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. D

    Question Regarding the Distributed Capacitance of a Winding

    First, take the case of a winding with a regular capacitor connected to both terminals of the winding. When a moving magnetic field induces a voltage therein, the cap becomes charged and a current flows in the conductor while cap is charging, producing a magnetic field. Next, take the case of a...
  2. hackhard

    Can Wood Be Used as a Capacitive Sensor in an LM358 Op-Amp Circuit?

    i connected pin 4 and 8 to gnd and +5 v respectively .when pin 2 touches wood, or hand pin 1 is high. i understand skin is capacitive , but wood?
  3. hackhard

    Fully discharge a metal plate?

    is there any way to fully neutralize a charged metal plate ? connecting it to metal body of relatively huge capacitance will make its charge negligible but not zero? is there any other way?
  4. Kfir Dolev

    Combine capacitors and inductors to be frequency independent

    Is it possible to combine (possibly infinite) capacitors and inductors to get a total impedance which is independent of frequency. If so, how?
  5. S

    Sample smaller than parallel plates in capacitance setup

    Hi, My setup for measuring parallel plate capacitance consists of an LCR meter and a parallel plate test fixture. The area of the parallel plates is much larger than the area of the samples I am measuring. From what I've seen in literature, and my physics classes, the sample area should be...
  6. D

    How to Convert Capacitance in cm to Farads for a Transmission Line Experiment?

    Homework Statement Basically I have to convert from Gaussian Units in a Transmission Line Experiment. I don't get what the point of expressing Capacitance in cm is and I find this very confusing. Homework Equations The equation I've been given is C'=Q'/V=w/4πh, (1) where the prime denotes "per...
  7. J

    Can someone explain the negative sign in the capacitance formula?

    Hi everyone, 1. Homework Statement It takes 18 J of energy to move a 0.30-mC charge from one plate of a capacitor to the other. How much charge is on each plate? Homework Equations -We/q = ΔV C = Q/V The Attempt at a Solution -18J/0.3.10-3 C * 15.10-6 F = Q Q = -0.9 C The problem is...
  8. G

    Capacitance Homework Help: Equations, Tips and a Helpful Datasheet

    Homework Statement Ive got these questions and I am not quite sure where to start. if someone could point me to the equations to use for Q1 and Q4, id appreciate it. As for Q2 and Q3, I need help with how to determine if its in series or parallel and how to properly add them. Homework...
  9. H

    What is the value of capacitance C?

    Homework Statement The equivalent capacitance of the capacitors shown in Figure 21-41 is 9.22 microFarads. Find the value of capacitance C.Homework Equations Ok so this has to deal with Ceq, and it is a combonation of series and parallel circuits. Knowing this, I can use for series...
  10. 1

    Discharging capasitors and it's general electrical propertie

    It's just that I've never properly sorted this confusion of mind, so i would like to sort it out so it 2 different capacitors are in series with a resistor: (let's say 0.15F, 0.45F, 150M ohm) 1. both current and voltage drops exponentially 2. capacitance on capacitors remain same over time? 2...
  11. C

    Capacitance vs. Resistance Proof

    Can somebody confirm if this is correct? I'm trying to use a wye-delta transformation on capacitors to solve for equivalent capacitance, but to be super-precise, I want to put capacitance in terms of resistance. I = C*(dV/dt) V = IR, so I = V/R V/R = C*(dV/dt) (V*dt) = R*C* dV Integrate both...
  12. goonking

    Capacitance and Inductance questions

    Homework Statement Homework EquationsThe Attempt at a Solution Here are two questions. I'm stuck on question 1, and need someone to check question 2. For the first question I see C2, C3, and C4 in parallel, so Ceq of those 3 are C2 + C3 + C4 = 3C but now how do I continue? seems like the 3...
  13. K

    Would there exist capacitance if the speed of light....

    ...were infinite? In other words: is capacitance a reflex of the finitude of the speed of light? Please elaborate. Thanks a lot in advance.
  14. kolleamm

    Wiring a capacitance sensor to a controller

    I'm trying to make a simple capacitance sensor to use with my controller. Do I need two resistors? And where do I place them? I drew a circuit diagram of what I'm thinking so far.
  15. Narayanan KR

    What is the estimated conductivity of Earth according to the USGS?

