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. K

    Capacitance (Potential Difference and Energy)

    Homework Statement In the figure C1 = 10.0 μF,C2 = 20.0 μF and C3 = 27.0 μF. If no capacitor can withstand a potential difference of more than 130 V without failure, what is (a) the magnitude of the maximum potential difference that can exist between points A and B and (b) the maximum energy...
  2. genxium

    How does surface capacitive touchscreen work in detail?

    It's pretty easy to find the "working principle" of surface capacitive touchscreen like http://www.elotouch.com/Technologies/SurfaceCapacitive/howitworks.asp Roughly speaking, I know that A surface capacitive touchscreen has a conductive layer to which some electrodes are attached The...
  3. A

    Capacitance of 3 Hollow Cylinders

    Homework Statement I'm trying to find the capacitance of a system of 3 concentric hollow cylinders. The first cylinder has radius R, the second radius 2R, the third radius 3R. Cylinders 1 and 3 are connected by a wire. In total, they have charge +λ, and the second cylinder has charge -λ. The...
  4. P

    Dielectric inserted in parallel plate capacitor

    Homework Statement When a dielectric is partially inserted between two plates of a capacitor that is disconnected, what will be it's motion? Also what will be its motion if the capacitor is connected to a battery? Homework EquationsThe Attempt at a Solution For the first part. I think the...
  5. P

    Capacitance of Connected Spheres: Finding dq1/dt

    Homework Statement 1) Two solid conducting spheres have radius r1 = 3 cm and r2 = 42 cm. The two spheres are connected by a thin conducting wire. Assume: - the wire is very long and thin, with negligible surface area, so it does not affect the capacitance of the system. - the switch is...
  6. G

    Capacitance per length unit of coaxial cable

    Homework Statement Coaxial cable has double layered dielectric. a=2mm, b=4mm, c=8mm, εr1=3, εr2=4.5, Uab=1000V 1. Calculate capacitance per unit length of a cable. 2. Calculate inner and outer charge densities of a cable. 2. Homework Equations Gauss law C'=Q'/U σ = nD σp = nP 3. The Attempt...
  7. G

    Capacitance per unit length - two cylindrical conductors

    Homework Statement Two cylindrical conductors, of distance between them d and radius a (a<<d), have dielectric layer of relative permitivitty εr and thickness a. Calculate capacitance per unit length of this system. Homework Equations Capacitance per unit length, C'=Q'/U Gauss law, cylindrical...
  8. R

    Electricity and Magnetism RC Circuit Conceptual Questions

    Edit: sorry i realized i posted this in the wrong section. Can some move it to hw? Thank you. Hey i was studying intro E&M recently and i started learning about RC circuits. Up until this point everything, concept wise, has made sense for me. Then I learned how RC circuits work, and the math...
  9. S

    Resistance and Capacitance of Two Metal Plates "Touching"

    Homework Statement Two conductive metal plates are bonded together. Upon bonding, an oxide layer forms between them. The oxide layer is negligible thus the overall resistance of the plates is small. However I am having trouble finding this overall resistance, i.e., trying to find Cp and Rp...
  10. N

    How to measure this capacitance using a capacitance meter

    I posted this picture in another thread but I now have a specific question for the capactance measurement between both transducers (the middle C). How can I measure the capacitance of this using an LCR meter. Where should the leads be put and what are some averages of a capacitance I should be...
  11. A

    Tapped Transformer With a Capacitor

    The question is the following: a) Determine all load voltages and currents b) Determine the resistance reflected into the primary. Answer a) R(l): 35V, 2.91A, X(cl): 15V, 1.5 A The answer to part (b) is apparently 34.5 Ohms. How did they arrive at these answers? I am not sure...
  12. N

    Capacitance of Transducers with a water-filled beaker

    I am making a sonoluminescence experiment and part of it is two transducers touching a glass beaker with water. However, I don't know exactly how to measure the capacitance. With the picture attached, the places marked in green with "C?" written on them are where I was told there would be...
  13. bananabandana

