# Voltage between two point charges

1. Oct 6, 2014

### mayer

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

2. Oct 6, 2014

### Staff: Mentor

Different from what?
I guess this part is your homework?

3. Oct 6, 2014

### mayer

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?

4. Oct 6, 2014

### mayer

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?

5. Oct 6, 2014

### sophiecentaur

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.

6. Oct 6, 2014

### mayer

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: Oct 6, 2014
7. Oct 6, 2014

### davenn

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