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Resistor is a current limiting element.?

  1. Aug 14, 2012 #1
    In a simple circuit having battery (6v) connected to two resistors (1K each) in series, 3mA would flow through the entire circuit.

    1) If resistor is a current limiting element, current following the first resistor should reduce its value (from 3mA to some other) before entering the another resistor right??

    2) If current reduces, then conservation of charges is violated right?

    3) As per ohm's law, V=IR. In the above circuit, voltage gets dropped (3V)across the first resistor. Ohm's law implies voltage varies linearly with current if resistance is constant. So this would mean drop in voltage across resistor should limit currewnt from 3mA to some other value right?

    if resistor is not a current limiter, then how does a resistor in screwdriver tester limits huge current to flow through our body?

    Thanks in advance
     
  2. jcsd
  3. Aug 14, 2012 #2

    NascentOxygen

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    Hi harsha2591! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Current does not diminish as it progresses through a circuit, it is the same at every point. Charge is conserved!! 3mA through each resistor, and 3v across each.
     
    Last edited by a moderator: May 6, 2017
  4. Aug 14, 2012 #3

    davenn

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    Yes, correct

    No, the overall resistance of the circuit ( that is the load across the battery/other PSU) sets the amount of current flowing in the circuit

    in your above circuit say there was just one 1k Ohm resistor then the current flowing in the circuit would be 6mA. You double the resistance and you will halve the current flowing. It doesnt matter if those two 1k resistors are separate ones or just a single 2 k resistor the current flowing in the circuit will be 3mA

    You mite have 10 resistors of different values in a circuit some in series some in parallel, you need to work out or measure the total value of resistance to find out what the current flowing will be

    Dave
     
  5. Aug 14, 2012 #4

    sophiecentaur

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    What you need to remember is that the situation as described in the above (and other circuit problems) is what happens after the system has settled down. On a long wire, some charge may, indeed, flow through the nearer components before the voltage pulse (at switch on) arrives at a further component and before it 'can know about' that distant resistor. But all this is not relevant to basic circuit theory, which starts to kick in within a few nanoseconds of switch on, by which time, all the components are interacting with each other and all the 'rules' start to apply, such as sharing of the Volts along a series chain.

    It's not like cars on a motorway, where a hold up, 20 miles away is reducing the flow of traffic there but not having any effect on the ones which are just joining it. Although, even in this case, the motorway would eventually jam up totally, given enough time, and the total number that could get onto the start section would be limited by the rate that they could exit at the other end.
     
  6. Aug 14, 2012 #5
    Resisters are current limiting things, not current dropping things.
    You connect 2K to 6V. The current is limited at 3mA. If you now hook 6K to 6V, current is further limited at 1mA. Thats it.
    If there were no resistance, <a direct short between battery terminals> then huge currents will flow (theoritically infinite), i.e. the current has no limit. But in practice, even if you short out the terminals, the resistance in the battery (internal resistance) and the shorting wire will limit the current within few Amperes.
     
  7. Aug 14, 2012 #6

    psparky

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    Kirchoffs current law.

    Current entering a node equals current leaving a node.

    Current leaving the source EQUALS current going back to the source.
     
  8. Aug 14, 2012 #7

    sophiecentaur

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    But that doesn't tell you what currents may be flowing in parts of the circuit in between. It's a simplification which works. Think of the current in a parallel resonant circuit, compared with the current into and out of the generator that's connected.
     
  9. Aug 14, 2012 #8

    psparky

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    Ok...well, once the circuit reaches steady state, the capacitor and inductor oscillate between eachother (the source sees and open circuit at the L and C) and all the current from the source goes thru the resistor and all the current returns to the source. Simplified....indeed.
     
  10. Aug 15, 2012 #9
    It depends on how you energize your system. If you use a current source, resistance might be called "voltage drop incrementing" element.
     
  11. Aug 15, 2012 #10

    sophiecentaur

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    All that K1 really says is that the sum of the current at a node is Zero. It doesn't, except by implication, say what you are saying - only if you can identify just two nodes, one each end of the power source.
     
  12. Aug 15, 2012 #11

    psparky

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    Great. We are in total agreement!
     
  13. Aug 15, 2012 #12

    sophiecentaur

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    This is bad. We'll have to find something to argue over soon!
     
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