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Question on concept of voltage.

  1. Jan 22, 2010 #1
    WARNING: Sorry about babbling on, the question is probably really simple and all this text is pointless, so for those of you who don't like to read, my main question is bolded.

    Alright, so I can apply the concepts of voltage, that isn't my issue, but there is something about voltage that has been constantly bugging me.

    I always find 2 different definitions of voltage (both here & in textbooks/classes, and I prefer here since things tend to be explained in reasonable terms!), one that says that it is electric potential (which I understand), and one that says it is potential difference. These aren't the same thing though!

    So, I finally think I may have thought of an explanation for it, but I'm wondering if I'm wrong (which I probably am), so could anyone correct me if I'm wrong.

    Voltage IS simply electric potential correct? However, you can't just say that something has "so and so voltage"; it has to be relative to something else right? So then instead of saying potential difference or electric potential, would it be possible to define voltage as "the potential energy (or electric potential, whatever you prefer) that one charge has relative to something else (another charge or something)".?

    Side note: I apologize if this seems dumb, but I'm just wondering if I can define voltage that way since it encompasses both electric potential and potential difference (in my opinion), and makes things much simpler (in my opinion as well - Everyone has their own little things to help them learn)
     
  2. jcsd
  3. Jan 22, 2010 #2
    Yes, they are the same thing. When you look at E = -grad(V) = -grad(V + C), then yes it is arbitrary up to some constant. But reference is usually infinity where V = 0 (far away from any source).
     
  4. Jan 22, 2010 #3
    Thank you.

    I had previously understood voltage quite well, but now I have a teacher who felt the need to draw pictures of cliffs and relate voltage to height many times which only served to confuse me. I think he was just over simplifying it. Sometimes concepts can become warped if one tries to break it down to far. Or maybe I just don't like comparing 2 completely unrelated things, which is probably the case.
     
  5. Jan 22, 2010 #4
    It's one of those things that if you're a layperson that thinks of voltages in terms of AA, AAA, C, and 9 V, then you have a decent understanding. On the other hand, once you've studied enough science, you also have a decent understanding. It's the middle ground in going from the former to the latter that gives people problems.

    When you take the formal definition of electrostatic potential (E = -grad V), then you are strictly speaking about the "absolute" electric/electrostatic potential. I say "absolute" in quotes because, as kcdodd said, V at every point in space can be varied by a constant without changing the electric field.

    When you write something like, say, Ohm's law (V=IR), then you are strictly speaking about a potential difference. That is to say that a potential difference must exist across the resistor for a current to flow. In engineering parlance we say a voltage is dropped across the resistor. I suppose then more correct way of writing Ohm's law then would be "delta V = IR".

    Like I said, if you've never heard of electromagnetism or you've studied it suficiently, then you "just know." In the meantime, just keep going at it!
     
  6. Jan 22, 2010 #5
    Your teacher maybe have mentioned the words "conservative force" in relation to voltage. What this means is that if you pick two points and find the difference in voltage, it does not matter how a charge gets from one point to the other. The change in voltage (ie, change in energy) will always be the same. That's why you can have a big mess of wires, but the "voltage difference" between the two ends will still be the same. That's also why your teacher drew the picture of hills and valleys, to try and convey the idea of path independence.
     
  7. Jan 23, 2010 #6
    Hmmm, I see where you're going, but sadly he never mentioned "conservative force", and never talked about "path dependence". In fact, he used height and voltage interchangeably once or twice (like says a problem is "asking for the height... the voltage.". I think he might just be trying TOO hard to use an analogy that people would understand. He said that its an analogy and it tends to help most people understand voltage, which I can understand. he did admit that the analogy might be complete crap for some people, which I'm glad he did.

    I do see what you mean though.

    Wow, I think you hit the nail on the head.

    My dad is an electrician, so I've been surrounded by electricians and have done electrical work my entire life. I have an understanding of how things work. He understands how everything works, but I can guarantee he doesn't know the formulas and the complete explanations behind it (in fact, I have met few electricians who do). However, he can give you a perfectly valid explanation for some phenomenon in laymens terms.

    Like you said, the problem for me is NOT the concepts, it's more of the transition from layperson knowledge to scientific knowledge that is getting me. It's not that I'm confused, it's that my mind is all over the place trying to make the switch, and then I have stuff like the teacher and the textbook and other people giving me some knowledge that, at this point, just confuses me more. It's also probably because I'm someone who actually tries to learn, understand, and be able to apply the concepts to the best of my abilities, unlike most of the kids who just want to get the hell out of class (and are content with just the formulas).
     
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