    Double Paralell Plate Capacitor with Dielectric

    Homework Statement Please see attached. Homework EquationsThe Attempt at a Solution I get a result of ## \alpha =\frac{1}{2} ## for part a) - which I think is correct. I'm stuck however on part b) - with the dielectrics. I have that the field in the region where the dielectric is ##E_{1}##...
  14. E

    Solving Capacitance Problem: Identical Answers Explained

    Homework Statement Why are both these problems identical? In both cases, c>b>a. Q1 Three concentric conducting shells A, B and C of radii a, b and c are arranged as shown. A dielectric of dielectric constant K is filled between A and B. Find the capacitance between A and C. Q2 A spherical...
  15. S

    Capacitance between two wires-not straight

    Hi, I would like to know how to calculate capacitance between two wires which are parallel but not necessarily straight.for instance the two wires might be parabolic..I would like to calculate for such general structures.i.e) two parallel wires that are not necessarily straight. Thanks in advance
  16. S

    Capacitance between two parallel wires not straight

    Hi, I would like to know how to calculate capacitance between two wires which are parallel but not necessarily straight.for instance the two wires might be parabolic..I would like to calculate for such general structures.i.e) two parallel wires that are not necessarily straight. Thanks in...
  17. L

    Capacitance, Potential difference

    Homework Statement Find the potential difference Va-Vb between points a and b. Homework EquationsThe Attempt at a Solution Each capacitors left plate will have difference amount of positive charge and right plate will have negative charge. After that i don't know how to proceed.
  18. F

    Estimate the Dielectric Breakdown of Air

    Homework Statement A capacitor is charged to V = 5 kV. A wire is then connected to one plate and is slowly connected to the other. Before actually touching the plate, a spark of length l = 1 cm shoots from the wire to the plate. Using this information, estimate the dielectric breakdown of air...
  19. S

    Is it possible to charge photons?

    I was wondering if there is any way to electrically charge photons?
  20. Suraj M

    Capacitance, unbalanced wheatstone.

    Homework Statement Find capacitance across AB So i divided the current into q1 and q2 as shown. then i assumed that q₂>q₁. Homework Equations ##C=\frac{Q}{V}## I don't know anything after KVL KCL and loop law. The Attempt at a Solution So i split the current at T into q₃ and q₂-q₃ by the...
  21. AnkurGarg

    Can we calculate capacitance of an isloated meta plate?

    i have done the capacitance of an isolated charged sphere (=4 pi E R).but can we find capacitance of an isolated ,infinite charged plate too??how?
  22. S

    Capacitance of an isolated spherical conductor

    So it says here that a conducting sphere of radius R with a charge Q uniformly distributed over its surface has V = Q/4πεR , using infinity as the reference point having zero potential,,V (∞) = 0. This gives C = Q/|ΔV| = Q/(Q/4πεR)=4πεR. Does ,V (∞) mean that you are taking the potential of a...
  23. R

    What is the capacitance of the system between P and Q?

    Homework Statement Four metallic plates each of surface area S are placed at some distance from each other as shown in figure. Then capacitance of the system between P and Q is A. ## \frac{ε_0S}{3d} ## B. ## \frac{ε_0S}{2d} ## C. ## \frac{3ε_0S}{2d} ## D. ## \frac{3ε_0S}{d} ## Homework...
  24. P

    Capacitance of four metal plates

    Homework Statement Four identical parallel metal plates with area $$S$$ and distance between each two $$d$$ are located in air. The external plates are connected by wire. What is the equivalent capacitance between A and B? This is problem from Irodov.Homework Equations $$q=UC$$[/B]The Attempt...
  25. M

    Capacitance of a spherical capacitor?

