# Basic Circuits Theory

Discussion in 'General Electronics Chat' started by jkele, Nov 19, 2010.

1. ### jkele Thread Starter New Member

Oct 17, 2010
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Interested in electrical stuff and am trying to learn about it myself (not the greatest idea i know, but its my hobby on the side, may take up a UNI course on it).

So voltage is the electrical force in a circuit, no voltage means no current flowing? Also, how can we create ground in an electrical circuit? I know it is 0V but how do we make 0V on a PCB?,

Given any circuit we assume that the wires have 0 resistance, then if we look at two points in a circuit in which there is no element connected in between them. Then wouldn't the voltage between these two points be 0 and if voltage is 0 then no current flows? Or should i be thinking of it as if the voltage between these two points is 0 then we have a short circuit therefore current flows. (This does not make sense to me as if V = 0, then I = 0 - no voltage to push electrons, no current).

In a short circuit the voltage - 0, then is current as large as you want or 0 (once again its this if no electrical force then no current thing which is causing me headaches). Also for an open circuit, if no current flows then why can we have a large voltage?

2. ### mbxs3 Senior Member

Oct 14, 2009
153
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I will try to answer what I think your questions are to the best of my inexperienced ability...I am sure a more knowledgeable person will come a long and correct me and clarify the things I can't explain as well.

First of all, yes, voltage has to be present in a circuit for current to flow.

As far as a ground goes, think of a ground as a "common point". So on a PCB, you would be creating a "common point" for the circuit.

The resistance of a wire in a circuit is usually negligible. Meaning, you wouldn't factor it into any calculations.

When you have a short circuit it any given circuit, usually this means that the wire is "shorted" to a ground before getting passed through a "load". This would cause the current to be extremely high and would be dependent on the amount of voltage. A short circuit is the type of thing that burns fuses, pops circuit breakers, or could start a fire.

In an open circuit, there is no voltage or current flowing through the circuit. There may be a voltage present on the power side of the circuit, but due to the "open", it is not getting to the load.

I hope this clarifies some of the things you were asking.

3. ### jkele Thread Starter New Member

Oct 17, 2010
10
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Do we have to connect the wire to ground for it to be shorted? I thought short just meant, no voltage drop across wire.

4. ### mbxs3 Senior Member

Oct 14, 2009
153
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A "short" is not something you desire to have in a circuit. A short can happen in many different ways in many different situations...for instance, you might accidentally solder across a component on a PCB...or a lead wire might accidentally come into contact with a chassis.

5. ### jkele Thread Starter New Member

Oct 17, 2010
10
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Thanks for that..

I can see that my text is not very good, i will put up a diagram better illustrating my question.

Oct 17, 2010
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7. ### mbohuntr Senior Member

Apr 6, 2009
431
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Try this approach... Current flow is what generates heat. Too much heat, and things melt... All wires contain some small resistance even though it's VERY small. Ohms law reads V= IR or volts = current x resistance. If you have say...12 volts, and 100 ohms of resistance, 12 volts/ 100 ohms = .12 amperes or 120 mA of current. Now... If we remove said resistance, and only have the tiny resistance in the wire...say .01 ohms, 12 volts/ .01 ohms equals 1200 amps, and your circuit is now a smoldering mess, and hopefully, you didn't get hurt...

Yes, short circuits are bad... and...no, you can't normally get that much current out of a supply, but it will generate enough heat to fry most circuits. I hope this helped.

After re reading your question, those two points on your drawing are (electrically) considered the same point because the amount of resistance in that tiny amount of wire is not able to provide enough resistance to change things. The rectangular box later in the circuit is (I think) representing a resistor which limits the total current so the circuit doesn't melt. If you make a direct path from source to ground without enough resistance, it is considered a short. One exception being that enough wire (very long) WILL add resistance to a circuit, and must be calculated into the design.

Last edited: Nov 20, 2010
8. ### jkele Thread Starter New Member

Oct 17, 2010
10
0
Thanks but it is still not so clear, if you look at the diagram again since it is a perfect wire, then the Voltage at A = Voltage at B (measured with respect to ground of course). So the difference between A and B is 0 right?

Isnt the chararestic of a short circuit a 0V voltage drop? Short circuits deliver a huge amount of current right? So the short circuit theory implies that a lot of current flows through there. However if voltage is defined as the electric force and we have a 0 electric force then shouldn't the electrons not move?

What is the fundamental thing that i have missed here?

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,508
1,272
The voltage is a potential difference, for example if we have on one end of a wire 300 electron missing and on the other end 500 electrons are missing.
Then we have the voltage between the two points.
Both these "points" are positive, but the first point (300 electrons missing) is negative with respect to the second point (500 missing electrons).
So everything is relative.
And if we connect these points with a conductive wire then current is star to flow. From more negative point to more positive point. And current will flow until the number of electrons on both ends will be equal (conservation of energy).

The term "short circuit" refers to the situation when we have voltage source and then we connect a "short" (zero resistance wire) to the voltage source terminal.
So we have a voltage but 0 resistance so from Ohm's law, the current will be infinitely great.

But if we have a close loop circuit and a voltage source then the current must flow becaues the voltage source.

V1 pushes out (repel) electrons from "negative" terminal and attracts electrons to "positive" terminal.
So current in close loop circuit will flow.
And know if we short one bulb by a short wire, current will still flow becaues voltage source repel/attract action and close loop circuit (Conservation of energy).

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10. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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Try a water flow analogy.

Voltage is the PRESSURE of the water supply. Current flow is dependent on the size of the water hose. The size of the water hose is similar to resistance. The smaller the hose, the higher the resistance.

So a hypodermic syringe filled with water would be a high voltage, very low amperage situation. While a fire hydrant with hose would be a lower voltage, but much higher amperage.

11. ### mbohuntr Senior Member

Apr 6, 2009
431
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If you measure the actual resistance of the wire from the source to each of these points, there would be an increase in resistance, but the difference in any practical circuit is so small as to not be measurable. So the drop in voltage if there is one would also not be measurable. If your talking about a mile long cable, then the resistance difference from one end of the cable to the other end is more important. Like jony 130 said, your measuring the "potential difference" and in that small length of wire, there isn't much.

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12. ### mbxs3 Senior Member

Oct 14, 2009
153
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Looking at your diagram, lets imply that the power source is a 12 volt battery. Therefore you would have 12 volts at point A in your diagram and you would also have 12 volts at point B in your diagram.

The way you have drawn your diagram doesn't depict a "short". Looks more like 2 points on a wire.