What is Capacitance: Definition and 899 Discussions

Capacitance is the ratio of the amount of electric charge stored on a conductor to a difference in electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. In this case the electric potential difference is measured between the object and ground. A material with a large self capacitance holds more electric charge at a given potential difference than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors). In a typical capacitor, two conductors are used to separate electric charge, with one conductor being positively charged and the other negatively charged, but the system having a total charge of zero. The ratio in this case is the magnitude of the electric charge on either conductor and the potential difference is that measured between the two conductors.
The capacitance is a function only of the geometry of the design (e.g. area of the plates and the distance between them) and the permittivity of the dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and thus the capacitance, is independent of the potential difference between the conductors and the total charge on them.
The SI unit of capacitance is the farad (symbol: F), named after the English physicist Michael Faraday. A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates. The reciprocal of capacitance is called elastance.

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  1. genxium

    Is there capacitance between non-aligned parallel plates?

    Our professor gave us a lecture about sensors tonight, but I'm kind of confused about the principals of capacitive touch screen. As the attached figure shows, when finger touches the screen , capacitance is formed between finger and the ITO bars, but is it true for all the ITO bars? It's...
  2. J

    Capacitance if inner sphere is earthed

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  3. M

    Jackson's Electrodynamics. Question on Capacitance definition

    In Jackson's book he defines the capacitance of a conductor, "...the total charge on the conductor when it is maintained at unit potential, all other conductors being held at zero potential." I'm trying to get a more concrete definition in my head rather than the standard definition of...
  4. S

    Microstrip (Derivation of Capacitance and Inductance)

    Could anyone help me to find out a book which has detailed explanation(derivations) for the capacitance, inductance of a mictropstrip line. It would be helpful if the book has detailed discusions on fringing fields,(effective width of the ground plane) effective permitivty etc. I have tried...
  5. G

    Capacitance of a System (Spherical Conducting Shells)

    Homework Statement A spherical conducting shell of radius 0.1 m has a free charge of -2 μC. It is surrounded by a concentric spherical conducting shell of radius 0.12 m carrying a free charge of -2 μC. Between the shells is a dielectric material of dielectric constant 10 εo. If the dielectric...
  6. O

    Proof that Capacitance Coefficient C21=C12

    Homework Statement Show that for two arbitrary conductors the coefficients of capacitance C12 and C21 are always equal. Hint: Consider a two-conductor system in which the two conductors have been charged so that their potentials are φ1f and φ2f respectively (f for ”final”). This condition...
  7. S

    [Quick] At what capacitance will the current in the resistor be a maximum?

    Homework Statement A series RLC circuit is connected across a 6.70 kHz source. The 1.50 H inductor and the 1.35×103 kohm resistor are fixed whereas the capacitor is variable. At what capacitance will the current in the resistor be a maximum? Homework Equations No idea.. The Attempt at a...
  8. R

    Capacitors in a circuit / Calculate total capacitance

    Homework Statement Capacitors are connected as shown in this diagram: http://o3xn.files.wordpress.com/2012/04/c4.png C1 = 2 μF, C2 = 1 μF. What is the total capacitance of the system in between the terminals A and B? Homework Equations For capacitors C1 and C2 connected in...
  9. A

    Capacitance concentric spherical shells

    Homework Statement Given two concentric spherical metal shells, with radii a and b (a < b), and surface charge densities Sa and Sb. Find the capacitance if Sa = - Sb. Homework Equations C = Q/V The Attempt at a Solution I would know how to solve this if the absolute values of the...
  10. P

    Capacitance and Other Traits of a Coaxial Cylinder

    This is for an electromagnetic theory class. Homework Statement Prompt can be seen here Homework Equations J = I/A, J = σE, V = Ed, C = εA/d The Attempt at a Solution I'm having trouble mostly with parts i and ii, as the rest are fairly simple to acquire after figuring out those two...
  11. D

    Question on capacitance and electric fields

    Consider a thin parallel plate capacitor of an area of of 2.0x10^-4 metres squared, with a thickness of 1.2x10^-4 metres. The top and bottom thirds of the capacitor are filled with a dielectric material with a relative dielectric permittivity εr1=4, and the central third, with another material...
  12. O

