1. Limited time only! Sign up for a free 30min personal 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!

B Why/how does distance affect capacitance?

  1. Aug 6, 2017 #1
    As distance between two capacitor plates decreases, capacitance increases - given that the dielectric and area of the capacitor plates remain the same.

    So, why does this occur?
     
  2. jcsd
  3. Aug 6, 2017 #2

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

  4. Aug 6, 2017 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Capacitance is the amount of charge imbalance (Q) for a given Potential difference. When the plates are closer together, the same PD, applied across the plates will bring opposite charges on the two plates closer together. They will attract each other more (reduced spacing) and so even more charges will be attracted to the faces (it takes only a minuscule field to make the charges in the metal move towards the face. So, by the way Capacitance is defined (more charge, same PD) , the Capacitance is increased.
    If you put a dielectric between the plates, the molecules in faces of the the dielectric are much nearer to each plate and the molecules inside the dielectric will polarise easily. So, again, the actual charge imbalance on the plates will be greater for a given PD. Another way I look upon a dielectric is that it's a bit like putting a conductor in between that's just a tiny bit narrower than the original gap. That is producing a capacitor with a much smaller gap and, hence, a greater Capacitance. The only problem is that such a narrow gap between metal plates could flash across with only a tiny PD. A dielectric will only allow Polarisation and not a flow of charge so no 'short circuit'.
     
  5. Aug 6, 2017 #4
    Does that mean that capacitance is:
    - inversely proportional to Electric field strength (E)
    - inversely proportional to Distance (d)
    - proportional to Charge (Q)
    - proportional to Area (A)

    And because distance affects capacitance and distance is proportional to the Electric field strength, is capacitance affected by the Electric field strength?
     
    Last edited: Aug 6, 2017
  6. Aug 6, 2017 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The Capacitance is a description of the Quality of an object. It depends on the dimensions and materials used and not the conditions it is exposed to. A mathematical formula is only a formula. It does however, tell you how to Calculate the Capacitance, if you measure the Charge and PD.

    PS. There are materials for which C is not constant for all values of charge and PD, in the same way that there are materials with a Resistance that is not the same for all currents (i.e. Non-Ohmic) But that is definitely not Introductory Level. :smile:
     
  7. Aug 6, 2017 #6
    If I, for example, have paper as the dielectric, and the distance between the two capacitor plates were decreased (so there are less "molecules" of paper), why and how does this increase capacitance? Shouldn't the plates hold more charge if there are more polarised molecules in the dielectric, as the pull on the nucleus will be greater (due to all of the electrons), and thus the atom's electrons will be pulled towards the nucleus with greater force, allowing more charges on the capacitor plates?
     
  8. Aug 6, 2017 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The best explanation for this is that there is no shortage of polarisable molecules in the dielectric and, of course, the applied Electric field (Volts per metre) would be Increased with a narrower gap.
     
  9. Aug 6, 2017 #8

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    The link that I pointed to gives a derivation, and the answers to your questions.

    Capacitance [itex]C[/itex] is defined as [itex]\frac{Q}{V}[/itex].

    The voltage between two parallel plates, one with a charge [itex]+Q[/itex] and one with a charge [itex]-Q[/itex] is computed as:

    [itex]V = E d[/itex]

    where [itex]E[/itex] is the electric field, and [itex]d[/itex] is the distance between the plates. The electric field, in turn, for two parallel plates is computed by:

    [itex]E = \frac{\sigma}{\epsilon}[/itex] where [itex]\sigma[/itex] is the charge density (charge per unit area).

    So putting it all together,

    [itex]C = \frac{Q}{V} = \frac{Q}{E d} = \frac{Q}{\frac{\sigma}{\epsilon} d} = \frac{Q}{\frac{Q}{A} \frac{d}{\epsilon}} = \frac{\epsilon A}{d}[/itex]

    where [itex]A[/itex] is the area of one plate, and [itex]d[/itex] is the distance between the plates. So the charge drops out, and the only quantities that [itex]C[/itex] depends on are the area of the plates, the distance between them, and the dielectric constant.
     