    1. In above image an insulated metal plate has been placed inside Earth (soil), and an voltage is applied between plate and Earth groung what will be the capacitance here?? 2. one plate is the metal and the other plate is entire earth, so it is a big assymetrical Capacitor or What? all...
  16. V

    How Does Introducing Different Dielectrics Affect Capacitance?

    Homework Statement A parallel plate capacitor having plates of area S and plate separation d, has capacitance C1 in air. When two dielectrics of different relative permittivities ( ε1=2 and ε2=4) are introduced between the two plates as shown in the figure, the capacitance becomes C2. the ratio...
  17. I

    Potential Difference Across Capacitor Regions and Work Done

    Homework Statement A parallel plate capacitor of area A and separation d is fully charged so that the charge on the top plate is Q. The battery is disconnected and two dielectric slabs of dielectric constant k are inserted with an air gap between them. Each region takes up one third of the...
  18. A

    No variation in capacitance with body load in comsol

    Hi Friends, I am trying to find variation in capacitance between two plates with applied body load. I defined an air box around the plates. Now I'm trying with electromechanics physics for applying body load and finding capacitance. I defined the two plates as linear elastic material. Applied...
  19. Y

    Why does a capacitor discharge slowly?

    Homework Statement Explain why a capacitor will discharge, although very slowly when there is high internal resistance? Homework Equations V=IR Q=V/C The Attempt at a Solution I am confused slightly by internal resistnace. Does that refer to regular resistance. From that high internal...
  20. K

    New capacitance when dielectric material covers bottom half

    Homework Statement The plates of a parallel plate capacitor of capacitance C0 are horizontal. Into the gap a slab of dielectric material with K=2 is placed, filling the bottom half of the gap between the plates. What is the resulting new capacitance? Homework Equations C = Q/V = κɛ0A/d The...
  21. W

    Solving voltage divider involving capacitors

    Homework Statement Given ZV1 = ZV2 = 100Ω, ZCCV = ZCV = 2000/j = -2000j, and VV/VC = 0.04789. I'm trying to get the given VV/VC result. Homework Equations I know that 1/ZV = 1/ZV1 + 1/ZV2 + 1/ZCV VV = ZV / (ZV + ZCCV) * VC The Attempt at a Solution 1/ZV = 1/ZV1 + 1/ZV2 + 1/ZCV = 1/100 +...
  22. H

    2 microstrip capacitance with dielectric

    Hi guys, I've been trying to figure out how the capacitance of two microstrips, separated a distance d, are influenced by the presence of a dielectric. The dielectric will be a height (h) above the strips and has restricted movement. Using the electric potential I am able to derive a function...
  23. Luek

    Energy stored in a LC circuit

    Homework Statement Energy in the circuit remains constant. When the current is flowing, the energy stored is all stored in the inductor. When the current stops flowing, it is because all the energy is stored in the capacitor. Find the fraction of energy stored in the inductor when half of the...
  24. R

    Capacitance Question - Total Energy, Potential Difference

    Homework Statement Two identical parallel-plate capacitors, each with capacitance 10.0 μF, are charged to potential difference 50.0 V and then disconnected from the battery. They are then connected to each other in parallel with plates of like sign connected. Finally, the plate separation in...
  25. recursive

    Capacitor Energy With Different Dielectrics

    Homework Statement Two parallel plates are connected to a battery which maintains a constant potential difference of 12.0 V. The distance between the plates can be varied and there are three plates of different dielectric materials available: rubber (0.40 mm, κ = 6.7), glass (4 mm, κ = 7) and...
  26. E

    Dielectric in a capacitor in equilibrium

    Homework Statement Consider the situation shown in the figure. The width of each plate is b. the capacitor plates are rigidly clamped in the lab and connected to a battery of emf E. All surfaces are frictionless. Calculate the value of M for which the dielectric will stay in equilibrium...
  27. E