    Capacitance power factor question

    Homework Statement A 50 KW load operates from a 60 Hz 10KV rms line with a power factor of 60% lagging. Determine the capacitance that must be placed in parallel with the load to achieve a 90% lagging power factor.The Attempt at a Solution for 60% p.f:- θ1 = Cos-1(0.6) = 53.13 degrees Q1 =...
  13. H

    Self capacitance of a multi layer coil

    Hi everyone, I am trying to figure out how to correctly calculate the self capacitance of a multi layer coil. I have been trying to find an online calculator or software which can do this but I don't really know where to look. Just hoping someone here can give me some insight. Any help...
  14. I

    Electric Potential, Capacitance and charge of a Geiger Counter

    Homework Statement Find the potential difference, capacitance of the Geiger Counter and the charge on the anode. So, we have a Geiger Counter with low-pressurized neon inside. What we want is to have an alpha particle or electron to travel into the Geiger Counter with enough energy to collide...
  15. M

    Parallel Vs. Series - Capacitance?

    What exactly is the reasoning behind deciding to use parallel vs series? What is the value behind capacitance, really, as opposed to a general electric circuit? Thanks.
  16. P

    Equivalent Capacitance: Distinguishing Parallel/Series

    Homework Statement I'm having trouble distinguishing exactly which combinations are in parallel and which are in series. The fact that both ends of the circuit are open is throwing me off. Can someone walk me through this?
  17. D

    Dielectric Breakdown and Capacitance

    If a capacitor's dielectric partially breaks down, as in some of the material has ionized and acted as a conductor, will there still be a capacitance to the component? Say its a high frequency AC signal on a capacitor with a small part of its dielectric material in a breakdown state. My...
  18. N

    Capacitance Calc for Co-Planar Comb Electrode

    can somebody give formula to calculate value of capacitance at co planar comb electrode as at the attachment..
  19. D

    Capacitance per foot? My foot

    So I left a thread of posts about this new coax I had made, trying to understand its electrical characteristics such as velocity factor and so on... Then I had an occasion to exchange emails with the MFG asking what the capacitance was per foot. The answer was something like 30pF per foot...
  20. S

    Stray or parasitic capacitance

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  21. S

    Does the universe has a capacitance?

    Does the universe has a capacitance?
  22. P

    What is kinetic capacitance in superconductors or in solid state in general?

    What is "kinetic capacitance" in superconductors or in solid state in general? Hi, I would appreciate it if anyone could explain to me what "kinetic capacitance" is, particularly in the context of superconductors or solids. I understand "kinetic inductance" which comes from the kinetic...
  23. D

    What is the Capacitance of a Capacitor with Two Different Dielectrics?

    Homework Statement A capacitor is constructed from two square metal plates. A dielectric \kappa = 4.61 fills the upper half of the capacitor and a dielectric \kappa = 6.9 fills the lower half of the capacitor. Neglect edge effects. Calculate the capacitance C of the device. Answer in...
  24. P

    Affecting Mutual Capacitance w/ an External Field

    I've been playing around with projected capacitive touchscreens - specifically, the iPhone's. I've attached a white paper that outlines the details, but the basic idea is as follows: An iPhone's touchscreen has a dielectric layer sandwiched between a layer of conductors arranged in rows and a...
  25. W

    How Does Adding Different Dielectrics Affect a Capacitor's Performance?

    Homework Statement A parallel plate capacitor of area 2 cm^2 is filled with three dielectrics with constants k1=3.5, k2=1.7 and k3=8.5. The dielectrics are placed as shown in the figure, each occupying half the total area, while the volume of the first double of the other two. If the...
  26. J

    Inducatnce and Capacitance calculation for rectangular spiral coil

    Hi, I am looking for a formula that calculates self inductance and capacitance of rectangular spiral coil, i came across many formulas for helical coil and circular spiral coil but couldn't find one for rectangular coil, any help is highly appreciated. Thanks
  27. R

    Question Relating Electric Field and Capacitance

    Homework Statement A parallel-plate capacitor has a plate area of 0.2 m2 and a plate separation of 0.1 mm. To obtain an electric field of 2.0 × 10^6 V/m between the plates, the magnitude of the charge on each plate should be Homework Equations Q=epsilon(E)(A) The Attempt at a...
  28. M

    What Is the Capacitance of Three Concentric Spherical Shells?

