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Lots of electricity confusion

  1. Mar 8, 2005 #1
    is there any site out there with decent complete explanations of how electricity (voltage, current, resistance) behaves in series and parallel circuits? if not... can soemone answer a few qs:

    1) is voltage constant in a series circuit? theres always the same amount of voltage "pushing" electrons through the circuit right? then why do voltmeters give different readings all over the place?

    2) what is a voltage drop? voltage doesn't actually fluctuate does it (series circuit)? Is it the minimum amount of voltage req'd to push the charge thru the resistance?

    3) why is voltage the same across a parallel circuit, that is why does the "voltage drop" stay the same as the source?

    TiA
     
  2. jcsd
  3. Mar 8, 2005 #2

    xanthym

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  4. Mar 8, 2005 #3

    chroot

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    No. Whenever current flows through a resistance, there is an inevitable drop in potential across that resistance. This is Ohm's law: V = I R.
    If you have two equal resistances connected to a battery which produces a potential V, the potential in the middle of the two resistances is V/2. Again, you cannot have current flowing through a resistance without losing potential across it.

    You can make an analogy with water hoses and sponges, if you'd like. If the city water supplies a certain pressure in your water hose, the hose will deliver a certain flow. If you shove a sponge into the hose, the current will drop dramatically -- but so will the pressure at the end of the hose.
    Look at the wires. Every point in a wire is defined to have the same potential, because ideal wires have no resistance. The wire leaves the positive terminal of the battery at some potential, and is connected to the ends of two or more resistances, with nothing in between -- just wire. The wire has the same potential everywhere, so all of the resistances are exposed to the same potential.

    - Warren
     
  5. Mar 8, 2005 #4
    for the second question then, why doesn't the emf become zero after the last resistor and current stop altogether? If I have a circuit with a source that gives 12v of potential difference with one load, that load uses all 12v right?
     
  6. Mar 8, 2005 #5

    chroot

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    The ideal wire has no resistance, so it takes no voltage to push electrons around inside it. Of course, real wires have very small (non-zero) resistance, but it's negligible.

    - Warren
     
  7. Mar 8, 2005 #6
    if current is the same throughout a series circuit then the voltage has to constantly push to keep the current flowing at a constant rate right? so then is it right to say no voltage (energy to push electrons) is used when electrons are moving through the wire (or at least that the amount used is negligible) , that only once they encounter the resistor(s) additional emf is required to push the electrons thru the resistor? how does voltage work exactly?... is there like an inital creation of potential difference and then the electrons start moving towards the other node with the defecit of electrons, then when they meet a resistor, additional potential difference is created so the current goes through at the same rate? why wouldn't some electrons be moving faster than others because not all of the electrons are flowing at one point in the circuit at once? isn't the potential difference created by the power source applied to the whole circuit all the time? sorry if im bothering you... thanks for the help so far
     
  8. Mar 8, 2005 #7

    chroot

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    Potential difference is not "energy," but your argument is roughly correct. An ideal wire has absolutely no resistance to electrical current, so it requires no force (emf) to push electrons around.
    Potential difference (voltage) is just another way of expressing an electric field. A battery produces an electric field all through the wires (and resistors) connected to it, and that electric field compels electrons everywhere inside the circuit to flow toward the positive terminal.
    The same number of electrons must be passing through any point of the series circuit as through any other -- that's what it means for current to be the same everywhere in a series circuit. The actual velocity of electrons, in moving from one battery terminal to another, is very slow -- on the order of a few centimeters per minute. The pressure that pushes those electrons, the electric field, propagates through the wire at nearly the speed of light.

    - Warren
     
  9. Mar 9, 2005 #8
    Thanks...
    I still want to know how the voltage acts when current flow meets a resistor... the electric field is compelling electrons to move to the +ve terminal... but there's a resistor in the way... so now what? I can imagine that current is moving constantly through the circuit but I cant picture how voltage acts.
     
  10. Mar 9, 2005 #9

    chroot

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    Potential difference drops proportionately with the resistance, as Ohm's law indicates:

    V = IR.

    - Warren
     
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