  10. Aug 6, 2017 #9
    Just to clarify, this derivation is for large i.e. infinite parallel plates.
     
  11. Aug 7, 2017 #10
    Let's say a charged capacitor A is two metal plates 1 cm apart. And a non-charged capacitor B is two metal plates 0.1 cm apart.

    When we let the side of capacitor B touch the side of capacitor A, what happens?

    Well, opposite charges like to be close to each other, so the charges go from the capacitor A to the capacitor B. We say capacitor B has larger capacitance than capacitor A, when charges prefer to be go from A to B.


    Or maybe this is better:

    Negative charges like to be far from each other, so some negative charges go from the negatively charged plate of A to the originally neutral plate of B. And positive charges like to be far from each other, so some positive charges go from the positively charged plate of A to the originally neutral plate of B. Seen from A the charged capacitor B looks quite neutral, because B contains equal amount of positive and negative charges almost at the same place. So almost all charges go from A to B.
     
  12. Aug 7, 2017 #11
    A slightly different way of looking at it:
    1. Charge the capacitor and disconnect it from the supply.
    2. Pull the plates apart by a method that does not allow charge to leak away. In order to do this work must be done in order to overcome the force of attraction between the plates. This results in an increase in V between the plates and since C=Q/V, C decreases.
     
  13. Aug 7, 2017 #12
    I am having trouble understanding why the voltage increases as the force of attraction is overcome.
    Could you please elaborate as to why this occurs?
     
  14. Aug 7, 2017 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The plates are attracted to each other. You exert a force against this and, moving away, you do Work. Like when you stretch a spring. SO the Energy per unit charge increases - that's Potential Difference.
     
  15. Aug 7, 2017 #14
    Could this relate to when capacitor plates are charged at a certain distance (x), then discharged, and charged at another distance (y), then discharged, and the capacitance at those two points differing? If so, how?
     
  16. Aug 7, 2017 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The Capacitance depends just on the geometry and the dielectric - not on the charge. to get the same charge on the plates with different separation you need different voltages.BUT putting the same PD across the plates at different distances will change the Charge.
    That's what follows from Q=CV.
     
  17. Aug 7, 2017 #16
    Just to clarify one more thing: Does the distance between the two capacitor plates, therefore (assuming PD is same), affects the charge on each plate?
     
  18. Aug 7, 2017 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Q=CV is the way to put it. C is the Capacitance. The "charge on each plate" will be equal and opposite if the whole thing started off at Earth Potential.
     
  19. Aug 7, 2017 #18
    Does this mean that electric fields affect capacitance, and distance affects electric fields (assuming voltage is the same)?
     
  20. Aug 7, 2017 #19

    jbriggs444

    User Avatar
    Science Advisor

    Electric fields do not affect capacitance. This has been explained already. A particular capacitor has the same capacitance whether it is fully charged, half charged or fully discharged. Capacitance is the ratio of charge to potential difference for that capacitor.

    If you take a fully charged capacitor and separate the two plates (doing work as @sophiecentaur suggests) while not permitting any charge to flow you will have left the electric field strength in the gap unchanged. The potential difference between the two plates is given by field strength times separation distance and will have increased. The charge on each plate will have remained unchanged. The ratio (##\frac{charge}{potential\ difference}##) will have decreased with the increase in potential difference. Capacitance is that ratio.
     
  21. Aug 7, 2017 #20
    It might help to go right back to first principles, forget about capacitors for a moment and consider charging any old lump of conductor. Put a charge Q on the conductor and it develops a potential V. As Q is increased so does V and in a directly proportional way (until, that is, the field strength reaches values which cause discharge to the surroundings). The capacitance C is a constant of proportionality connecting Q and V as in:
    Q=CV

    The capacitance of an isolated lump of conductor depends on its geometry but is extremely small. But capacitance can be increased enormously by having two conductors. Two conductors make a capacitor and the capacitance of a capacitor depends on its structure and geometry as described in the posts above.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why/how does distance affect capacitance?
Loading...