somehow voltage will suck as well as push current when it comes to AC. Are we constantly recycling current? Can the same group of electrons keep moving back and forth over long runs?

Why doesn't voltage return in the same manner? Why is it depleted whereas current(electrons) is not?

#19
12-31-08, 08:01 PM
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Just a question to let us know what level we need to start on.How much formal training do you have in electrical/electronics ?If your still in school let us know what classes you have completed.We can not teach you to run if you have not learned to walk yet.
"The negative is actually a hot as well, not just the black positive wires? "
You seem to be confusing AC and DC.The " black wire " in your home electrical is not + or - .It is the HOT wire , where power is coming in.It's measured in AC volts < rms > not + or - .
DC volts can be + or - .

Last edited by craftsman_50; 12-31-08 at 08:11 PM. Reason: adding more
#20
12-31-08, 08:33 PM
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I didn't realize this. The negative is actually a hot as well, not just the black positive wires? You could get zapped just by touching the negative wires, the grey(white) wires? We don't even need to touch the black and white wires together to make a complete circuit and short/get shocked?
Sorry, but this comment is way off the mark. The black wire is not "positive" and the white wire is not "negative". There is no positive wire. Both the black and white wires are positive 60 times a second and negative 60 times a second.

There is no "pause" in power.
Actually, there is a pause in power. When the voltage sine wave crosses zero, at that instant, no power is being delivered. As has been mentioned, 120 volts is the "average" power (more specifically, the root mean average). Sometimes it's above 120 volts, and sometimes it's below. But the power delivered averaged over time is the same as a steady 120 volts.

It's best to separate in your mind what is happening instantaneously from the effect of this over time. What's happening at any one instant is not very important, unless you're a motor designer or building a power plant.

#21
12-31-08, 09:14 PM
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Originally Posted by John Nelson
The black wire is not "positive" and the white wire is not "negative". There is no positive wire. Both the black and white wires are positive 60 times a second and negative 60 times a second.
Why do we have a hot and a neutral for our pair of wires? I mean, why do we give the two wires different names if they are doing the same thing then? Both wires bring in voltage and current, one of them when it's positive and then the other when it's negative voltage. I mean, when it's pushing, it might start with the black, and then when it's pulling, it might start with the white. In the form of where the first electron actually starts moving from.

Or is that it? The hot is the one that does all the pushing and pulling and the neutral is just reacting to it.

#22
12-31-08, 09:20 PM
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Yes, they are doing exactly the same thing. But we give them names nevertheless. We humans like to name things so that we can talk about them.

Even though they do exactly the same thing, they are not exactly the same. The "hot" wire has voltage relative to ground. The "neutral" wire doesn't (most of the time). The only reason this makes any difference is that we humans spend most of our time in contact with the ground. So the effect is different on us when we touch the wires. If you touch the hot wire, it will hurt a lot more than if you touch the neutral wire.

If you spent all your time floating in the air touching nothing, then neither wire could hurt you (as long as you only touched one of them at a time). That's why birds can sit on electrical wires without harm.

Finally, if you spent all your time in contact with the hot wire and not in contact with ground, then it would be the neutral wire that could hurt you and touching a hot wire would cause you no harm.

Everything is "relative". In fact, voltage is defined as a "relative" difference in electrical potential.

Let me compare this with gravity again. Normally, if you are on top of a high building, you can get hurt if you fall to the ground. But if you are already are on the ground, you cannot get hurt by falling--unless you fall down a mine shaft. So it's all "relative"--where you are vs. where you might fall to.

#23
12-31-08, 10:12 PM
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Originally Posted by John Nelson
That's why birds can sit on electrical wires without harm.
Finally, if you spent all your time in contact with the hot wire and not in contact with ground, then it would be the neutral wire that could hurt you and touching a hot wire would cause you no harm.
I wondered about this before. A bird touches a wire with both legs which made me think that they would be creating a short circuit going through them. I think I see now how you're saying that since the bird is not grounded, that there is no difference of potential between it's two legs and that is why it doesn't get electrocuted?

#24
01-01-09, 03:06 AM
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[QUOTE=craftsman_50;1492540]Just a question to let us know what level we need to start on.How much formal training do you have in electrical/electronics ?If your still in school let us know what classes you have completed.We can not teach you to run if you have not learned to walk yet.
Please answer the question.It's starting to become apparent that you have no knowledge of "electrical theory" at all.

#25
01-01-09, 03:20 AM
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Originally Posted by foolios
While learning that voltage is lost to resistance and length of wire, I came up with a question. I noticed that a length of 50' 12AWG wire could cause a 2% voltage drop.
Does this voltage drop start from the beginning of the wire or at the end of the wire?
Also, does the current that passes through a wire and a device like a lamp on the wire still have the same amount of amps from beginning to end? Is the amps used by the device? What actually powers the device if no electrons are lost?

Here's the absolute answer to your original question. A POWER SOURCE provides energy. If you study OHM's LAW it will give you the anwer to the rest of the original post.

#26
01-01-09, 06:35 AM
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I have just started an apprenticeship program. I start taking courses at the beginning of the year.
I have no previous experience in electrical work. I am currently reading every article on the net that I can find.
I have read Electricity Demystified, and Shaum's Basic Electricity. They do discuss Ohm's Law. But they do not answer all the questions I have. I have asked some questions from experienced electricians, and they don't have the answers to some of them. I was hoping I could ask here. I really want to learn.
Thanks for getting my head around some walls, i really felt like I was doing this

#27
01-01-09, 03:06 PM
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Originally Posted by foolios
I have just started an apprenticeship program. I start taking courses at the beginning of the year.
Congratulations and best of luck to you! I think you'll do great.

(FYI my experience is limited to minor electrical projects at home and some simple electronics).

#28
01-01-09, 03:25 PM
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foolios...just an opinion here, take it for what it's worth. I taught electronics repair and other things in the Navy for many years off and on.

You have classes ahead of you I'm sure, learn what THEY want you to learn, then explore on your own after the class is over. Most classes have criteria you must meet. If the instructor doesn't understand, comprehend, or agree with your answer (no matter how complicated and correct it may be), you lose. Thats a shame, but it is reality.

Thirst for learning is all well and good, but when its appropriate.

I could tell you about several times when I got slammed for knowing more than the person at the front, but it won't make any difference in the long run.

As I said..just my opinion, for what it's worth.

#29
01-01-09, 03:28 PM
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I'll take your advice, I was already beginning to feel overwhelmed trying to get ahead. There is a lot just to know, not to mention how much effort it will take to understand that knowing.
Those were some serious words of wisdom that just helped me to calm down a lil'.

THanks

#30
01-01-09, 09:03 PM
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I'm not sure how it is down south., but through my apprenticeship here in canada, the order of learning was like this. In first year you'll learn DC fundementals. Second year you'll learn SIngle Phase AC fundementals. Third year you'll learn Three Phase AC, motors, alternators ect., and 4th year will be all over the place from high voltage to PLC's to computers, ect. All 4 terms you'll do some power electronics, motor controls and some code as well. Now, these guys are right. Concentrate on what's on your plate at the moment. You aren't going to learn it all on your own, and if you try to, it'll be to your detriment. Don't breeze over dc ccts and try and take on 3phase ac because you'll certainly fail. these guys are right again, get to know ohms law and take your time. that's my advise. sid

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