Capacitor Definition and 1000 Threads

A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals.
The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit. The capacitor was originally known as a condenser or condensator. This name and its cognates are still widely used in many languages, but rarely in English, one notable exception being condenser microphones, also called capacitor microphones.
The physical form and construction of practical capacitors vary widely and many types of capacitor are in common use. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. A conductor may be a foil, thin film, sintered bead of metal, or an electrolyte. The nonconducting dielectric acts to increase the capacitor's charge capacity. Materials commonly used as dielectrics include glass, ceramic, plastic film, paper, mica, air, and oxide layers. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy, although real-life capacitors do dissipate a small amount (see Non-ideal behavior). When an electric potential (a voltage) is applied across the terminals of a capacitor, for example when a capacitor is connected across a battery, an electric field develops across the dielectric, causing a net positive charge to collect on one plate and net negative charge to collect on the other plate. No current actually flows through the dielectric. However, there is a flow of charge through the source circuit. If the condition is maintained sufficiently long, the current through the source circuit ceases. If a time-varying voltage is applied across the leads of the capacitor, the source experiences an ongoing current due to the charging and discharging cycles of the capacitor.
The earliest forms of capacitors were created in the 1740s, when European experimenters discovered that electric charge could be stored in water-filled glass jars that came to be known as Leyden jars. Today, capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems, they stabilize voltage and power flow. The property of energy storage in capacitors was exploited as dynamic memory in early digital computers, and still is in modern DRAM.

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  1. J

    Doubt about old electrolytic capacitors

    I have stored here electronic devices with aluminum electrolytic capacitors used and manufactured in the early 90s. Here the temperature varies between 33-35C. These electronic devices need to be energized what time and frequency to prevent failures in these aluminum electrolytic capacitors...
  2. user2539

    DC-Link Capacitor Current Modeling for a Voltage Source Inverter

    Hello I'm reading this research paper about obtaining an accurate DC-link capacitor current equation. I've attached the paper. Im struggling to understand a few things and was hoping someone could help with them. 1) How is the the value marked in the bracket after each V vector describing the...
  3. Z

    Magnitude of magnetic field at different points near a capacitor.

    Here is a picture depicting the capacitor and the points of interest. I approached this problem by applying the Ampere-Maxwell law. For each point I used an circular Amperian loop that I denote by ##P##, enclosing a circular surface ##S##. Thus, for point ##b## we have...
  4. E

    What explains the current flow in a LC circuit?

    I'm just not able to grasp the concept of a LC circuit intuitively, and yet I have found zero answers to my doubts. I can't understand why does the current keeps flowing counter-clockwise between the 3rd and the 4th circuit (see image attached) I know that when the capacitor has 0 charge, in...
  5. M

    B Why do capacitors not discharge everything immediately in LC circuits?

    I just started taking LC circuits, and I was wondering, when capacitors charge without a resistance, they charge immediately, so when they discharge in an LC circuit, why don't they "send" all of their charge immediately? Is it because that's how capacitors work in general, or is it because of...
  6. A

    B Can charges move without a field? How charges redistribute without it?

    So from Gauss theorem, electric field at any point inside a uniformly charged sphereical shell is zero. Thus there is no electrostatic force on the inner sphere. From what I have learnt, a field is necessary to move charges. But in this case the inner sphere acquires a charge q without any...
  7. Philip Koeck

    A An electrolytic capacitor keeps charging by itself

    This is related to an earlier post (https://www.physicsforums.com/threads/an-electrolytic-capacitor-charges-by-itself.1008156/), but there's a new angle to it, therefore the new thread. I have two 100 microF electrolyte capacitors in my office, one is connected to a volt meter, the other is...
  8. S

    Why Do Capacitors Behave Differently in Series and Parallel Configurations?

    Consider the above diagram. Once the first capacitor is charged, clearly it will have a voltage ##E##. Then when the switch is flipped, the cell no longer matters (there is no complete circuit which goes through the cell), so we have the first capacitor connected to the second one, and it looks...
  9. Tofuwu6

    Final Charge of a Capacitor in a Circuit with Capacitors and Resistors

    I had two trains of thought. One is that the capacitor will fully charge when t = infinity, so when you plug t = infinity into the equation of charge as a function of time you get 1.68E-4, which you also could've gotten from Q = CΔV where ΔV = 42V. My other train of thought was that when t =...
  10. gabriel109

    I Charging Capacitor with one terminal grounded

    I'm reading the capacity chapter of Serway's book, and I had a question about the charging of a parallel plate capacitor. Let's assume the following situation with a modification of the circuit in the figure: we connect the negative terminal of the battery and one of the capacitor plates to...
  11. electricx

    What causes voltage between AC Mains phase and me?

