# Charge on a capacitor

Discussion in 'Electronic Design' started by ChrisGibboGibson, Nov 9, 2004.

1. ### ChrisGibboGibsonGuest

If we charge an air-spaced cap to 100 volts. Then increase the spacing between
the plates, what happens to the voltage?

Gibbo

2. ### John PopelishGuest

It goes up. There are several ways to imagine this without math.

The opposite charges on the plates cause them to attract each other.
It takes work to pull them apart against this attractive force. The
increase in distance also lowers the capacitance. The electrical
energy stored in the cap is 1/2 * C * V^2. The only way the energy
can go up (because mechanical work was done on the cap, conservation
of energy and all that) in spite of the drop in capacitance is if the
voltage increases.

Another way to visualize this is to have two similar capacitors except
for the difference in spacing. Apply enough voltage across each to
force the same charge to be displaced through each. The one with the
bigger spacing (lower capacitance) will require more voltage to force
the same charge to move. If there were some way to get them so far
apart (or otherwise change the plates) that there was zero capacitance
between them it would take an infinite voltage to move that charge.

3. ### PN2222AGuest

Q = C V

C = epsilon A / D (ignoring fringing effects)

V = D / epsilon A (where epsilon A is a constant)

so V rises directly as D increases

4. ### ChrisGibboGibsonGuest

Thank you John and PN

Now what, exactly, causes the potential difference between the 2 plates?

Gibbo

5. ### John PopelishGuest

The electric field caused by the separated charges and the distance

6. ### PN2222AGuest

Um, the electrons?
I'm not sure I understand the question.

When opposite electric charges are separated, the electric field exerts a
force on those charges. That force is the "electromotive force" -- e.g.
the voltage.

ft = 300MHZ minimum.

7. ### nospamGuest

It goes up, and so does the stored energy to balance the work done pulling
the plates apart.

8. ### Rich The PhilosophizerGuest

Whatever it was you used to charge the cap in the first place. Cheers!
Rich

9. ### BanGuest

this question would be more appropriate in s.e.b.
The metal conductor of which the plates consist has free electrons, which
are vibrating between the grid of the nucleusses. In an uncharged state both
plates have the same density of free electrons. A voltage source will "pump"
some of those electrons from one plate to the other until the resulting
electric field is in equilibrium with the applied voltage.
Now one plate has a higher electron density than the other. If you remove
the voltage source, this distribution stays the same. We call this
"potential difference" and the amount of moved electrons "charge". The
situation creates an electrostatic field.
It is similar when you move a mass upwards, the energy is transformed into a
potential energy, which is released when you drop the mass. The weight of
the mass(or the charge Q) stays the same, but the energy(or voltage V)
augments, the higher the mass is moved.
Compare this to the formulas of PN, and you hopefully might have a few more
insights.

10. ### Robert BaerGuest

Lemme see.. Q = C * V
So, decrease C and V goes up.
Seems that has been done before in a rotary fashion to make rather
high voltages.
There may even be a patent somewhere on them thar new-fangled
sparkers...

11. ### Active8Guest

Wrong. Force is force. The voltage potential is the work (F.d)
needed to move the charge divided by the amount of charge,
Joules/Coulomb.

12. ### CBarn24050Guest

This is completely wrong Ban, it's hard to imagine a more fundamental
missunderstanding.

13. ### BanGuest

I do not know what exactly you refer to, but the gravitational field and the
electrostatic field have a lot in common. Look at Newtons Law of
Gravitation:
F = (-G* m* M)/r^2 F=force G=const. m= mass1 M= mass2
and Coulombs Law:
F = (k* q* Q)/r^2 F= force k= const. q= charge1 Q= charge2
Now charge has a sign, whereas gravity is alway attracting, but the fields
have similarities.

14. ### MarlboroGuest

Or.. becauses human calls that the voltage, the energy space for what
human called charge 15. ### CBarn24050Guest

Yes quite so Ban, but that isn't what you said at the start of your post.

16. ### James MeyerGuest

Doesn't anti-matter fall UP in a gravity field produced by ordinary
matter?

Jim

17. ### John PopelishGuest

I don't think the experiment has been definitively done, yet.
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatterFall.html

But it has been pretty well tested that antimatter has the same sort
of momentum as matter. It couldn't be contained in accelerators if
this was not the case.

18. ### Tim WescottGuest

"So what we would really like to have is a laboratory experiment where
we simply drop some antimatter in a lab, and see how fast it falls. "

But they don't mention the difficulty of keeping one's lab notebooks
intact when it touches down...

19. ### Rich The PhilosophizerGuest

Use The Force, Luke.

;^j
Rich

20. ### Rich The PhilosophizerGuest

So, it's not really anti-"matter", but just matter with the opposite
charge?

stuff like, can the equations that describe a proton be mapped onto
the equations that describe a black hole?

And I don't have the maths, which might be a good thing, since my  