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

I Voltage measurement of a point charge

  1. Mar 16, 2019 #1
    If we have a point charge of significant value, would osciloscope measure voltage if we just point the electrodes in two diffrent point in space?
     
  2. jcsd
  3. Mar 16, 2019 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Your question has been posed in an unusual way. What is the actual experiment that you are proposing?
    You could be referring to the effect on an oscilloscope inputs of a nearby charge. An oscilloscope has an input resistance of, say 10MΩ. (i.e. 'very finite') and it provides a conductive path which will allow current to pass until the field between the probes is zero. It's true to say that waving a scope probe around near a high voltage source can cause the trace to move up and down a bit but holding the probes stationary will result in a 0V DC reading on the trace. (The reading on the trace depends on contributions from all over the connecting wires.)
     
  4. Mar 16, 2019 #3

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    A very strange description, especially on why one would use an oscilloscope.

    Let me put it this way. If you use a voltmeter, then yes, there's a good chance you'll see a potential difference between two points in space if you don't happen to hit both ends of the probe on the same equipotential surface.

    Zz.
     
  5. Mar 16, 2019 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    A particular form of voltmeter with an extremely high input resistance would show movements of the needle. They are available with Hundreds of MΩ input impedances and tend to cost a bit. I approve of the idea of using a scope because you can see the history of your movements on the trace - but not with the sensitivity that you would need for 'everyday' electrostatic situations.
     
  6. Mar 16, 2019 #5
    So technicaly osciloscope or voltmeter measure only voltage where current is possible? If we try to measure 12 V battery with either one we get result, but if we have have for example Van de Graaffs generator with a lot of voltage, than the metters wouldn show anything or very little, altrught we can also calculate voltage, betwen lets say 0,5 m and 5 m from Van de Graffs generator.

    If we would than use a metalic plate and put it in the electric field of Van de Graaf generator(in correct geometrical orientation) than the plate would get polarised and if we than measure voltage of this plate(phyysicaly connected) we would again get correct value?
     
  7. Mar 16, 2019 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Best not to use a VDG generator with normal test equipment, it can damage equipment that's designed to work at a few hundred volts maximum if the ball charges up fully.
    Your proposals are not clear. Perhaps a diagram would get the message across.
     
  8. Mar 16, 2019 #7
    I am asking this from theoretical point of view. And the quetion I had was basicaly does oscciloscope or volmeter meassure voltage created from static charge at two diffrend distances of this card ,(Calculated V=Q/4*pi*e0*r) and the answer in as I understand pretty much no.
     
  9. Mar 16, 2019 #8
    How about this: any measurement requires some transfer of energy. Therefore any electrical measurement requires movement of charge (current) through a potential (voltage). A gold leaf electrometer will measure very high potentials using a tiny amount of charge. A galvanometer will see tiny voltages but requires some current. This could be a very long list but the fundamental premise is, I believe, correct. So you choose what you wish to measure.
     
  10. Mar 16, 2019 #9

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    No?

    Then how do you explain how this type of experiment that is common in general physics labs works?

    http://ipl.physics.harvard.edu/wp-uploads/2013/03/ps3_s08_1.pdf

    Zz.
     
  11. Mar 17, 2019 at 7:58 PM #10
  12. Mar 17, 2019 at 9:01 PM #11

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    I have my students do that in their labs!

    Did you miss the fact that this is a rather COMMON experiment in schools?

    Zz.
     
  13. Mar 18, 2019 at 4:53 PM #12
    I do know the exact procedure that you do, but if I undestand correctly you say that if we have a charge on Van de Graffs generator of let say 1nC and we measure the potential using votlmeter at 1 and 5 m voltmeter would show 7V?
     

    Attached Files:

  14. Mar 18, 2019 at 8:50 PM #13
     
  15. Mar 18, 2019 at 8:55 PM #14
    Not with any voltmeter I know.

    Please indicate the procedure. I don't get it.
     
  16. Mar 19, 2019 at 4:31 AM #15
    I would use a Electrostatic voltmeter to measure the VDG, though I haven't tried it myself yet.
     
  17. Mar 19, 2019 at 6:10 AM #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It's often convenient to measure a voltage without passing any current if you use a 'potentiometer' circuit. You balance the test voltage against a reference voltage for zero detectable current . That way you can obtain a very high effective input resistance for the 'Voltmeter' system.
     
  18. Mar 19, 2019 at 7:25 AM #17

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2018 Award

    What don't you get? The link that I gave is a lab instruction. The procedure is all there.

    Zz.
     
  19. Mar 19, 2019 at 10:38 AM #18
    I see so if you have a sufficient current constant voltage source and a 2D problem you can use carbon paper and a DVM. That's a fine lab demo....
    Not exactly similar to measuring the 3D field of point charge with an oscilloscope probe, which was the original question! Hence my confusion.
     
  20. Mar 22, 2019 at 7:00 PM #19
    I found the video where guy mesures electric field with oscilloscope(start at 1.45) of the plasma ball. So is this possible because plasma has AC current or would it work with Van Den Graaf also?



     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?