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. J

    Capacitance in Series: Find Charge, Voltage, Energy

    [b]1. ok, there are 2 capacitors in series in a DC network, with A having a capacitance of 1.6 nanofarads and B having 5 nanofarads capacitance, but how do i find the charge, voltage and energy stored in each if the supply voltage is 24 volts. if anyone could help within the day that would be...
  2. F

    Simulation of a double layer capacitance and a warburg impedance with comsol

    Hallo, i'm trying to implement a comsol simulation of a impedance spectroscopy experiment. But unfortunatly I'm an absolut beginner in Comsol and i have many problems with this work. First of all i want to say that I'm using the AC/DC module, more precisely the transient analysis. At the...
  3. I

    Capacitance and inductance of cylindrical shell

    i am having trouble with finding inductance and capacitance of a cylindrical shell with gap. say inner radius is a, outer radius is b, and filling dielectric between a and b, and its not a exact circle which have a small gap say d. its like a split ring but only one shell. How can i...
  4. H

    How to calculate capacitance and electric energy for an axon membrane?

    Capacitance using an axon.. :)? Homework Statement An axon membrane is 8.0×10-9 m thick. a) Find the dielectric constant of the membrane, if the capacitance C is 100 nF for an area A = 10.0 square mm. b) If the resting potential across the membrane is ΔV = - 90 mV, what is the charge ±Q on...
  5. N

    Determine the instantaneous power absorbed by the capacitance

    1. For the circuit of Figure 1, VT = 100 V, R = 5000 Ω and C = 400 μF; switch S is closed at t = 0. a) Determine the instantaneous power absorbed by the capacitance. b) Obtain an expression for the instantaneous power dissipated in the resistance. c) Determine the voltage...
  6. H

    To find the capacitance of 2 concentric spherical shells

    Homework Statement There are 2 concentric shells. The outer shell of radius a is given a charge, while inner shell of radius b is earthed. We need to find the capacitance of the system. Homework Equations Delta V = - integral E.dl (for some funny reason, latex is...
  7. S

    Capacitance of a landing plane in a thunderstorm

    Homework Statement An airplane flies through a thundercloud and picks up an excess charge of 1 microcoulomb. When the plane comes into land, its wings form approximately a parallel plate capacitor with the ground. What is the capacitance of this system? What is the potential of the plane...
  8. T

    Calculating Capacitance for a Given Energy and Voltage?

    Homework Statement A capacitor stores 0.12 J energy when connected to 6 V voltage source. What is the capacitance of this capacitor? (UNIT:uF) I am just not sure how to go about answering this question. The formula for capacitance is C=(Q/V), but I am not sure how to incorporate energy...
  9. B

    How to Calculate Capacitance for Two Slanted Conducting Plates?

    Homework Statement Determine the capacitance of two conducting plates facing each other at an angle as shown (in link) . Plate A and B have dimensions of 10x20 cm and 10x (20sec30) (angles in degrees) Homework Equations C=( \epsilon A)/d The Attempt at a Solution not quite sure can...
  10. Z

    Total Capacitance. Parallel or Series?

    Homework Statement In the attached question about total capacitance of the network, because of the layout I wasn't sure if this should be totalled as capacitors in parallel or capacitors in series. I was thinking perhaps parallel, but am not sure. Thank you! :smile: Homework...
  11. K

    Calculate capacitance to adjust current in ac circuit

    Homework Statement A Purely resistive resistive 100w, 110v lamp is to be supplied by an a.c. source of 230v r.m.s at 50Hz. what value of pure capacitance could be put in series to adjust the current flow to a suitable value? Homework Equations p=iv q=cv v=ir i know a number of other...
  12. V

    Heat Capacitance Homework: Compute Work & Internal Energy Increase

    Homework Statement When water is boilded under a pressure of 2 atm, the heat of vaporization is 2.2 x 10^6 J/kg and the boiling point is 120 degree C. At this pressure, 1 kg of water has a volume of 10^-3 cubic meter and 1 kg of steam has a volume of 0.824 cubic meter. a.) Compute the...
  13. S

