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Some DC Circuit quick questions

  1. Jun 2, 2004 #1
    i just finished reading the circuit chapter in my book hoping it would answer a few of the questions i had in mind, it helped, but not as much as i had hoped...here are a few questions

    how quickly is a current established throughout a circuit when the switch is closed? (now i suppose the answer would be instantly, but im not positive)

    what factors influence the resistance to a flow of a current? (would it be whether its in series or parallel?...alittle confused here) and how does that compare with the resistance to waterflow in a hydraulic systtem? (no clue on this one)


    -thanks ahead of time for any responses
     
  2. jcsd
  3. Jun 2, 2004 #2

    chroot

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    Electric fields propagate through wires generally around 3/4 of the speed of light. The exact speed is dependent on the dieletric medium surrounding the wire. The current 'begins' at a spot in the wire as soon as the electric field is non-zero there.

    The flow of current depends upon resistance. When you put resistors inline in series, you increase the total resistance, and decrease current. When you put resistors in parallel, you offer more paths for the current to flow, and thus decrease total resistance and increase current.

    BTW if these are homework problems, this is not the appropriate forum, nor the appropriate way to ask them.

    - Warren
     
  4. Jun 3, 2004 #3

    Gza

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    not to be a pain in the neck, but charge flows, not current. :wink:
     
  5. Jun 3, 2004 #4
    Perhaps, but "current flow" is the standard terminology used to describe the movement of electrons in a conventional electrical circuit such as house wiring, lamps, etc...
     
  6. Jun 7, 2004 #5
    Total Circuit Resistance Rt

    Total circuit resistance in a DC circuit is irrelevant of any combination or groupings of series, parallel, or complex series-parallel branches. Rather, it is the actual total resistance of the circuit that determines the total resistance. That sounds very blunt and straightforward but that’s the truth.

    In other words: It doesn't matter if you have 10 parallel and 5 series in your circuit. Its the resistance of those circuits that make it count.

    The total resistance of each branch is calculated:

    Total Resistance in Series is the sum of all the resistance for each component. R1 + R2 + R3

    Total Resistance in a Parallel circuit is each components resistance’s reciprocal summed. 1/R1 + 1/R2 + 1/R3 ...

    The statement:
    "When you put resistors inline in series, you increase the total resistance, and decrease current. When you put resistors in parallel, you offer more paths for the current to flow, and thus decrease total resistance and increase current."

    Is utterly wrong and misleading. You do not inherently increase/decrease current because of the type of branch. Ohm's Law decides that. If a parallel branch has higher resistance, then yes, it will have less current; but only if.

    I would suggest if you are learning anything about electricity, to memorize Ohm's Law. There are other laws you will need to learn later when you study parallel circuits and complex series-parallel circuits. Such as Kerchiefs Law.
     
  7. Jun 7, 2004 #6

    Gokul43201

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    Yes there are similiarities between "current flow" and viscous flow in a pipe.

    Ohm's Law for current : V = IR or I = V/R ,where I : current, V : Voltage difference between ends of conductor, R : resistance of the conductor

    Fluid Flow : Q = P/Z , where Q : flow rate, P : pressure difference across ends of pipe, Z : impedance of the pipe

    Nature of R : R = const*L/A , where L : conductor length, A : cross section area

    Nature of Z : Z = contant*L/A^n , where L : length of straight pipe, A : cross section area , n : typically between 2 and 4 depending on whether the flow is laminar or turbulent.

    Series and Parallel Conductors : R = R1 + R2 + R3 +... (Series)
    1/R = 1/R1 + 1/R2 + ... (Parallel)

    Series and Parallel Pipes : Z = Z1 + Z2 + Z3 + ... (Series)
    1/Z = 1/Z1 + 1/Z2 + ... (Parallel)

    These similarities often make it useful to treat hydraulic systems like circuits and use "circuit analysis" to figure out different quantities at various points in the system.
     
  8. Jun 7, 2004 #7

    chroot

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    No, actually, it isn't wrong or misleading. Ohm's law says that when resistance increases, current decreases, and vice versa. This is precisely what I said.

    - Warren
     
  9. Jun 8, 2004 #8
    Is Kercheif's first name Hans? :)
     
  10. Jun 8, 2004 #9
    RE: "Total Resistance in a Parallel circuit is each components resistance’s reciprocal summed. 1/R1 + 1/R2 + 1/R3 ..."

    Hmmmm... so the total resistance of a parallel circuit is measured in mho's? :)
     
  11. Jun 8, 2004 #10
    In my opinion the waterfall analogy is off a little. I instead envision a stream of ball bearings, each in contact with each and free to roll. If this stream stretches across an incline, then this is analagous to electrons sensing a potential difference between two points. The balls will roll, which means a current is developed.

    But if you like the waterfall analogy, consider what happens in a stream if you dredge out another channel for the water to flow. The amount of water that can pass a certain point per unit time increases. In this sense, we say that the total resistance drops.

    I think the problem with our understanding of resistance is that we associate it too closely with regular friction. Instead consider the resistance in terms of the current. When the current increases, we say that the resistance decreases. Clearing another path for the electrons to travel (that is, attaching another resistance in parallel to the original circuit) allows us to pass more electrons by a point per unit time. In this sense the fact that the overall resistance falls makes a little more sense.
     
  12. Jun 8, 2004 #11
    :biggrin:
    Perhaps Perseus is utterly wrong and misleading? :rofl:
     
  13. Jun 8, 2004 #12
    Very funny. The math is right the wording is wrong.

    :grumpy:

    At least I admit that I am wrong. Funny no one will stand up to an Admin. Im sure he doesn't mind being corrected. This is a professional forum of adults. Constructive critisim is what this forum is all about.
     
    Last edited: Jun 8, 2004
  14. Jun 8, 2004 #13
    However that doesn't take away that chRoot was misleading:

    I will state it again. Just by him saying that by putting a circuit in series or parallel doesn't mean anything to the total circuit resistance without taking into context the resitances of each branch. Omh's law states nothing about when you place somthing in series or parallel that it has, by default, lower or higher resistance.

    But anyways. Im sure we both know what we are talking about we are just saying in it two different ways. Besides why does it really matter as long as the person understands that the design of the circuit doesn't matter so much as the total circuit resistance does when thinking about total circuit resistance. If one was considering why the total circuit resistance was at X ammount, then one would be more apt to look at the design of that circuit; parallel, series, etc.

    Geeze, lets just relax and have a cold one.
     
  15. Jun 8, 2004 #14

    chroot

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    Take your own advice, champ. Also, please recognize that no one, least of all me, ever claimed that resistors magically change resistance when moved from one place in a circuit to another!

    - Warren
     
  16. Jun 8, 2004 #15
    thanks everybody, i didnt expect to get overwhelmed with this many replies
     
  17. Jun 8, 2004 #16
    Perseus,

    Words mean things. A misleading statement can really confuse someone, so I think we should always point out situations where a description is incorrect, no matter how slightly.

    I certainly didn't intend to look like I was attacking you. But keep in mind that you took on chroot in a fairly aggressive manner, so I don't think you can really complain.

    I am not taking on chroot because I think his statements are accurate. When we consider the overall resistance between resistors when placed in series as opposed to parallel, we assume that the resistors have the same corresponding values between the two situations. Otherwise it would be nearly impossible to communicate without applying burdensome qualifiers to every statement.

    I agree that we should all relax.
     
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