Capacitance Definition and 903 Threads

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. P

    What will trigger a capacitance touch sensor?

    I've got an iPod Touch and am trying to create an electrical circuit that, when turned on, will trigger the touch sensor in a specific spot (the spot that whatever component of the circuit provides capacitance is resting on, I guess). For the life of me, I cannot figure out how to trigger a...
  2. S

    Capacitance and distance between plates

    Homework Statement The question is at http://dl.getdropbox.com/u/175564/capacitance.JPG Homework Equations Q = CV C = \epsilon_{0} A / d The Attempt at a Solution The right solution is Q^2 / C that is (h). I know that C is proportional to the inverse of the distance between...
  3. J

    Are amp-seconds the same as capacitance?

    Homework Statement What is an amp hour? Are they saying that batteries in series maintain the same capacitance when they say amp-hours? When connecting your batteries in Series you are doubling the voltage while maintaining the same capacity rating (amp hours). Homework Equations None...
  4. R

    Calculation of capacitance of conductors

    Hello, I would like to get a method for calculation of capacitance of conductors. Let say, I have several conductors in space (2D). I know voltage everywhere between and on conductors, and I need to know the value of capacitance between 2 conductors among several. I'm looking for a general...
  5. D

    Deriving value of capacitance using E = V/d

    This formula (E = V/d) has been used to derive the value of capacitance...but how is this applicable?...this is for point charges and here we're dealing with plates. Any derivation to prove this is also applicable for plates?
  6. B

    About infinite inductance, zero capacitance

    Hi Friends, :smile: Two particularly abstract queries have surfaced in my mind (this seems to be a recurring problem with me! ) :biggrin: CASE 1 :- Infinite Inductance Let us consider an 'ideal' solenoid coil (zero resistance, pure inductance). It is connected to an AC supply of say...
  7. D

    Engineering Help with Circuits Capacitance & Inductance

    The images tell it all. Please help someone. http://image.cramster.com/answer-board/image/200941884006337564080009162509346.jpg on this diagram, the expression IL(t) should read I(t). http://image.cramster.com/answer-board/image/2009418845296337564112974787505740.jpg
  8. R

    Energy stored in capacitance of a squid

    Homework Statement The giant axon of a squid is 0.5 mm in diameter, 10 cmlong, and not myelinated. Unmyelinated cell membranes behave as capacitors with 1 microF capacitance per square centimeter of membrane area. When the axon is charged to the -70 mV resting potential, what is the energy...
  9. Hemmer

    Capacitance of Concentric Shells (different charges)

    Homework Statement A thin conducting shell of radius a has charge q. Concentric to this is another shell of radius b (b > a) with a different charge Q. How is the charge distributed and what is the capacitance of the two shells. No hint to the relative polarity of the charges is given...
  10. E

    Capacitance from a separated plate disk

    Homework Statement Ok, let's say that I have two plate disk, with an area of A and they are separated with a distance d apart from each other. I need to measure the half-life of exponential decay to calculate capacitance. I need to graph C vs x = 1/d, however how can I get C ?? Homework...
  11. J

    Capacitance problem (verify answer)

    Find the capacitance of a parrell plate capacitor in pF with A=0.04 m^2 distance of separation between plates = 2mm relative permittivity of dielectric = 6 and permittivity of free space = 8.854x10^-12 C = (6)(8.854x10^-12)(0.04) / 0.002 = 1.06248x10^-9 F in pF = 1062.48 pF Thanks
  12. Z

    How to Determine Fringing Capacitance in a Cylindrical Rod-Ground Plane Setup?

    Hi there I've encounted a problem in which I'm required to calculate the total capacitance between the end of a cylindrial rod and an infinite ground plane. I've attached a schematic drawing here. The gap between the rod if far smaller than the diameter...
  13. I

    Capacitance in Parallel and in Series

    Homework Statement In the FIGURE (shown below in "attempt at solution" section), find the equivalent capacitance of the combination. Assume that C1=10 nF; C2 = 5 nF; and C3 = 4 nF. Homework Equations For parallel capacitors: Ceq = C1 + C2 + C3 +... For series capacitors...
  14. M

    Why does adding a dielectric to a capacitor increase its charge storage?

    I was getting ready for a test in my physics class and have a question that I cannot figure out. Why does adding a dielectric to a capacitor cause the amount of charge that can be stored to increase. I understand what a dielectric is and how a capacitor works but am just not sure why this...
  15. S

    What is the correct voltage current eq. for a variable capacitance capacitor?