    Homework Statement Hi, I would really appreciate clarification on the definition of capacitance in this context: Suppose I have 3 concentric metal spherical shells and they have charges Q1,Q2,Q3 and potentials 0,V,0 and radii a,b,c respectively, what is the capacitance of the configuration...
  29. G

    Capacitance Combination for 9V Potential Difference

    Homework Statement An electronic engineer is designing a circuit in which a capacitor of capacitance of 4700 microFarads is to connected across a potential difference of 9.0V. He has four 4700 microFarads, 6V capacitors available. Draw a diagram to show how the four capacitors could be used...
  30. F

    What's the capacitance of a typical car's battery?

    Question for fun. A car's battery is not a capacitor, but it seems to "accumulates" charge the same way. So, if you take a typical fully charged car's battery, what would be the equivalent capacitance of a 12V capacitor that would have the same charge? For an extra kick - try to...
  31. X

    Solve Capacitance Problem: Voltage, Charge & Dielectric Effects

    I do not know whether this is the correct section to post this. This is in my graduate level Electromagnetism class but the problem is sort of a "throw back" to the freshman level course. So I very much apologize if this is not the correct place! This problem shows a parallel plate capacitor...
  32. P

    What Happens to Charge When Capacitor Plate Separation Doubles?

    Homework Statement If the separation between the plates of a parallel plate capacitor is doubled and the potential difference is held constant by a battery, the magnitude of the charge on the plates will: *a. double b. quadruple c. be cut in half d. not change Homework...
  33. M

    Why are the charges on capacitors A and B the same, and C and D equivalent?

    http://capa.physics.mcmaster.ca/figures/sb/Graph26/sb-pic2654.png This is more of a concept understanding if anything but I'm interested to know why the charge on capacitors A and B are the same and similarly, the charge on capacitor C and D are equivalent.
  34. R

    Flux, Capacitance, Difference Potential, and Volt

    I have a question about electricity and magnetism. My teacher went through these chapters in 3 days, and I am completely confused. Can some body please explain what exactly is flux, capacitance, difference potential, and voltage? And can you also explain to my what the number stands for when...
  35. S

    Engineering Circuits - using equivalent capacitance

    Homework Statement C1 = 1μC ; C2 = 2μC ; C3 = 3μC ; C4 = 4μC V = 12.0V If both switches are closed, what is the charge on C1? Homework Equations q = CV In parallel - V is constant across capacitors Cxy = Cx + Cy In series - q is constant across capacitors Cxy-1 = Cx-1 + Cy-1The Attempt...
  36. J

    How to add capacitance in between two points end points of spiral coil?

    Hi All, I am at present working on coil design for RFID tag, i want to know how you add capacitance in spiral coil i.e. how you join two ends in spiral structure with capacitance in between, i have already calculated capacitance value needed to resonant at particular frequency. Any help is...
  37. M

    Capacitance by energy or time current product?

    I want a capacitor bank capable of transferring a 350 kJ, 500 ms pulse. Or, rather, a bank capable of supplying 70 A @ 10 kV for 0.5 s. I have located a 2000 μf 10kV capacitor which looks like it might do the job. Unfortunately they are very expensive ($1500 USD) and I can only afford 2 or maybe...
  38. dlgoff

    Remembering Capacitance Electronic Disc (CED)

    The TMC channel is showing "Close Encounters of the Third Kind" which brings back the memory of RCAs Capacitance Electronic Disc. Anyone remember these? http://upload.wikimedia.org/wikipedia/en/thumb/e/ed/Ced_cart2.jpg/670px-Ced_cart2.jpg The first "video disk" I ever purchased/watched was:
  39. L

