Voltage between two point charges

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Discussion Overview

The discussion revolves around the behavior of voltage between point charges and parallel plate capacitors, particularly focusing on how voltage changes with distance and the implications of constant voltage in closed circuits. Participants explore conceptual understandings and relationships between electric fields, charge, and potential difference.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants question whether the voltage between two point charges changes as the distance between them increases, suggesting that this might depend on the specific setup.
  • One participant proposes that, conceptually, the voltage remains constant when increasing the distance between two point charges, as the electric field decreases but the distance traversed by a test charge increases.
  • Another participant expresses confusion about why increasing the distance between capacitor plates connected to a battery results in less charge on the plates, despite understanding that the electric field decreases.
  • It is noted that if point charges or capacitor plates are not in a circuit, work must be done to separate them, which implies an increase in voltage.
  • A later reply clarifies that for a constant voltage in a closed circuit, if capacitance changes due to increased distance, the charge on the plates must also change according to the relationship Q = CV.

Areas of Agreement / Disagreement

Participants express differing views on how voltage behaves with distance in both point charges and capacitors. There is no consensus on the conceptual understanding of these relationships, indicating ongoing debate and exploration of the topic.

Contextual Notes

Participants highlight the difference between point charges and parallel plate capacitors, suggesting that the fundamental principles governing their behavior may differ. The discussion includes unresolved questions about the implications of changing distance on electric fields and charge accumulation.

Who May Find This Useful

This discussion may be useful for students preparing for physics exams, particularly those reviewing concepts related to electric potential, capacitance, and the behavior of electric fields in different configurations.

mayer
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HI,

Would the voltage change between two point charges say + and - as you increase the distance between them? When you have two charge parallel plate capacitors, that was charged but a battery then disconnected, its a different story right? The voltage increases?

Thanks
 
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mayer said:
Would the voltage change between two point charges say + and - as you increase the distance between them?
That depends on your setup.
mayer said:
When you have two charge parallel plate capacitors, that was charged but a battery then disconnected, its a different story right?
Different from what?
mayer said:
The voltage increases?
I guess this part is your homework?
 
Nope, haven't taken a physics course in two years, and reviewing for the MCAT. Just toying with certain concepts that I don't think I considered while taking the class. For the first question. Purely conceptual, two point charges, one positive the other minus. Along the line between the two, the test charge is going from the positive charge to the negative charge. Since I am avoiding most of the heavy math and just thinking of things from a conceptual perspective, it seems to me like voltage will stay constant when you increase the distance between the two test charges because as you increase distance, the electric field the test charge experiences is smaller but the distance the test charge has to traverse is longer?
 
All of these questions are arising from my confusion as to why increasing distance between two capacitor plates attached to a battery, which gives it a constant voltage, results in less charge on the plates. I understand why the electric field decreases in such a situation, but doesn't increased distance with the same charges on each plate achieve the same purpose of decreasing electric field? My questions are basically subquestions to try to answer this question. It has to do with some fundamental difference between point charges and plates of charges correct?
 
The plates (or point charges) will be attracted to each other. If they are not in a circuit so that no charge can flow, you will need to do work to pull them apart. That means that you are changing (increasing) the voltage (Potential Difference) between them. Potential is defined in terms of work.
 
Thanks for the reply! I thought the potential difference was defined by the work that a point charge would require to be pushed from lower to higher potential. Or is that the same idea? But seeing it in this manner does make more sense. How about for a constant voltage, in a closed circuit. Why does the charge capable of accumulating on each plate decrease as you increase distance?
 
Last edited:
mayer said:
...How about for a constant voltage, in a closed circuit. Why does the charge capable of accumulating on each plate decrease as you increase distance?

If you have a constant voltage and you change the capacitance, then the charge must change

From Q = CV, we know that if C changes (by changing the separation distance or adding a dielectric), then so does the charge on the plates.
If you increase the distance, that will decrease C and with a constant voltage (V), then the charge (Q) will also decrease

cheers
Dave
 

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