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. B

    Medical Defibrillator/pacemaker affected by device measuring capacitance?

    would a 1.5 volt electronic device that measures capacitance to turn on or off a regulator that controls water flow affect a defibrillator or pacemaker? Another way to ask this, A kitchen faucet that is touch sensitive would be safe or not safe for a heart patient who has defibrillator and...
  2. F

    How in the world did I get more capacitance? Violating First Law of Thermodynamics?

    Homework Statement Basically we did a lab on charges, capacitors and voltages. We used a 2200uF capacitor and we hooked it up with a 10kΩ Resistor connected in series with a DC Supply. The DC Supply was turned down to 0V and all the other senors were "zeroed" out. Simultaneously, when...
  3. D

    RLC Circuit Find inductance and capacitance

    The energy of an RLC circuit decreases by 1.00% during each oscillation when R=2.00 ohms. If this resistance is removed, the resulting LC circuit oscillates at a frequency of 1.00 kHz. Find the values of inductance and capacitance.
  4. B

    How Is Capacitance Calculated in a Cloud-to-Ground Model?

    1. The base of a cloud has an area of 2*10^7 metres squared and is on average 1km above the ground. treating the base of the cloud, the ground and the intervening air gap as a parallel plate capacitor, calculate the capacitance.[relative permittivity of air = 1.0] 2. I tried to say C...
  5. M

    Variable Capacitance of a Radio Dial

    Homework Statement The capacitance of the variable capacitor of a radio can be changed from 100 to 350 pF by turning the dial from 0° to 180°. With the dial set at 180°, the capacitor is connected to a 130 V battery. After charging, the capacitor is disconnected from the battery and the dial...
  6. G

    Capacitance between two coplanar electrodes

    Homework Statement I have two coplanar electrodes (180 deg between them) and one of them is under voltage causing circular electric field between them. The target is to calculate the capacitance. Homework Equations Using the capacitance equation C = \frac{\epsilon_{0}A}{l} I say that...
  7. F

    What happens to capacitance and voltage when a battery is disconnected?

    hello i need some explanation on this . I know that when a battery is connected it provides constant voltage across the capacitors. But the happens when the battery is disconnected. What happens to the voltage, the capicitance, and what is the importance of dielectric. I have read the book...
  8. Q

    How Do You Calculate Capacitor C1's Value After Connection?

    Homework Statement Capacitor C1 is charged so that potential difference between its plates is 20 V. Another capacitor C2=33µF has potential difference of 4 V between its plates. After plates of the capacitors that carry the charge of opposite sign were connected the potential difference became...
  9. H

    Capacitance Question: Charge Change in Circuit

    Homework Statement After the switches have been closed, in which circuit will the charge on the left hand capacitor increase? decrease? stay the same? Picture: [PLAIN]http://img714.imageshack.us/img714/3369/capacitance.png Homework Equations (1/Ceq) = (1/c1) + (1/c2) q = cv...
  10. C

    What is the equivalent capacitance of the capacitors shown in the figure?

    Homework Statement The equivalent capacitance of the capacitors shown in the figure is 9.23 MicroF Homework Equations http://upload.wikimedia.org/math/2/f/5/2f5b978fd081ea63e204909addf3d028.png http://upload.wikimedia.org/math/f/a/3/fa34ce461e6988fd973081401cc9790c.png The...
  11. B

    Equivalent Capacitance of capacitors

    Homework Statement The equivalent capacitance of the capacitors shown in the figure is 9.26 uF http://session.masteringphysics.com/problemAsset/1122585/1/Walker.21.58.jpg Homework Equations Cseries = 1/Cs = 1/c1 + 1/c2 + ... Cparallel = Cp = C1 + C2 + ... The Attempt at a...
  12. A

    Capacitance of a variable capacitor

    Homework Statement Hi again! I've been handed an assignment in which we are to find the capacitance of a variable capacitor consisting of seven semicircular plates with a radius of 2 cm spaced 1 mm from each other. Every other plate is connected to a common +terminal, and every other to a...
  13. Z

    How to measure the Capacitance of this circuit

    Homework Statement Hi, I have virtually no background in electronics and I have been given the following task. I have to measure the capacitance of the (real) circuit below (using labview, but it is the theory of how to do this that I am interested in). I am not able to use the...
  14. C

    Help Me Find the Capacitance: A Stumped Student's Story

    Hi again! I got stuck in this question (Pic), the original question is the fig. 1. I then arranged the charges on the plates (fig 2) and then separated the plates to form three capacitors. Now I just don’t know what the capacitance will be and how to find it! First I thought to ignore the...
  15. C

    Calculating Capacitance: Series vs Parallel

    Hi all, This question was given in my physics book. Given that: C1 = 4uF and C2 = 6uF (u=micro) ... it comes out to be C1 + C2 = 10uF I used the general method ... you know ... assign charges, assume potentials... solve equations. My question is that aren't the two capacitors...
  16. F

    Capacitance between two crossed wires

    Homework Statement I have two wires (length L) which are separated by a distance d. They are also oriented perpendicularly (but I am curious about the orientation impact, part of the question). By simplification, I assume them to be cylinders of radius R (in actuality they are likely...
  17. M

    Find Inductance and Capacitance

    Hello, I need to construct an AM radio's RC circuit that: 1. resonates to frequency f= 580 Hz 2. has a maximum current of I = 0.2 A 3. the capacitor has a maximum potential energy U = 2 x 10^-5 I need to find: The inductance and capacitance (L & C) of this RC circuit. Please help...
  18. J

    Capacitance matrix and integral equation method

    Dear all, I'm having some trouble calculating the capacitance matrix, as outlined below. So first of all, the lecture notes I'm using use an integral equation method (method of moments) to determine the capacitance of two infinitely long and thin parallel strips in vacuum, at a distance d of...
  19. Z

    How can I measure capacitance without expensive equipment?

