New guy with Voltage/Current question

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Old 10-02-13, 07:47 AM
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New guy with Voltage/Current question

I'm a novice, know enough to get stuff done round the house (or shoot my foot off sometimes).

Question: One thing that has always escaped my ability to get under, is the "correct" (for lack of a better word) relativity between voltage and current. I know Ohms law but .........

I.e. I realize voltage is pressure (or Potential) and current is the amount (or speed?) of electron flow thru a conductor and load.

But when I look at the base level explanations of electron charge "build up" at a point, I get this gut revelation that voltage is nothing more than a higher amount of electrons at a given point over the conductor path (?). So the more electron build up you have, the more voltage you would have. But obviously I'm still missing the whole picture since on an induction load you can have the voltage (potential electron build up) become "less" and the amps (amount of electron flow) become more.

This blows me back to square one, since there is "more" electron flow coming from a decreased voltage (electron build up ?). From reading on induction, I get faint revelations that "more" voltage is being transformed across the motor coils (voltage that did not come from the line feed) so I'm guessing this maybe is how more current would still apply to my gut feeling. But I don't completely understand it enough to be sure. Please try to illuminate my ignorance with descriptions or good study links that are specific to this question.

If my confusion has come across ambiguous please say so and I will attempt to word more constrictive.
 
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Old 10-02-13, 11:30 AM
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Welcome to the forums!

Drawing an analogy between a residential electrical system and a residential plumbing system is pretty straightforward, and can aid in understanding how the components in each system need to be installed and connected:

Cold water supply pipe = ungrounded conductor, or "hot wire;"
Regular drain = grounded conductor, or "neutral wire;"
Overflow drain = equipment grounding conductor. or EGC or "ground wire."

Trying to draw a parallel analogy between the behavior of water in the plumbing system and electricity in the electrical system doesn't work as well, though. Yes, we can say that voltage is roughly equivalent to water pressure and an ampere is roughly equivalent to an ounce of water and a watt is roughly equivalent to a tablespoon of water. The reason this analogy doesn't work well, though, is that no mass is moved when an electrical circuit is closed and the electricity is performing work. When you say that
voltage is pressure (or Potential) and current is the amount (or speed?) of electron flow thru a conductor and load.
you're not seeing one of the traps in the electricity-to-water analogy.

Here's the trap: The analogy is too inexact to work because water is a physical substance and electricity is a force. When you open your main cutoff and let water into your supply pipes, they fill with water. When you close your main breaker and energize your panel, nothing moves. The conductors acquire electrical potential - they become energized - but the number of electrons in them is the same as it was before you closed the breaker. When you open a tap and use water, that water leaves the pipe. Mass is moved. But when you plug in a hair dryer or close a light switch and use electricity, nothing physical leaves the conductor. No mass is moved. The number of electrons in the conductors is the same as it was before you closed the circuit by adding a load..

The electrons in an electrical conductor may dance a bit when electricity is being used but they don't go anywhere. What "flows" in an electrical system is the potential - the force or energy.
 
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Old 10-02-13, 03:56 PM
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Thanks for the reply, I might have aimed my question in the wrong direction by using the term "house". Actually most of my troubleshooting is on the car and motorcycle (in the garage). So it's really DC current that I'm mostly using. However you're analogy will still apply, and while it is informative it does not really help me much on understanding how the current can go up on (say a starter circuit) when a low voltage occurs. Obviously something is happening that is in addition to the ohms law of E=I*R. <~edited typo
Let's just say that a starter circuit is pulling 40amps under load. The inital voltage is 12.x volts but lets say it's 12 even. That gives a load resistance of 0.3 ohms per ohms law (trying to stay away from impedance right now).

Now if (for some reason) the line voltage to the starter (while under load) drops to 9v.
By straight ohms law the current would now be 30amps (lower not higher) ?

I suspect what I'm missing is the lower voltage somehow changes the resistance, possibly indirectly like as the lower voltage causes a slower armature revolution and this "approaches" a locked roto state and decreases resistance thereby raising current flow ?????.

Appreciate any input
 
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Old 10-02-13, 05:33 PM
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I suspect what I'm missing is the lower voltage somehow changes the resistance, possibly indirectly like as the lower voltage causes a slower armature revolution and this "approaches" a locked roto state and decreases resistance thereby raising current flow ?????.
I don't think that's it. I think that what you may be missing is that the load controls in an electrical system.

So we'll say that you have a starter that draws 40A and a battery that supplies 12V DC and unlimited A/hr. So that's 480W of consumption when you're cranking the starter. And let's the output from the battery drops to 9V DC. (It wouldn't, of course, since it's the amps, or milli-amps, that get drawn down, but we'll say it does.)

In order to supply the 480W draw that is the starter load, a 9V battery has to supply 53.33A. Wouldn't work, of course, which is why batteries are built and rated for constant voltage, but it's an example.
 
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Old 10-03-13, 05:41 AM
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R_W_B, resistance isn't important to the answer you want. Since you know the load is 40 amps at 12 volts, resistance is assumed to be constant.

Use the power formula P=IE to calculate the change in amps when voltage changes.
P=Watts
To illustrate what Nashkat1 said above:
IE=P
12 x 40 = 480 watts
480 / 9 = 53.3 amps
 
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Old 10-03-13, 05:45 PM
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Ok thanks for the replies. I have two questions at this point.
1. How can I change my forum settings so that the thread replies/posts show at bottom? On mine they show at top and I can't seem to find a setting to change this.

2. If I wanted to buy a 115v AC to 12.Xv DC transformer/power generator(?) (hopefully I vary the voltage) so I could mount it on plywood and wire experimental circuits to it, where would I go to buy such a thing? And what exactly would I ask for?

At this point I feel this would server me better than a small bread board setup.
 
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Old 10-03-13, 07:06 PM
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If I wanted to buy a 115v AC to 12.Xv DC transformer
Radio Shack or almost any old device with a wall-wart power supply (the little black box that sticks into the outlet). You can get variable power supplies, but to start off, the cheaper option should work just fine.

This won't run many automobile devices, if you want to play around with alternators, starter motors, etc, you'll need a much beefier power supply. You'll also want to take more precautions as a few amps at 12vdc can still give a good shock.

Good luck!
 
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Old 10-03-13, 07:13 PM
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Thanks, I will go there and ask around. I just want to experiment with voltage drop scenarios on normal resistance load circuits now (no motors yet). When I do experiment with the other type loads I will probably just hook a battery up to it and beef up the wire and put in a fuse.
 
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Old 10-03-13, 08:10 PM
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To answer your first question:

At the top of the page put your cursor on Forum Actions
In the drop down box choose General Settings
Scroll down to Thread Display Mode
Choose Linear-Oldest First
Scroll to bottom of page and click on Save Changes
 
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Old 10-04-13, 05:23 AM
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Thank you, I am reading at the bottom now. In retrospect seems I should have spotted that setting as I looked there previously, but something appeared different to me than the forums I'm used to seeing. I thank you for your patience.
 
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