1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Current flow

  1. Oct 29, 2011 #1
    If you had a negatively charged plate connected to a wire that had a resistor and on the other side of the resistor was a ground connection would electrons not flow?

    I tried this at home with one side of a battery and couldn't measure any current on my voltmeter.
     
  2. jcsd
  3. Oct 29, 2011 #2

    gneill

    User Avatar

    Staff: Mentor

    No continuous current will flow unless there is a circuit -- literally a closed path, from the positive battery terminal to the negative battery terminal. Any initial (very) brief current that flows when the wire is first connected to ground would be due to static electric potential difference between the battery as a whole and ground. This is akin to rubbing your feet across a carpet and touching a conductive object at a different static potential (such as a doorknob or your annoying brother). Note that the battery, taken as a whole, is a net neutral object unless you put a static charge on it.
     
  4. Oct 29, 2011 #3
    So it must be a closed path for a battery but what if I had just a negatively charged plate(no battery) connected in the same way. Shouldn't current flow to ground through the resistor until the plate is nuetral correct?
     
  5. Oct 29, 2011 #4

    gneill

    User Avatar

    Staff: Mentor

    Current would flow until the potentials equalize, yes. It would have to be an enormous plate for the current to last more than a brief instant (think "spark").
     
  6. Oct 29, 2011 #5
    Wouldn't the resistor prevent the charge from distributing instantly? If it was all conducting and 0 resistance I would agree...
     
  7. Oct 29, 2011 #6

    gneill

    User Avatar

    Staff: Mentor

    Sure. But for a realistically-sized plate and the amount of charge you can put on it without it leaking off into the air by corona discharge, even a really large resistance (practically an insulator) won't give you a very impressive time constant.

    For example, the self-capacitance of a circular disk is given by [itex] 8 \epsilon_o r[/itex]. How large must the radius be to give a capacitance of 1μf?
     
  8. Oct 29, 2011 #7
    Ok wow that's really helpful. Gonna do some problems related to this and get back to you
     
  9. Oct 29, 2011 #8
    So even at 3*10^6 volts the amount of charge on a 1 meter radius disc is microcoloumbs... I had no idea.

    So if the circuit is not closed and the voltage cannot continuously replenish itself like in the case of a battery you have nothing.
     
  10. Oct 29, 2011 #9

    gneill

    User Avatar

    Staff: Mentor

    Well, not nothing exactly, but certainly something that comes pretty close to it :smile:

    Of course, static electric sparks, however brief and however little charge and current involved, can be pretty noticeable on a dry winter's day!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook