Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Putting a ground in between circuit

  1. Jan 15, 2008 #1
    [SOLVED] Putting a ground in between circuit

    If i put a ground in between the two resisters that are in series of a circuit, will the current go through the ground wire and NOT the 2nd resister. Example..

    this is not a hwk question by the way. I just drew this right now on paint

    http://img247.imageshack.us/img247/503/45634000nf9.jpg [Broken]

    I didnt have time to test this out in my circuits lab yesterday but i was wondering what it will do

    A1 will have current measurement for sure

    but A2 and A3

    ?:S? what will be the readings like will A3 even have a reading or not..
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 15, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    No current will flow through A2 unless you also ground one of the sides of the battery.
    I assume that IS a battery? if it is a generic voltage source than the result will depends on whether or not the negative terminal is connected to ground.

    Ground is is REFERENCE POINT in a circuit, nothing more.
    Current doesn't go to ground unless it can flow in a loop (e.g. flowing back to the negative terminal of a grounded source).
  4. Jan 15, 2008 #3
    so if the ground was also connected to the negative terminal then all of the current would go through the ground wire?
  5. Jan 15, 2008 #4


    User Avatar

    Staff: Mentor

    No current will flow though the ground connection as you have drawn the circuit. Quiz question -- why not?

    BTW, this is still coursework, so I'll move it to the Homework Help forum.
  6. Jan 15, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper

    Yes if the negative terminal of the 'battery' was actually an earthed PSU then the current would flow through A2.
    You also have to be careful when using an oscilloscope since the clips on the probes are earthed. It's a common question in electronics labs how to measure the signal through R1/R2 with a scope without shorting out the circuit.
  7. Apr 15, 2008 #6

    Berkeman could you please answer the quiz? Is there a good theoretical or physical explanation why the current ''chooses'' not to travel towards the ground?
  8. Apr 15, 2008 #7


    User Avatar

    Staff: Mentor

    mgb answered the Quiz Question with his post:

    A single Earth ground connection anywhere in a floating circuit does nothing. If you make more than one connection in the circuit to Earth ground, then that forms a parallel path for currents to flow.
  9. Apr 15, 2008 #8
    Yes, I can understand why this is true. The quiz question has not been answered though. Imagine you are an electron in the circuit drawn above and as you travel you encounter two separate paths. One leads eventually to the ground while the other keeps you within the loop. Why do you choose the second?
    Please correct me if my question is not well-posed. Thank you.
  10. Apr 15, 2008 #9


    User Avatar

    Staff: Mentor

    Since there is only one Earth ground connection shown, call the voltage at that point 0V. The ammeters A1 and A2 are short circuits, so the points between A1, A2 and R2 and including that end of R1 on the other side of A1 are also at 0V (relative to Earth).

    The battery imposes a voltage potential across R1 and R2, with 0V in between the two resistors. The absolute voltages on the battery side of R1 and R2 are determined by the battery voltage and the resistor divider ratio, again with 0V being forced in between the two resistors.

    There is no "loop" to the Earth ground branch as drawn -- there is no return to another place in the circuit from Earth ground. Current has to flow in loops, so there is no loop for an electron to follow toward Earth ground when it is flowing between R1 and R2.
  11. Apr 16, 2008 #10
    So I see. No loop means no current. I guess what was puzzling me was the following statement:

    ''In theory, if current has a choice between a zero-resistance path and a finite resistance path, all the current takes the path of zero resistance; a path with zero resistance, however, is an idealization.''
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook