Why isn't electricity flowing through this wire?

  1. Mar 18, 2013 #1
    [​IMG]

    In the top circuit, electricity doesn't flow through section 2, but it does in section 1.
    In the bottom circuit however, electricity flows through both sections 3 and 4. Why does
    simply adding a resistor stop the flow of electricity in 2?

    Much appreciated!
     
  2. jcsd
  3. Mar 18, 2013 #2
    current will flow through the easiest path ( without resistance) or with less resistance and as the wire doesn't have any resistance, all the current will flow through it.
     
  4. Mar 18, 2013 #3
    In the top circuit, if I add a resistor to section 1 that has less resistance than section 2, electricity will still flow through both, won't it?
     
  5. Mar 18, 2013 #4
    yeh it will, but the one with more resistance will have less current flowing through it.
     
  6. Mar 18, 2013 #5
    SO, why isn't there just less current (instead of zero current) in section 2 compared to section 1 since it has more resistance?

    Thanks for the help by the way, I greatly appreciate it.
     
  7. Mar 18, 2013 #6
    it's just how it is, because current has to flow through both of the sections and that most current will flow through the section with less resistance.
     
  8. Mar 18, 2013 #7
    im understanding it now. Thank you very much sir.
     
  9. Mar 18, 2013 #8
    glad you understood it, your welcome.
     
  10. Mar 18, 2013 #9
    You are right, there is SOME current in section 2. The ratio of the currents is the same as the ratio of the resistances; 10cm of wire might have a resistance of 0.006Ω so if R3 is 100Ω and 0.1A is flowing through the circuit, 0.1A x 0.006 / 100 = 6μA will be flowing through R3.
     
  11. Mar 18, 2013 #10

    davenn

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    yup, but the main point is that the resistor in section 2 is effectively being shortcircuited

    Dave
     
  12. Mar 18, 2013 #11

    russ_watters

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    Staff: Mentor

    The problem here is one of starting assumptions. If the assumption of the problem is that there is no resistance in the wires (a common assumption), then all of the current has to go through the short. But in real life, there is a finite resistance even in a wire, so a certain amount of current will flow through each branch.
     
  13. Mar 18, 2013 #12
    If you measure the voltage at 2 points on a wire without resistance you get the same result. By short circuiting the resistor in section 2 you are making both ends the same voltage.

    The current in an element like a resistor obeys the following:

    [Voltage (one end) - Voltage (other end) ] / resistance = current

    So as you can see if the voltages are the same you get zero current.
     
  14. Mar 19, 2013 #13
    Look up "Current Divider" on Wikipedia. It will explain what is going on quantitatively. If you have a mind for math, this may help you understand better.
     
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