    What is the correct voltage - current equation for a variable capacitance capacitor? If the capacitor has a fix value, C, then the following expression holds: i(t)=C*dv/dt, where i is the current that charges the capacitor and v the voltage across it. If C=C(t) then i(t)=C(t)*dv/dt...
  16. C

    Capacitance: Equivalent C of a Network

    Homework Statement A capacitor network is shown below. http://img99.imageshack.us/img99/9967/79727282.gif What is the equivalent capacitance between points y and z of the entire capacitor network? Homework Equations In parallel: C = c1 + c2 + ... In series: 1/C = 1/c1 + 1/c2 +...
  17. M

    Capacitors in Series: Finding Equivalent Capacitance and Stored Charge

    1. Two capacitors, one 7.5 µF the other 15 µF, are connected in series across a 12-V battery. (a) Find the equivalent capacitance of the two capacitors. (b) Which capacitor stores more charge? Explain. (c) Find the charge stored on each capacitor. 2. Cequiv = ∑C C = Q/V 3. (a) Cequiv = 7.5µF +...
  18. J

    Electric potential and capacitance (semicircle problem)

    Homework Statement A uniformly charged insulating rod of length 14cm is bent into the shape of a semicircle (looks like a "C"). The rod has a total charge of (-7.50E-6C). find the electric potential at O, the center of the semicircle Homework Equations v= ke\intdq/r The Attempt at...
  19. B

    Defibrillator: charge, capacitance, power and current problem

    Homework Statement A heart dysfunction that can cause death is ventricular fibrillation. This is an uncoordinated quivering of the heart as opposed to regular beating. An electric shock to the chest can cause momentary paralysis of the heart muscle, after which the heart will sometimes...
  20. D

    Capacitance of circuit & RC time constant

    1. The problem statement http://s685.photobucket.com/albums/vv212/dewdr0p714/?action=view&current=5circuittakehometest1.jpg 1) Determine the equivalent capacitance of the circuit 2) How many RC time constants are needed for the charge on the capacitors to reach 75% of their final...
  21. H

    What is the Charge and Error in a Parallel Plate Capacitor?

    A capacitor with capacitance of C = 8.5±1 μF has a potential difference of V = 4.5±0.4 Volts. What are the charge on the capacitor and the error in this value, in μC? To find charge I just use Q = C*V correct? And how do I calculate the error? Thank you!
  22. B

    Lightning Strike: Examining Electric Field, Capacitance & Power

    Homework Statement Lighting strike: Storm clouds build up large negative charges that dwell in a charge center. Suppose a storm cloud has -25 C in a charge center located 10 km above the ground. The negative charge center attracts an equal amount of positive charge that is spread on the...
  23. P

    Capacitance (farads) am I doing this right?

    1. To make a parallel plate capacitor, you have available two flat plates of aluminum (area = 190 cm2), a sheet of paper (thickness = 0.10 mm, κ = 3.5), a sheet of glass (thickness = 2.0 mm, κ. = 7.0), and a slab of paraffin (thickness = 10.0 mm, κ = 2.0). Find each's capacitance; then enter the...
  24. P

    Calculate Capacitance of Flat Plate Capacitor

    the problem is: To make a parallel plate capacitor, you have available two flat plates of aluminum (area = 190 cm2), a sheet of paper (thickness = 0.10 mm, κ = 3.5), a sheet of glass (thickness = 2.0 mm, κ. = 7.0), and a slab of paraffin (thickness = 10.0 mm, κ = 2.0). Find each's...
  25. R

    How Does Inserting a Dielectric Affect Capacitor Voltage?

    Homework Statement An empty capacitor is connected to a 12.0-V battery and charged up. The capacitor is then disconnected from the battery, and a slab of dielectric material (K = 2.8) is inserted between the plates. Find the amount by which the potential difference across the plates changes...
  26. S

    Calculating Capacitance of Misaligned Plates

    Calculating the Capacitance of "out of alignment" finite parallel plate capacitors Homework Statement Homework Equations The Attempt at a Solution
  27. F

    How Do You Calculate Equivalent Capacitance in Complex Circuits?

    Homework Statement I need to find the equivalent capacitance between points 1 and 2 in the following circuit, with each capacitor having capacitance C: Homework Equations For n capacitors in series: 1/C_t = 1/C_1 + 1/C_2 + ... + 1/C_n For n capacitors in parallel: C_t = C_1 + C_2 + ... +...
  28. B

    Adding capacitance to an impedance

    Homework Statement If I have an impedance of: 10+j4 ohms and was told to add a capacitance of -j15 in parallel to this, would this make the impedance 10-j11 ohms??
  29. M

    What is the capacitance of a wire mesh parallel to a solid plane?

    Hi Guys, I seem to have forgotten most of EM, but I was hoping for some help on estimating the capacitance of a wire mesh/grid parallel to a solid plane. I'm guessing that it will follow the form of a simple parallel plate (with the appropriate area factor) if the geometries are such that the...
  30. F

    Doubling the cell diameter increases the capacitance by what factor?

    1.If I double the diameter of a cell, by what factor does the capacitance of the membrane change? 2. Help! The Attempt at a Solution
  31. T

    Capacitance and Resistance of a fluid

    Homework Statement A parallel plate air-spaced capacitor has a capacitance of 100pF. It is immersed in a fluid of resistivity 10 m. Calculate the resistance between the plates. Homework Equations C = \frac{\epsilon_0A}{d} R = \frac{\rho l}{A} The Attempt at a Solution I...
  32. J

    What is the equation for capacitance of a coaxial cable?