    Solving Potential Diff. Between Points a & b: Part (b) & (c)

    Homework Statement Find the potential difference between points a & b. Diagrams attached below. I have doubts in part (b) and (c). Homework Equations Q = CV, kirchhoff's laws The Attempt at a Solution I honestly don't get how the current can flow in opposite directions in part(b). if we...
  28. vetgirl1990

    Understanding Capacitance and Impedance in Series Circuits

    Homework Statement This is not necessarily a homework question, just a conceptual one. I was reading up on capacitors, and an example said that a 25uF capacitor has TWICE the impedence of a 50uF capacitor. Can someone explain what this means? I would have thought that a capacitor with higher...
  29. kostoglotov

    Thevenin Equiv with just Capacitors

    Homework Statement imgur link: http://i.imgur.com/TWB5rRu.png Homework EquationsThe Attempt at a Solution I can find the equivalent capacitance, 10F, but I'm not really sure how to find the equiv. Thev. voltage. I figured that with the voltage source present, half the voltage will be lost...
  30. W

    Capacitors in Series and Parallel

    Homework Statement Fig. 25-39 shows a 12.0 V battery and four uncharged capacitors of capacitances C1 = 1.00 µF, C2 = 2.00 µF, C3 = 3.00 µF, and C4 = 4.00 µF. If only switch S1 is closed, what is the charge on (a) capacitor 1, (b) capacitor 2, (c) capacitor 3, and (d) capacitor 4? (figure at...
  31. gracy

    What Happens to the Voltage Across a Capacitor When the Plates are Separated?

    A capacitor has a capacitance C and is charged using a battery. After disconnecting the battery the two plates are separated until they are twice as far apart as originally. What happens to the voltage across the capacitor? I think answer should be voltage increases . As battery has been...
  32. B

    Capacitance in a Complicated Mixed Circuit

    Homework Statement Find the capacitance of the circuit if C1 = 10.3, C2 = 41.1, C3 = 95.5, C4 = 299.9, and C5 = 525.7, with all the units being in microfarads. Homework Equations 1/Series Ctotal = 1/c1 + 1/c2 ... Parallel Ctotal = C1 + C2 + ... The Attempt at a Solution I tried to combine all...
  33. B

    Calculating Capacitance in a Mixed Circuit?

    Homework Statement Please calculate the equivalent capacitance if C1 = 10.3 microfarads, C2 = 41.1 microfarads, and C3 = 95.5 microfarads in the diagram. Homework Equations 1 / Ctotal for Parallel = (1/C1) + (1/C2) + ... Ctotal for Series = C1 + C2 + ... The Attempt at a Solution So I tried...
  34. Phy_TR

    Question regarding Energy in Circuits with Capacitors

    Hey everyone! I have a doubt that relates to energy stored in different capacitors (when they're in a circuit). I understand how in capacitors connected in parallel, the capacitor with the biggest capacitance stores the most energy because: E = 1/2 CV2 So naturally the bigger the capacitance...
  35. Marcin H

    Equivalent Capacitance and Voltage Across

    Homework Statement Homework Equations Series: 1/Ceq=1/C1+1/C2 Parallel: Ceq =C1+C2 The Attempt at a Solution So to find the equivalent resistance do I short (put a wire through the open terminals) the circuit and start adding up the capacitors? Once I do that do I just create a new circuit...
  36. vetgirl1990

    Equivalent capacitance of circuit

    Homework Statement I am having trouble simplifying a circuit of capacitors that I was given. I am asked to determine the equivalent capacitance of the combination below (attached photo). Homework Equations Capacitors in series: 1/Ceq = 1/C1+1/C2+... Capacitors in parallel: Ceq = C1 + C2 +...
  37. V

    Please help me with this doubt from electrostatics

    in deriving the capacitance of a parallel plate capacitor why do we take the field between the capacitors as σ/∈ and not 2σ/∈,when there are two plates and in the region between the capacitors, field must add up so shouldn't it be 2σ/∈?
  38. kostoglotov

    Marginal inductance and capacitance of a real wire?