    Hello, I am new to this forum (and no physics expert). I apologize if this topic is in the wrong place, but I would really appreciate help on the abovementioned topic. I am investigating the effect of temperature on the capacitance of capacitors for a school project. Since C =...
  20. T

    Capacitance of two long parallel wire

    Homework Statement Consider two long parallel and oppositely charged thick wires of radius d with their central axes separate by a distance D apart. Obtain an expression for the capacitance per unit length of this pair of wires Homework Equations \oint \vec{E}\cdot d\vec{a}...
  21. F

    Calculate Total Capacitance: Step-by-Step Guide

    Total Capacitance[SOLVED] Homework Statement how do i calculate the total capacitance of the image attached, as there is series and parallel capacitor in the same circuit The Attempt at a Solution i did try using 1/8uF + 8uF but then could not get the answer. can someone please...
  22. T

    Equivalent Capacitance of Concentric Spheres

    Hey guys! This problem has been perplexing me for ages, and everything I do to try to solve it doesn't seem to work. So I was wondering if you guys could help point me in the right direction. Anyway: Homework Statement The figure below shows six concentric conducting spheres, a, b, c, d, e...
  23. O

    How Do You Calculate Time Constant and Capacitance in an RC Circuit?

    Homework Statement A battery with emf = 2V is connected to a RC circuit with a 450Ω resistor and an unknown capacitor. After 3.8 milliseconds, the current is 2.2mA. (a) Find the time constant and the capacitance. (b) Now, 2.9 ms after the charging begins, find the current I, the charge...
  24. T

    How does exactly higher capacitance have more bang for your buck ?

    How does exactly higher capacitance have "more bang for your buck"? Hi! I'm trying to really grasp the concept of capacitance. Whenever I see a description of the definition, C=Q/V, they always say either: "More charge stored per potential", or "more bang for your buck". I don't really...
  25. N

    Help! Calculating Capacitance from Voltage & Charge

    Capacitance (Please Help!) Homework Statement The charge on a capacitor increases by 21 \mu(micro) C when the voltage across it increases from 99 V to 127 V. What is the capacitance of the capacitor?Homework Equations There is Q=cV, or C=Q/v. But there are 2 unknowns and no number for the...
  26. T

    Frequency, inductance, and capacitance

    Homework Statement The oscillator of a 96.9 MHz FM station has an inductance of 1.4 μH. What is the value of the capacitance? Homework Equations I haven't the slightest idea. I've looked through the section in my physics book and looked all over online and I can't find anything. The Attempt...
  27. L

    Accurate Capacitance Meter Reccomendations

    I need a very accurate capacitance meter with capabilities down to + .01 puff. Any reccomendations?
  28. A

    Finding equivalent capacitance

    Homework Statement Hi, i was working on finding the equivalent capacitance, but I'm not sure if I'm doing it right This is the figure : http://i8.photobucket.com/albums/a8/ryu0919/physicsproblem2.jpg? Homework Equations Find the equivalent capacitance? The Attempt at a Solution...
  29. S

    Find the equivalence capacitance of the below circuit

    Homework Statement All the capacitors have the same value. Need to find the capacitance between ab. Homework Equations The Attempt at a Solution
  30. S

    Electic field / capacitance of parallel plates

    Upon calculating the electric field for a plate of charge, you arrive at: E = Q/Ae (Q the charge on the plate, A the area of the plate, e the permittivity of the dielectic) But then, isn't the total electric field between the plates twice this because of the equal but opposite charge on the...
  31. P

    Parallel plate capacitor with dielectric. Find capacitance

    Homework Statement The plates of a parallel plate capacitor measure 15cm x 15cm, and they are separated by 6cm. The capacitor is charged with a potential difference of 18V, then removed from the voltage source and kept isolated. A 15cm x 15cm x 3cm slab of dielectric material (k = 2.5) is...
  32. T

    Equivalent Capacitance Question

    Homework Statement What is the equivalent capacitance of the four capacitors in the figure below? Take C1 = 2.6 µF, C2 = 4.1 µF, C3 = 3.9 µF, and C4 = 1.1 µF. Homework Equations Cequiv of capacitor in series = (1/C1)+(1/C2)+(1/C3)+(1/C4) Then take the reciprocal of the answer. The...
  33. A

    Finding equivalent capacitance

    Homework Statement http://yfrog.com/mrcapdg http://img819.imageshack.us/img819/1172/capd.th.gif Find equivalent capacitance answer is 4C/3 Homework Equations whatever is in parenthesis is subscript formula1: series: 1/C(eq)= 1/C(1) + 1/C(2) + 1/C(3) + ... formula2: parallel...
  34. A

    Parasitic capacitance in generator

    Hi, can anyone tell me how to measure the parasitic capacitance in a generator?
  35. K

    How to measure capacitance of capacitor using these materials only?