    So my question is that if i put other lead of multimeter to my hand and other lead to socket 230 phase wire why it shows 150 volts when i stand at floor and 80 volts when i stand at carpet? what causes those voltages? And yes i know doing that could be dangerous. It also shows weird voltage if i...
  12. Z

    Why can we model spherical capacitor with two dielectrics as two capacitors in series?

    I solved this problem by simply applying the formula for capacitance. The potential difference between a point on the inner shell and the outer shell is computed by considering the electric fields to be ##\frac{E_0}{\kappa_1}## between radius ##a## and ##b## and ##\frac{E_0}{\kappa_2}## between...
  13. Z

    Two different dielectrics between parallel-plate capacitor

    We have a parallel plate capacitor with two different dielectrics It seems to be the case that the potential difference on each half of the capacitor is the same. Initially, the electric field was ##\vec{E_0}=\frac{2\sigma_+}{\epsilon_0}\hat{j}##. If we were to insert a single dielectric...
  14. Z

    Charge on inner/outer surfaces of two large parallel conducting plates

    Let me first think about a simpler case. Suppose we have a capacitor. That is, the two plates have charges of equal magnitude and opposite signs. Consider the purple rectangle which represents a Gaussian pillbox. The electric field due to one of the plates individually has field lines...
  15. A

    I Where is the lost energy in this example?

    We connect the charged capacitor to the no-charged capacitor (consider the wires to be ideal R=0), the final energy is less than the initial energy of the system. Where is the lost energy? (see example blew)
  16. A

    I Do the plates of the capacitor exert a force on each other?

    Do the plates of the capacitor exert a force on each other due to opposite electrical charges? Consider a planet capacitor. A simple calculation shows that this force must be very large. If you are not convinced of the magnitude of the force, I will give a simple example.
  17. ka_reem13

    Electromagnetism question: Current flowing between concentric spheres

    I know that my solution is time dependant, and I initially tried to use a capacitor model of sorts, but I realised as it was filled with a conductive medium, I cannot use a capacitor model. So now I am very stuck on this
  18. A

    Calculations involving different Dielectrics and Capacitors

    TL;DR Summary: Need dielectric constant for given capacitor Given a 7.4 pF air-filled capacitor, you are asked to convert it to a capacitor that can store up to 7.4 mJ with a maximum potential difference of 652 V. Which dielectric in Table 25-1 should you use to fill the gap in the capacitor...
  19. Simobartz

    Positioning of Capacitance in Ionization Chamber's Equivalent Circuit

    I'm delving into the topic of ionization chambers, but as someone without a background in electrical engineering, I'm finding the equivalent circuits a bit challenging to comprehend. Specifically, I'm puzzled by the placement of the chamber's capacitance and any parallel capacitance in the...
  20. S

    Calculating low frequency cutoff for this R-C microphone circuit

    I want to ask about question (c). My idea is to compare the period and time constant. The period is 0.05 s and time constant is 0.005 s. Time constant is the time needed for capacitor to discharge until the charge stored in it becomes 37% of initial charge. But I don't know how to relate the...
  21. PhysicsTest

    Capacitor equation to be used in electrical analysis

    I am confused with the equation to be used for capacitor in electrical analysis The standard equation we have is Q=CV -> 1 the other equation is is V = Z*I ohms law Z is the impedance of the capacitor. Both are giving me voltage, which one to use ?
  22. PhysicsTest

    How Does a Capacitor Introduce Phase Delay in a Circuit?

    I simulated the below circuit to capture the phase delay between input voltage and output current in LTSpice How do i measure the phase delay introduced due to capacitor?
  23. Bruce Ratcliffe

    My variable speed drill switches turn into ON/OFF switches :-(

    My students build "eCars" that use wheelchair motors and two 12 volt AGM lead acid batteries in series. A pair of Bosch 25Amp, 24 VDC, drill switches control power to each wheel. Tons of fun (see here): The switches last in some cases for years, but quite a few have burned out the "variable"...
  24. K

    I How do we know that both plates of a capacitor have the same charge?