    1-capacitance of sphere with radius r : 4πεr 2-capacitance of 2 concentric spheres with inner radius a and outer radius b : 4πε(a.b/b-a) 3-when the outer the outer sphere is earthed it gives the same capacitance as in number 2 4-when the inner sphere is earthed it gives the sum of capacitances...
  26. O

    Gold Electrodes and Capacitance

    Homework Statement Well its 2 questions: Immersing a pair of gold electrodes in a saline solution in the presence of a fixed voltage causes the capacitance to: IncreaseDecreaseRemain unalteredIncrease briefly and then return to its original value Immersing a pair of gold electrodes in a...
  27. Marcis Rancans

    Parasitic capacitance influence on lead voltage

    V' changes over time. How can I determine the influence from parasitic capacitance (~50 pF) on V? R wire ~ 100 ohm. If I need to read data (V' can change rapidly) from this at 50hz, does it influence reading accuracy? How can I determine values from which I need to be careful when reading data...
  28. mooncrater

    Two oppositely charged wires and their capacitance

    Homework Statement Two long wires with the same cross section are arranged in air PARALLEL to each other . Assume both have opposite charge densities +λ and -λ. The distance between the axis of the wire is η times larger than the radius(r) of wires cross section . What would be the capacitance...
  29. A

    Solve Unknown Capacitance from Series Capacitors & Battery Charge

    Homework Statement A 197-pF capacitor is connected in series with an unknown capacitor, and as a series combination they are connected to a 25.4-V battery. If the 197-pF capacitor stores 143 pC of charge on its plates, what is the unknown capacitance? Homework Equations Q = C*V (charge stored...
  30. A

    Calculating Effective Capacitance of a 3-Plate System with Varying Distances

    Homework Statement Three conducting plates, each of area A = 3.5 cm2, are connected in such a way that the center plate can be moved between the two fixed outer plates. If the distances between the outer plates and the center plate are d1 = 0.35 mm and d2 = 0.65 mm, find the effective...
  31. GoldWing

    Capacitor Network: C1,C2 & C3 in Series & Parallel

    Homework Statement Capacitors C1 and C2 are connected in series and this combination is connected in parallel with capacitor C3. Homework Equations Q=CV Ceq=C1+C2+C3 equivalent capacitance for capacitors in parallel 1/Ceq=1/C1+1/C2+1/C3 equivalent capacitance for capacitors in series The...
  32. J

    Marx Generator Capacitance and Charge Storing Question

    Hello, I have had a question on my mind that a generic google search has so far been unable to answer for me. I know that marx generators multiple voltage by cascading capacitor discharges so that you get a final voltage of n x Vinit - where n is number of capacitors and Vinit is your initial...
  33. S

    Capacitor with magnetic dielectric

    Odd question, would anybody know what would happen if a parallel plate capacitor (good conductor) had a soft magnetic material (fairly poor conductor) as a dielectric? What would be the DC/AC electrical characteristics of this?
  34. Destroxia

    Capacitance of a Spherical Capacitor?

    Homework Statement The plates of a spherical capacitor have radii 51.8 mm and 55.0 mm. (a) Calculate the capacitance. (b) What must be the plate area of a parallel-plate capacitor with the same plate separation and capacitance? (givens) a = 51.8 mm b = 55.0 mm d = 3.2 mm ε0 = 8.85E-12...
  35. PratyashAstro

    How Do You Measure Absolute Capacitance with a Ballistic Galvanometer?

    My question is : What is the absolute capacitance?In my electricity lab, I was assigned to prepare a report on measuring absolute capacitance of a capacitor using ballistic galvanometer. I am familiar to the term self capacitance but not the absolute one. My mentor asked me to find it out by...
  36. M

    Charge flowing through/across a Capacitor?