    Determining Capacitance in Different Kinds of Capacitors

    Homework Statement Determine the equivalent capacitance (in terms of C0) between points x and y of each network of capacitors shown. Homework Equations For capacitors connected in parallel, Ceq=C1+C2+...Cn For capacitors connected in series, Ceq=1/C1+1/C2+...1/CnThe Attempt at a Solution...
  14. M

    Capacitance, charge and potential

    Homework Statement The battery has a potential difference of 20 V. Find: a. the equivalent capacitance of all of the capacitors and b. the charge stored on that equivalent capacitance. Find the potential across and charge on c. capacitor 1, d. #2, and e. #3. See paint file for an...
  15. M

    Understanding the Impact of Dielectric Insertion on Capacitance

    A derivation of the capacitance for a parallel plate capacitor is as follows: [ assume uniform field, plate area is infinite in relation to the separation, the conductors are perfect conductors ] So, V=Ed where V=voltage difference between plates E= electric field (uniform)...
  16. M

    Determination of signs in basic capacitance problems

    In my electromagnetics textbook ("Fundamentals of Engineering Electromagnetics" by DK Cheng) there is a typical problem with capacitance, conductors and cylindrical or spherical symmetry which pops up with different variations several times in the problems section of the electrostatics chapter...
  17. Topher925

    Methods to accuractly measure capacitance

    I need to measure the capacitance of something as accurately and as quickly as possible. I also need the method to be very noise tolerant and not depend upon signal attenuation. I've been looking at several different methods so far on how to do this and so far I have found methods like the UTI...
  18. A

    Intercapacitance: Can it Cause 230 V Contactor to Latch Momentarily?

    A pump did not stop when operator pressed the emergnecy push button. He ran towards another start/stop station and pressed the stop button. Even then the pump did not stop. After a while according to operator the pump stopped on its own. Electrical expert says that this can happen because of...
  19. S

    Square Plate Parallel, with angle, Capacitance Problem

    Given two parallel plates, length a. One being on the x axis, starting at the origin. The other starting a distance, d up the y-axis at an angle theta (parallel to the x axis), for theta being small. I need to find an equation for an approximation of the capacitance. I know that if the...
  20. V

    Capacitance of infinite cylinders

    Homework Statement I have two coaxial infinite cylinders and I must find their capacitance, where r_{1} < r_{2} Homework Equations The Attempt at a Solution I got an answer (for finite cylinders) that is inversely proportional to Ln (r_{1}/r_{2}). Assuming this answer is correct (if someone...
  21. H

    Maximizing Wind Power: Calculating Capacitance for Energy Storage

    Homework Statement A wind power plant will not produce as much energy when the wind slows down. In order to provide power during these time periods it is proposed to store some power when it is operating in high winds in a capacitor. If it is desired to store the energy to power a town at 20...
  22. 1

    Capacitance with a Geiger Tube

    Homework Statement The radius and the length of the central wire in a Geiger tube are 0.180 mm and 10.0 cm, respectively. The outer surface of the tube is a conducting cylindrical shell that has an inner radius of 1.50 cm. The shell is coaxial with the wire and has the same length (10.0 cm)...
  23. T

    Capacitance Comparison for Spherical Shells and Solid Spheres

    Would the capacitance of the system be the same if we had one spherical shell around another (radii a and b) , versus a solid sphere of radius a surrounded by shell of radius b? I mean...if the charge we add to the solid sphere distributes itself completely on the surface, isn't the inner solid...
  24. E

    Projected Capacitance Touchscreen, confused by how the capacitance is formed.

