# The Basic of Electric Circuit

Discussion in 'Physics' started by anthonylauly, Jan 8, 2017.

1. ### anthonylauly Thread Starter New Member

Jan 7, 2017
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Hi, I was just a new member here who started to learn here, I was just reviewing my knowledge about electric circuit, so I decided to learn about Vol. I - Direct Current (DC) from the textbook provided by AAC. But I found difficulties to differentiate between voltage and potential difference, and how they to related to each other. Please anyone explain it in briefly, thank you

2. ### Papabravo Expert

Feb 24, 2006
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They are the same thing. You can only measure a voltage (potential difference) across some drvice in which current can flow. This device could be a battery, a resistor, an inductor, a capacitor, or a piece of wire. Ohm's law tells us that when the current through the device is zero, the voltage (potential difference) is also zero. Any non-zero current flow through the device will result in a non-zero voltage (potential difference) across the device.

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3. ### MrChips Moderator

Oct 2, 2009
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Voltage
Voltage difference
Voltage drop
Voltage across
Potential
Potential difference

They are all the same thing.
When you measure a voltage, it is always a voltage difference, or a voltage at a point with respect to a reference point.
In other words, it is always some form of VA - VB.

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4. ### wayneh Expert

Sep 9, 2010
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I disagree with that interpretation. High voltage is like a lake at high elevation. The potential for power is there whether there is a path for current or not. It's the current that varies with the resistance in Ohm's law. Infinite resistance - no path down the mountain - results in no current, but the potential is there and constant.

It's a distinction without a difference, I suppose, for the TS. As noted, all voltages are relative to a reference (like elevation above sea level, or above the bottom of the mountain).

5. ### Papabravo Expert

Feb 24, 2006
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Ignore for a moment that infinite resistance cannot exist. The attempt to measure it introduces a lower resistance which allows a current to flow.

6. ### WBahn Moderator

Mar 31, 2012
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Ever heard of a null voltage measurement?

There is no need for the possibility for current to flow for their to be a potential difference. Two completely different concepts. There only needs to be an electric field since voltage, by definition, is merely the path integral of the electric field from one point to another.

7. ### WBahn Moderator

Mar 31, 2012
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We often talk of the voltage at a point or the voltage between two points. Both are really the same thing with the caveat that when we talk about the voltage at a point we are really talking about the voltage difference between that point and some agreed upon reference point (that we usually call "ground" or "common").

The word "potential" and the word "voltage" are pretty much synonymous.

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8. ### Papabravo Expert

Feb 24, 2006
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Nope -- should I have heard of this?
Maybe I should ask the U of M for my degree money back.

9. ### WBahn Moderator

Mar 31, 2012
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Perhaps. It is generally one of the early concepts presented in a Physics II course.

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10. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I disagree for those cases when ohms law does not apply, such as when I have a fresh battery sitting on a shelf. The current thru it may be zero but the voltage across it is far from zero.

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11. ### Papabravo Expert

Feb 24, 2006
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It is hard to argue that you have a circuit in that case. I agree that Ohm's law does not apply when there is no circuit.

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12. ### WBahn Moderator

Mar 31, 2012
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Well, is this a circuit?

If R1 and R2 are chosen so that there is no current in the ammeter, does that mean that V1 can have no voltage across it? If that isn't the case, then how do you reconcile that with your claim that, "Ohm's law tells us that when the current through the device is zero, the voltage (potential difference) is also zero."?

Jan 7, 2017
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Jan 7, 2017
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Jan 7, 2017
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16. ### MrAl Distinguished Member

Jun 17, 2014
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Hi there,

Something does not sound right here and i think that is why others have disagreed with this. I have a feeling you just did not word it the way you really intended to. The reason i say this is because you mention a battery as an allowed device in the discussion, yet you assert that it can not have a voltage across it unless it also has current flow through it. I agree that a resistor has to have a current through it in order to show a voltage across it, but a battery or capacitor can have voltage across it with no current through it.

This is so obvious that i am sure you just wrote it a little too fast and if you reworded it that would clear this up quick

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17. ### Papabravo Expert

Feb 24, 2006
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I was not thinking of components like a battery in isolation, which are incapable of making a circuit. I did however not appreciate the circuit with two two loops and no current flowing in one of the loops like a a whetstone bridge or the circuit in post #12. I was also not familiar with the term "null voltage measurement". I'm writing my letter to the U of M asking for a partial refund of all those tuition dollars with interest for the last 47 years.

18. ### WBahn Moderator

Mar 31, 2012
18,087
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Just imagine if you could actually get it -- now THAT would have been a good investment! Well, not as good as investing in the market, but not bad, either.

19. ### MrAl Distinguished Member

Jun 17, 2014
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515
Hello again,

I think i see what is happening here and why there is a slight misunderstanding.

First, there's no way they did not teach nulling techniques, which are the basis for all measurements, although they might not have used that specific phrase. Nulling techniques, as far as i know, originated more than 5000 years ago with the invention of the balance scale. When the weight on both sides is equal the two weights must be the same. The electronic version is the balanced bridge and in more recent years, the op amp.

But anyway, i think what is happening is the way we are interpreting the word, "circuit".

It must be obvious that a balanced bridge, in balance, is still a circuit even though there is no current flowing through the central measurement device which BTW can be either a voltage measuring device or a current measuring device. Even if that element had zero current we'd still call it a circuit. To get really picky though we could probably say that it is impossible to get a true zero current because we cant measure zero current because we cant measure zero anything without taking extreme measures, we can only know when the measurement equipment tells us it is close to zero, and that could mean very close to zero. Using an analog meter means there will always be some sticking friction if not anything else, which will prevent a perfect reading.

But what about a "circuit" that is not turned on yet? We often call that a circuit too even though it has no energy yet.

So maybe this is just another one of those semantics issues. Depending on what we want to call a circuit the meaning could be slightly different.

20. ### Papabravo Expert

Feb 24, 2006
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Maybe they did and I just forgot. I'll let deeper thinkers than myself determine the number of angels that can dance on the head of a pin.