    Getting weird formula for Capacitance

    Okay, so I know C=Q/ΔV And ΔV is the sum of the electric fields multiplied by the distance between the charges, so if the first charge has a charge of Q and the other has -Q with R distance between, the electric potential/voltage is: ΔV = [(kQ/r^2) + (kQ/r^2)]R so ΔV = 2kQ/R. And C = Q/ΔV...
  40. T

    Capacitance between two adjacent long thin plates

    Hi. I am working on a project for the design of a capacitive sensor. In order to do that, I am required to compute capacitance using a simulation software like COMSOL. As I want to compare my simulation results with the theoretical results, I would like to know the formula to be used for the...
  41. D

    Understanding Capacitance: Exploring the Relationship Between Charge and Voltage

    Hi. I'm trying to understand a couple of things. When you put a piece of metal on, say the cathode of a battery, I suppose there is a charge transfer from the battery to the metal because the electrons on the cathode have a high potential energy? And so if I connect a piece of metal to both...
  42. A

    Calculation of capacitance of transmission lines

    Capacitance between two plates is defined as the charge accumulated per unit potential difference between those plates. But in the case of transmission lines how can we define it? The potential difference any two conductors in a group of several conductors can be derived in terms of the charges...
  43. T

    Finding R in a RC circuit given battery, capacitance, and time.

    Homework Statement A 10.0 uF capacitor is charged by a 10-V battery through a resistance R. The capacitor reaches a potential difference of 4.00-V in a time interval of 3.00 s after charging begins. Find R. Homework Equations C = Q/ΔV q = εC(1-e(-t/RC) The Attempt at a Solution...
  44. S

    Capacitance nd Spherical cells

    Hey all I'd like to help me.. thnx in advance here z tha Q's : 1) Two plane parallel conducting plates each have area S and are separated by a distance b. One carries a charge +Q ; the other carries a charge -Q. Neglect edge effects a) What is the charge per unit area on each plate ? where...
  45. B

    Record-Breaking Capacitance: What is the Highest Ever Achieved?

    What is the highest capacitance ever made/achieved? I am asking this because, when I am solving some time domain circuits, the values are given like 3 F etc. And I know that is absurd, and I really hate it because, if I didn't take electronics at home, I wouldn't have the feeling for...
  46. P

    Choosing a capacitance to limit voltage across a switch

    Hi everyone. My problem is for an assignment in a network analysis course. I attached the homework page as a pdf. Homework Statement My problem involves an R-L circuit with a switch. There is a 100 V source, and switch that is initially closed, followed by a .25 H inductor and a 200 Ohm...
  47. K

    How Do You Calculate Capacitance from a Voltage Decay Curve?

    Only just started ac currents today and this was on my first quiz, I got 8/10 for it but got this question wrong. Homework Statement A 100 ohm resistor is connected in series with a capacitor and a power supply, and the supply voltage is brought up to 200 mV. The graph shows- for three...
  48. P

    Equivalent capacitance A hard one?

    Equivalent capacitance... A hard one!? Homework Statement Calculate Equivalent capacitance between A and B: [PLAIN]http://scsupport.org/moodle/file.php/13/Chapter26/imgch26q14.gif EDIT: If you can't see the picture, see 3rd post for file attachment, sorry!Homework Equations C1 = 3.55 µF C2 =...
  49. T

    Calculating Capacitance for Pulsed Nitrogen Laser Design: Tips and Tricks

    Homework Statement You are part of an engineering research team that is designing a pulsed nitrogen laser. To create the high energy densities needed to operate such a laser, the electrical discharge from a high-voltage capacitor is used. Typically, the energy requirement per pulse (i.e., per...
  50. H

    Series capacitance added to parallel resonant tank changing the resonant freq?

    HI all, A parallel tank circuit at resonance has a theoretical infinite impedance. However due to the resistance of L and C some power is dissipated so a small current will flow from the source. If a parallel tank circuit is oscillated at it's resonant frequency and a capacitor (C2) is...
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