    Homework Statement Dry air has a resistivity of about 3*10^13 Ωm. A capacitor has square plates 10cm on a side separated by 1.2mm of dry air. The capacitor is charged to 250V. Assuming the potential difference does not change as the charge flows, what fraction of the charge will flow across the...
  37. R

    Change in total energy of capacitance

    Homework Statement When an initially charged capacitor is connected in a circuit with an uncharged capacitor, why would the total energy of both capacitors be less than the initial energy of the charged capacitor? Homework Equations This was a question asking for an explanation, so I'm not...
  38. B

    Homemade Supercapacitor Capacitance test --

    Hi all, My friend and I have created a homemade supercapacitor. It can power a LED light and small alarm clock. We charged it up to 1.5 Volts. I was wondering if anyone knew how to find the exact capacitance in Farads, any help will be appreciated. I have added the steps that we have taken in...
  39. S

    Find the capacitance from 2 energies stored, not knowing voltage

    Hi there, new to this website so please bare with me if I break any rules and let me know for future. I am a maths and physics tutor but occasionally I come across a question I just cannot work the answer for. In this case I know the answer but I can't get to it. 1. Homework Statement...
  40. D

    How to switch on and off capacitance on a piece of conducting material?

    I don't know if this is the right way of describing it, but I need to activate an area of capacitance on a touchscreen by applying some current to a piece of conducting material like tinfoil. I am needing to remotely turn pages on a touchscreen ereading device with a microcontroller, and this...
  41. 1

    What is the equivalent capacitance of this formation?

    http://<a href="http://imgur.com/UwulXqg"><img src="http://i.imgur.com/UwulXqg.png" title="source: imgur.com" /></a>[/PLAIN] 1. Homework Statement Find equivalent capacitance of this formation from a to b, C1=4.1 uF, C2=6.86 uF, C3=5.03 uF, C4=5.92 uF http://imgur.com/UwulXqg Homework...
  42. N

    Capacitance of an asymmetrical parallel plate capacitor

    I've been looking all over and to my great surprise have not be able to find an answer to this question. What is the capacitance between to parallel plates if one plate is larger than the other? For example, the capacitance between a 1" diameter plate and a 4" diameter plate. Thanks.
  43. K

    Inserting metal to parallel plate capacitor

    If there is a parallel plate capacitor with plate area, A, plate separation, d, and a slab of copper of thickness, b, is inserted exactly halfway between the plates. How to find it's new capacitance, C', and new potential difference, V' ? Answer: V' = E(d-b) and C' = εA / (d-b) I...
  44. A

    Three plate capacitor and charge distribution

    In the figure given the capacitance for the two pair would be same. Can anybody please explain me how the charge would be distributed across each plate? I think that the middle plate being isolated should have +Q on one side and -Q on other, but how would be the charge distribution about the...
  45. jake jungle

    What is the voltage across C2 in this circuit?

    Homework Statement Find the pd accros C2 Homework Equations Q=CV The Attempt at a Solution i find C23 = (C2C3)/ (C2 + C3) then find Q23 = V x C23 Q23 = Q2 = C2V2 => V2 i used this and got V2 = 15V , however the answer is 20V , and i even didnt use the data of C1 . am i wrong somewhere ?
  46. H

    Proofing Capacitance: Understanding Electrostatics

    Hello everyone, I'm an engineer, and though I highly doubt I'd use this in my mechanic courses...I do truly want to understand what it is I'm putting into application here: So I'm going to explain my understanding of capacitance, be prepared for an extensive reading. I'm doing this because I...
  47. G

    Where does inductive reactance go?

    Lenz law states the induced emf in any circuit is always in a direction to oppose the effect that produced it. So my question is, 1. Does this mean xl creates a difference in potential and actual electrons flowing in the opposite direction or just oppose the current direction that created it...
  48. E

    Calculating Capacitance for Optimal Light-Bulb Performance

    Homework Statement Imagine that you have a light-bulb that has a resistance of about 10 ohms and that can tolerate a maximum voltage of 3 volts. Imagine that you want to connect this to a charged capacitor large enough to keep the bulb glowing reasonably brightly for more than 10 seconds...
  49. EinsteinKreuz

    Vacuum Tube & Thyratron plate capacitance

    What factors determine the anode plate-to-grid capacitance? I know that the capacitance of a regular capacitor is determined by the surface area of the plates and is inversely proportional to the distance between them. Does this hold for vacuum tube grids and anode plates? The reason I'm asking...
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