    Hello, I have a question that's really bothering me. So I've been reading about projected capacitance touchscreens (the kind that's in so many cell phones, the iphone etc) and I don't understand what the physical property of my finger is that allows the screen to function. From what I've...
  25. I

    Capacitance: 24µF, 30V - Calculate Charge

    Homework Statement A 24-µF capacitor has an electric potential difference of 30 V across it. What is the charge on the capacitor? Homework Equations kq1q2/r2?, q/change in V? The Attempt at a Solution ive tried looking through my notes but i was absent don't understand this if you could...
  26. H

    What is the capacitance of the two metal spheres

    Homework Statement What is the capacitance of the two metal spheres shown in the figure ? sphere one has 20nC of charge and the other has -20nC of charge. The change in voltage is 100V Homework Equations C=Q/(change in V) The Attempt at a Solution My question is how do yo...
  27. O

    SIMPLE e&m question: finding equivalent capacitance

    I am missing something very simple... in my book, it gives, for capacitors in series, that: 1/C(eq) = 1/C(1) + 1/C(2) using numbers, it says... 1/C(eq) = 1/6.0 uF + 1/3.0 uF ... C(eq) = 2.0 uF I cannot figure out how they got C(eq) = 2.0 uF Thanks
  28. C

    How Do You Calculate the Capacitance of a Homemade Capacitor?

    Homework Statement A science-fair radio uses a homemade capacitor made of two 45 45 sheets of aluminum foil separated by a 0.30-mm-thick sheet of paper. Assume dielectric constant of paper is 3.0. What is its capacitance? Homework Equations C=k e0 A / d The Attempt at a...
  29. ?

    How Does Changing Capacitor Characteristics Affect Its Capacitance?

    Please help! Capacitance question! Does the capacitance increase, decrease, or stay the same if: 1. the charge on the plate doubles? 2. the potential difference across the plates triples? 3. the distance between the plates is halved? 4. area of the plates is doubled? 5. electric field is...
  30. E

    Charge on Capacitor in Pure Capacitance Circuit?

    Homework Statement I apologize for having to upload an attachment but I couldn't describe this circuit. Basically, there is set of capacitors and the question is asking for the charge on one of them, when a 72 volt source is attached. All capacitances are 8.9 microFarads. Homework...
  31. N

    Calculate the capacitance per unit length of the cable

    Homework Statement The coaxial cable from a television or other device has an inner conductor with diameter of 1 mm and the shield of diameter 5 mm. Assume a plausible value for the dielectric constant (3 ≤ κ ≤ 5) of the dielectric separating the two conductors and calculate the capacitance...
  32. S

    Capacitance of a cylindrical capacitor

    Homework Statement A coaxial cable used in a transmission line has an inner radius of 0.15 mm and an outer radius of 0.65 mm. Calculate the capacitance per meter for the cable. Assume that the space between the conductors is filled with polystyrene. (Also assume that the outer conductor is...
  33. S

    What is the estimated capacitance of the capacitor in a photographic flash unit?

    Homework Statement A capacitor, previously charged by a 1000 V source, is discharged through a photographic flash unit. The capacitor delivered an average power of 1000 W and the flash lasts 0.040 s. What is the estimated capacitance of the capacitor a. 40 x 10-6 F b. 20 x 10-6 F c. 60 x...
  34. D

    Relation between capacitance and frequrency

    what is the relatiob between capacitance and frequency?.. Can anyone please me in simple words..couse i m confused about it, in some sites they says its proportional..and other says inverse...:(
  35. R

    Estimate its capacitance to earth

    Homework Statement A telephone wire of diameter 1mm is suspended 10m above the ground. Estimate its capacitance to Earth per unit length Homework Equations The Attempt at a Solution I've tried to solve the problem by using the formula for a cylindrical capacitor and got the answer...
  36. E

    Why is the charge split evenly between the two capacitors in a series?

    Homework Statement My question is mainly just conceptual not an actual homework problem. So let's say there are two capacitors in a series. Their C equiv= (1/c1 + 1/c2) So to find the charge on either c1 or c2, you would do C equiv*V=Q. Why is it that Q is the charge for each of...
  37. B

    Capacitance of a parallel plate capacitor with 2 dielectrics

    Homework Statement Find the capacitance of the parallel plate capacitor with 2 dielectrics below. Given that the parallel plate capacitor has area A = WL and the separation between plates is d. The Attempt at a Solution My method is to use integration. First solve for two small...
  38. A