    Homework Statement http://smg.photobucket.com/albums/v68/jumpboyb/?action=view&current=scan0001.jpg" Pretty much looking for a general equation for capacitance of a coaxial cable per meter length. given: -a diagram (click link) -two radii -dielectric material -outer conductor is...
  33. N

    Atom Self Capacitance: Electron Energy Levels

    can electron energy levels just be considered the self capacitance of an atom?
  34. A

    Inductance and Capacitance per Temperature

    I am studying the relationship between transmission speed in a Transmission Cable and the temperature of the cable. This speed is given by (as per discussion in the Feynman lectures) v=\frac{1}{\sqrt{LC}} where L is the inductance per unit length C is the capacitance per unit length...
  35. Q

    Equivalent capacitance in complex circuit

    Homework Statement Hello, I was given this problem as a homework assigment. Each capacitor in the figure has capacitance C. What is the equivalent capacitance between points a and b? The answer is given in "C_eq/C" Homework Equations I used the...
  36. S

    Concentric Spheres Capacitance Question- How to use area

    Homework Statement I am trying to solve a question where I have three concentric metal shells at different radii. I am treating them as two capacitors in series. I would like to use the formula C= enot*area/distance between capacitors, but I have a few questions. Homework Equations...
  37. Q

    How Does Capacitance and Resistance Impact Pixel Voltage in Devices?

    Each pixel in a device acts like a parallel plate capaciter with the transistor updating at a voltage of 50 Hz. The change in voltage over 1 frame is 1 % and dominated by the electrical resistance of the material. The material has a constant dielectric of 20. Find the electrical resistivty of...
  38. K

    Does actual voltage (potential) affect capacitance?

    If I replace a 16 volt rated capacitor of 47uf with a 64 volt rated capacitor of 47uf, will the working capacitance be the same or does this depend on the specific design and materials of the capacitor? I'm thinking that the capacitance is proportional to the voltage applied and that the...
  39. C

    Calculating the capacitance problem

    Homework Statement If an uncharged parallel-plate capacitor (capacitance C) is connected to a battery, one plate becomes negatively charged as electrons move to the plate face (area A). The depth d from which the electrons come in the plate in a particular capacitor is plotted against a range...
  40. Z

    Determine the range of capacitance

    Homework Statement FM radio stations use radio waves with frequencies from 88.0 to 108 MHz to broadcast their signals. Assuming that the inductance in an LC-tuned circuit has a value of 7.60 x 10-7 H, determine the range of capacitance values that are needed so the antenna can pick up all...
  41. D

    Multiple electrodes - capacitance.

    Hey, I have a length of pipe. Inside the pipe walls I have placed four electrodes. If one is giving out x volts, and I know the dielectric constant of the material between them, is there an easy way of finding the capacitance on the other three electrodes? I'm looking for hints as well as...
  42. J

    Capacitance question What is C_eq?

    Homework Statement C2<C2<C3<C4 for the capacitor combination shown. What is C_eq? a) less than C1 b) more than C4 c) between C2 & C3 d) less than C2 e) any value since it depends on the voltage applied Homework Equations Capacitors in series: 1/Ceq = 1/C1 + 1/C2 +... 1/Cn The...
  43. M

    Find Capacitance From Charged Capacitors

    Homework Statement A 13-µF capacitor and a capacitor of unknown capacitance are both charged to 4.00 kV. After charging, the two capacitors are disconnected from the voltage source. The capacitors are then connected to each other--positive plate to negative plate and negative plate to positive...
  44. B

    What's the Difference Between Heat Capacity and Heat Capacitance?

    1What is the difference between heat capacity and heat capacitance?
  45. A

    Vacuum Capacitance: Value & Dependency?

    Does anyone know what the value of capacitance in a vacuum (C_{0}) is? Would this be a constant or dependent on the particular capacitor? Thanks
  46. A

    Capacitance - Dielectric Constant Relation?

    Hi, Does anyone know if there is a relation between a materials capacitance (C) and its dielectric constant (K)? Thanks,
  47. G

    Capacitance, switches, and lightbulbs oh my

    Homework Statement The capacitor in the figure below has a large capacitance and is initially uncharged. After the switch is closed, what happens to the lightbulb? You can't paste the figure but it is a circuit with a capacitor paralled to a battery and in series with a lightbulb. a...
  48. K

    How does a dielectric increase capacitance, conceptually.

    Hi all, I'm a little stumped with how the dielectric increases capacitance. The general solutions all involve using the formulas, but don't really treat it conceptually. I realize that inserting a dielectric decreases field strength, and hence decreases the voltage across the capacitor. But...
  49. I

    Capacitance and inductance of passive filter

    I'm having some trouble calculating the values I need for a capacitor and inductor for a passive filter. The values that I know to use are current, voltage and frequency. But I have a voltage <1 volt and a frequency of 60Hz but no current. Can someone help me figure out what value of capacitor...
  50. F

    How Does Flipping a Coil Affect Capacitor Voltage?

    Homework Statement The magnetic field at one place on the earth’s surface is 55 μT in strength and tilted 60° down from horizontal. A 200-turn coil having a diameter of 4.0 cm and a resistance of 2.0 Ω is connected to a 1.0 μF capacitor rather than to a current meter. The coil is held in a...
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