    Hi, How can I measure the capacitance of a capacitor, using the following: a battery, a digital voltmeter (which records voltage as a function of time), a digital ammeter (which records current as a function of time), an industrial capacitor, two copper plates, and several...
  36. S

    Kirchhoffs law and capacitance

    Homework Statement The problem considers the diagram in the photo that I have attached. Apologies for the poor quality, but sketching it on my computer would have been even worse! a) Write down Kirchhoff's laws for the circuit and show that when the switch is closed, the charge on the...
  37. R

    Why do some books use dQ/dV for capacitance?

    Hi guys, Today was the second time in a textbook that they calculated capacitance using C = \frac{dQ}{dV}. In my electromagnetics book and any sources I find online they use C = \frac{Q}{V}. Can anyone help me understand why they can do it like this? This type of calculation was done to...
  38. C

    Capacitance and resistance question

    Homework Statement [PLAIN]http://img806.imageshack.us/img806/2373/compquesti.jpg The Attempt at a Solution dont know where to start... i know i have to use complex numbers... advice needed please!
  39. I

    Charging 10 1F Capacitors: Output Voltage and Capacitance?

    Can you tell me if I'm right with this. If you charge ten 1F capacitors in series at 10 volts. Each capacitor has a voltage of 1 volt and a charged capacitance of 0.1F. So if you discharged the ten capacitors in parallel the output would be equal to 1F of capacitance charge and 1 volt. or would...
  40. M

    How Is Capacitance Multiplied in an Op Amp Circuit?

    Homework Statement Please, I would like to know how to find the factor by which the capacitance is multiplied A picture of the Op Amp is here: http://en.wikipedia.org/wiki/File:Cap-mult-op.svg Zc is complex impedance of C Homework Equations none given The Attempt at a Solution...
  41. E

    Calculating the capacitance of a metal strip?

    Does anyone know how to calculate the capacitance of a metal strip of width 2a and infinitesimal thickness? The length of the metal strip is much larger than its width. Thanks a lot!
  42. C

    Calculating Voltage Across Capacitor: EMF and Internal Resistance Question"

    Homework Statement At time t=0, a cell of EMF E and internal resistance r is connected to the capacitor of Capacitance C. i) Derive an expression for the voltage across the capacitor at subsequent times. ii) If the capacitance is 5000uF and the voltage across the capacitor reaches 73 V...
  43. S

    Does the Electric Field Change when a Dielectric is Inserted into a Capacitor?

    Homework Statement This is not a problem from any book or any kind of source. I was just thinking on this. Suppose u have a capacitor attached to a battery. And then u insert the dielectric slab. Then do the electric field between the plates changes? Homework Equations The Attempt...
  44. P

    Calculating Capacitance for a Water-Filled Parallel-Plate Capacitor"

    Homework Statement The capacitance of a water-filled parallel-plate capacitor with plate area 3.0×10^−3m^2 and separation 4.0×10^−2 mm is 0.66 uF 1.3 nF 53 nF 660 nFHomework Equations C=E_0A/d The Attempt at a Solution C=8.85*10^-12*3*10^-3/4*10^-5 C=6.64*10^-10 This is none of the...
  45. C

    Litz conductor surrounded by drain wires = huge capacitance?

    this type of cable is used to transfer power underground: http://www.mahanson.com/images/Hendrix%20Cable.jpg see the drain wires? i would think that those would be a huge problem because they are grounded and are right next to the conductor, i suppose any shielding would have this problem...
  46. S

    Capacitance, voltage, distance and charge?

    NOT HOME WORK QUESTION! If a capacitor is attached to a battery, initially the separation between the capacitor plates is D then the distance is doubled(2D). The voltage should remain the same because it is still attached to a battery. C=E0A/D so the capacitance decrease by factor of 2. my...
  47. M

    Finding Capacitance of a capacitor

    Homework Statement A 100 μμF capacitor, C1, is charged to a potential difference of 50 Volts. The charging battery is disconnected and the capacitor is connected to a second uncharged capacitor, C2. If the potential across C1 drops to 35 volts, what is the capacitance of C2? Homework...
  48. A

    Solving Electrostatic Field & Calculating Capacitance of Microstrip

    Homework Statement I am trying to solve the electrostatic field (2D) for a microstrip laid on a dielectric with a ground plane on the other side. I am also calculating the capacitance of the microstrip by evaluating a closed surface integral to get the charge on the conductors. I am using FDM...
  49. O

    Capacitance in concentric shells

    Homework Statement Consider four concentric spherical metal shells of radii r, 2r, 3r, and 4r. The middle two shells are connected by wire, as shown. What is the total capacitance of the system? In your solution, clearly explain your method. Homework Equations u= 1/2cv^2 The...
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