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Question about constructing Circuits

  1. Feb 3, 2010 #1
    A battery doesn't work if you connect it negative to negative correct? It's not a proper circuit.

    Well, in Physics today we we making circuits blah blah, and instead of using a power supply today, we just used something of a battery pack (not sure exactly what it's called, but you put in some batteries , and the thing that you put them in has two wires that come out of a different part of the "pack", which I assume are connected to something else. I haven't seen inside it so I'm not sure). Well, point being is that if I connect those two wires to a Volt meter or Ammeter or whatnot, it doesn't matter what wire I connect to the positive terminal of the Ammeter. Why?

    Is it just because that I'm connecting a wire to the Ammeter? If I somehow connected the negative side of a battery to the ammeter, then the positive terminal of the Ammeter to a resistor, and then the resistor to the positive side of a battery, would it work??

    Basically, I'm wondering how come I can't connect a battery negative to negative, but when I connected the wires to the Ammeter I didn't have to worry about negative/positive...

    P.S sorry that it's worded really badly, I don't really know how to describe the "battery pack".
     
  2. jcsd
  3. Feb 3, 2010 #2

    turin

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    I assume that your ammeter measures "positive current" just as well as "negative currect". In that case, if you connect the terminals in the opposite order, then the ammeter should (ideally) tell you the same magnitude of current, but with the opposite direction (sign). I suppose you can consider four "proper" ways to hook up your circuit:

    batt(+) ----- amm(+)
    amm(-) ----- res ----- batt(-)

    batt(+) ----- res ----- amm(+)
    amm(-) ----- batt(-)

    batt(+) ----- amm(-)
    amm(+) ----- res ----- batt(-)

    batt(+) ----- res ----- amm(-)
    amm(+) ----- batt(-)

    The "-----" represent connections, "batt()" and "amm()" represent battery and ammeter terminals respectively (with the sign of the terminal in the parenthesis), and "res" represents the resistor (with two terminals). I suppose that you could argue that the first two configurations are the truly proper ones, and the last two configurations are "backwards". It depends on the ammeter, but most of them will just give you the same value of current with the opposite sign if you connect them "backwards".

    One word of caution, though. There are circuits in which the polarity of the terminals can be of the utmost importance. For example, when jumping a car battery, there is only one proper way to make the connections.
     
  4. Feb 4, 2010 #3
    Thank you for the diagram, it was helpful! And yup, when i reversed the connections it was reading negative.

    I think that I'm just hellbent on making sure I connect terminals correctly, since most of my exposure to circuits its just layman's electric & mechanical work, where I've always been told to do something a certain way if I don't want to end up in the hospital.

    One more question then: If you had to guess, would you say there are more objects that, if connected to a circuit, have terminals in which the polarity does matter, or more objects where the polarity doesn't matter?
     
  5. Feb 4, 2010 #4
    Hmm I'd probably say there are more elements that depend on the direction of current flow.
     
  6. Feb 4, 2010 #5

    turin

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    Homework Helper

    That is a really difficult question to answer. I would say that, since your low-level physics course is probably not designed to expose you to nonlinear circuit elements and such, it probably doesn't matter. But, in a complex real-world application it does. Actually, both the polarity, and even the actual procedure and order in which you make electrical connections can be a matter of life or death in the real world.

    Anything from cell phones to your car all contain thousands of transistors, each with at least three very different kinds of terminals. Your computer has millions. So, if you count each one of these transistors as an object, then the number far outweighs objects in which it doesn't matter.

    I think that you have the right attitude towards trying to understand which way to connect the circuit. It just turns out that, probably in your physics course you will not encounter a circuit in which the order and direction matters.
     
    Last edited: Feb 4, 2010
  7. Feb 4, 2010 #6

    sophiecentaur

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    Science Advisor
    Gold Member

    "end up in hospital" eh?
    Is he the violent type, your teacher?
     
  8. Feb 5, 2010 #7
    haha nooooo! I meant that I have done electrical work (quite often actually), mechanical work, some carpentry and some other stuff and I've always been told to do everything a certain way, and the people I've worked with have always seemed to feel the need to remind me that if I don't do everything correctly I'm liable to get hurt. Because of that I'm very careful about what I do, so when my teacher told me that the way I connected the terminals didn't matter, i was quite skeptical.

    you made me laugh though xD
     
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