    Capacitance of a forward biased diode

    How does a forward biased p-n junction display capacitive effect? I understand that a reverse biased diode can exhibit capacitance with the depletion region acting as a dielectric, but in a forward biased junction, depletion region does not exist,so how does it show capacitive behaviour?
  39. B

    Capacitance of three coaxial metal tubes

    Homework Statement Find the capacitance per unit length of three long coaxial metal tubes, with radii a < b < c . A wire connects the innermost and outermost tubes (radii a and c). Homework Equations The Attempt at a Solution I'm a little confused as to how I should set this...
  40. J

    Capacitance of concentric metal spheres

    Homework Statement A solid metal sphere has a radius of 10.0 cm and a concentric metal sphere has a radius 10.5 cm. The solid sphere has a charge 5.00 nC. (a) Estimate the energy stored in the electric field in the region between the sphere. Hint: you can treat the pheres essentially as...
  41. T

    How Does a Capacitor Behave in a Simple Circuit?

    A working circuit contains a 3V power supply, a 5000uF Capacitor, and a 200ohm resistor. a) What is the time constant for the circuit? b) What is the charge in coloumbs on the capacitor at a potential difference of 3V? c) How much energy is stored on the Capacitor at a potential difference...
  42. C

    Equivalent Capacitance of a network

    Homework Statement To find the equivalent capacitance of the following network (image attached at the end of the post). The blue capacitors have capacitance of 2C and the black ones have a capacitance of C. Homework Equations Q = CV where Q is the charge on a capacitor, C it's capacitance...
  43. V

    Digital Capacitance meter using microcontrollers

    Hey friends, I am trying to make a digital capacitance meter by using a HSC08 microcontroller chip (MC9S08QG8). Unfortunately this is one of those courses where one doesn't know anything because of lack of pre-requisites. Anyhow (enough complaining) what I have been trying to do is the...
  44. N

    Capacitance Homework: Show C=(ϵ_0 A)/d

    Homework Statement If the electric field is E and div.ξ=ρ/ϵ_0 where ξ=-dV/dx show that the capacitance is C=(ϵ_0 A)/d because i don't know how to type it I'm representing ξ with E for the electric field The Attempt at a Solution I've done this as two two...
  45. J

    Help with understanding capacitance

    my book does not fare well in explanations, thus I have a few questions if the capacitor plates hold energy, how does this energy become released in certain applications? what are some common applications of capacitors? if the voltage difference in parallel capacitors are the same, why is...
  46. fluidistic

    Calculate the capacitance

    Homework Statement Calculate the capacitance of a spherical capacitor such that its center and up to R_2 is vacuum. Then from R_2 up to R_3 there's a dielectric material of constant \kappa _2. Then from R_3 up to R_1 there's a material of constant \kappa _1. Homework Equations None...
  47. L

    Improving power factor using capacitors (what capacitance?)

    Homework Statement The power factor of a circuit has to be improved from 0,6 to 0,9 by adding capacitors. Originally, it only has an inductor L, a resistence R and a generator V. The circuit requires an active power of 3000 VA, and that should remain the same. Vef = 220V. The Attempt at...
  48. T

    What is the capacitance of the circuit problem.

    Homework Statement An LC circuit consists of a 1mH inductor. The capacitor is fully charged. After 2.1 ms, the energy in the capacitor is ½ of its original value. What is the capacitance of the circuit? Homework Equations (Q/C)+ L(d^2Q/dt^2)=0 d^2Q/dt^2= -(1/LC)Q The Attempt at...
  49. S

    Capacitance Formula Explanation: Discover the Origin and Meaning

    Dear Sir, I would like to know the origin for the following formula for capacitance. A textbook which describes this formula will be better so that I can buy it. Please see the attachment for better understanding. C= (2*pi*Epsilon o* Epsilon r* l)/ln(2*h)/r...
  50. A

    Solving 0.2uf & 0.1uf Capacitance Connected in Series to a 9V Battery

    Homework Statement A 0.2uf and 0.1uf capacitor are connected in series to a 9V battery. Calculate a) the potential difference across each capacitor and b) the charge on each. Homework Equations Not sure The Attempt at a Solution I don't even know where to begin, sorry :(
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