    Speaking just of long straight wires, not loops, coils or solenoids. Ideal wires have no resistance. Of course they would have no capacitance. And real wires have some very small amount of inductance, no? Firstly, would an ideal wire have some ideal nonzero inductance? Why/why not? After all...
  39. D

    How to derive formula for capacitance of spherical capacitor

    Please explain in detail how to derive formula for capacitance of spherical capacitor?
  40. marciokoko

    Electromagnetic radiation vs Capacitance and Inductance

    Im trying to compare, visually in my head, the difference between transmitting data vs something like radio or microwaves vs how inductance and capacitance. I understand that the way the wave is modulated for example, AM or FM, can be interpreted as 0s or 1s. Im trying to compare this to...
  41. gracy

    Equivalent capacitance problem

    Homework Statement If 100 volts of potential difference is applied between a and b in the circuit .Find the potential difference between c and d. 2. Homework Equations For Equivalent Capacitance in series ##\frac{1}{C}##=##\frac{1}{C_1}##+##\frac{1}{C_2}##For Equivalent Capacitance in...
  42. gracy

    Equivalent capacitance homework problem

    Homework Statement Find the equivalent capacitance of the combination between A and B in the figure. Homework Equations For Equivalent Capacitance in series ##\frac{1}{C}##=##\frac{1}{C_1}##+##\frac{1}{C_2}##For Equivalent Capacitance in parallel ##C##=##C_1##+##C_2##... The Attempt at a...
  43. gracy

    Equivalent capacitance, circular arrangement

    Mentor's note: Post moved to new thread since it's a new problem. One more problem Homework Statement Find the equivalent capacitance between A and B. Homework Equations The Attempt at a Solution I think all the Capacitors are in series but according to my book only three capacitors are in...
  44. gracy

    Equivalent Capacitance: Where to Place the Equivalent Capacitor Between A and B?

    Homework Statement I need to find equivalent capacitance between A and B. Homework Equations ##C##=##C_1##+##C_2##... It was for parallel connection. ##\frac{1}{C}##=##\frac{1}{C_1}##+##\frac{1}{C_2}##... It was for series connection. The Attempt at a Solution I tried to solve the problem...
  45. A

    Self-Capacitance of an Insulating vs. Conductive Sphere

    Hey, guys! I had a brief electromag question. If capacitance is defined as the amount of charge that can be stored in a capacitor per unit of potential difference, then technically can capacitors made from insulators still have a capacitance? The problem I'm considering is calculating the...
  46. L

    A couple Q's about parasitic capacitance in an inductor

    First question; I was measuring capacitance in various guitar pickup coils that have the same diameter of wire with an Extech LCR Meter, and it seemed to be that the more winds, the higher the inductance as expected, but I would get lower capacitance readings, too, both at 120 kHZ and 1k Hz...
  47. Weightofananvil

    Finding Voltage from capacitance, current and time

    Homework Statement The question asks the current through a 2uF capacitor is show. sketch the capacitor voltage Vc between t = 0ms give that the voltage at t=0ms is 0V. Your graph must be correctly scaled with numerical values. There is a graph included showing a current rise for 0-2...
  48. G

    Is My Capacitor Fully Charged?

    suppose i have a circuit,in which a capacitor and resistances are connected in series with a DC voltage source. then after a long interval of time we know that capacitor will have same voltage across its terminal as that of DC voltage source and no current flow in the circuit. so, can i say at...
  49. A

    Capacitors and Calculating for Capacitance involving Plates

    A sheet of gold foil (negligible thickness) is placed between the plates of a capacitor and has the same area as each of the plates. The foil is parallel to the plates, at a position 1/2 of the way from one to the other. Before the foil is inserted, the capacitance is 7.9μF. What is the...
  50. N

    Capacitors in a series circuit W/ one dielectric

    Problem: When a dielectric slab is inserted between the plates of one of the two identical capacitors in Fig. 25-23, do the following properties of that capacitor increase, decrease, or remain the same: (a) capacitance, (b) charge, (c) potential difference (d) How about the same properties of...
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