    Suppose we have two conductors ( can be of different shapes) and connect them to battery. Why would equal amounts of charge appear on the two conductors?
  25. L

    How Do You Calculate Capacitance in a DC Circuit with a Fully Charged Capacitor?

    Hi, I am not sure if I have calculated the task correctly I have now assumed that the capacitor does not need to be charged and is therefore fully charged. In a DC circuit, a capacitor acts like an infinitely large resistor or like an open switch, so I assumed that it is a parallel circuit...
  26. Edy56

    Engineering Calculate time for capacitor to charge and discharge

    I really don't get this one. Why do I need V? Since I didn't use it, I assume my work Is incorrect.
  27. T

    I Why is a Dielectric slab ejected from the capacitor when energized?

    A capacitor consisting of 2 square metal plates placed at a certain distance is connected to a potential difference generator V. A slab of dielectric material is inserted into the space between the armatures. By doing the calculation of the derivative of the electrostatic energy with respect...
  28. PhysicsRock

    Why Capacitors in Parallels vs. Series: Coaxial Capacitor Case

    So my idea was to separate the capacitor into two individual ones, one of length ##l - a## filled with a vacuum and one of length ##a## filled with the glass tube. The capacitances then are $$ C_0 = \frac{2 \pi \varepsilon_0 (l-a)}{\displaystyle \ln\left( \frac{r_2}{r_1} \right)} $$ for the...
  29. PhysicsRock

    Capacitor 1/3 filled with a dielectric

    My attempt would be to calculate the electric fields of the vacuum and dielectric part seperately and then use superpositioning to obtain the full solution. However, I don't see an ##x##-dependency coming along that path. The assignment suggests that there must be one though. Unfortunately, this...
  30. V

    Potential difference between 2 points in a capacitor circuit

    In the given circuit, a transient current will flow and when this current finally stops at equilibrium, the charges ##q_1## and ##q_2## are assumed to deposit at the capacitor plates as shown below. The dashed line indicates an isolated system that will have it's total charge conserved. If I...
  31. L

    Discharging a capacitor -- Calculate the current as a function of time

    Hi, I am not sure if I have calculated the task b correctly. I always interpret an open switch as an infinitely large resistor, which is why no current is flowing through this "resistor". So there is no current in the red circle, as it was the case in task part a, but only in the blue circle...
  32. V

    Polarities of capacitor plates in a complex circuit

    (a) I think the top plate of C5 could end up with either + or - charge, and not necessarily + charge as shown. This is because the connected plates of C1, C5 and C3 form an isolated system to which we can apply the law of conservation of charge i.e. Total charge just before transient currents...
  33. milkism

    Non-linear isotropic dielectric capacitor

    Question: Solution first part: Have I done it right? I don't know how to begin with second part since the dielectric is non-lineair, and most formulas like $$ D=\epsilon E$$ and $$P= \epsilon_0 \xhi_e E$$, only apply for lineair dielectrics. What to do?
  34. A

    Engineering Derive the governing equation for the voltage Vc across the capacitor

    I was thinking about doing KVL around the circuit at the right but I noticed when the switch opens, the current through the circuit at the right is not the same throughout -5 + Ic*2*1-^3 + Ic*10^3 = -Vc Ic is not the same around the right circuit so I am stuck....
  35. I

    Need a voltage divider for a capacitor source voltage

    The end goal is I need to convert a sinusoidal into a square wave using a zero crossing circuit. I have a voltage that ranges from 0 to 400vrms @70kHz from a capacitor that is part of an induction heater tank circuit. My comparator has a peak differential input of +/- 35v so I need to reduce the...
  36. ananonanunes

    Finding Current in Resistors & Charge of Capacitor

    Suppose the switch has been closed for a long time so that the capacitor is fully charged and current is constant. a)Find the current in each resistor and charge Q of the capacitor. b)The switch is now opened at t=0s. Write the equation for the current for the resistor of 15kΩ as a function of...
  37. K

    Calculating electric charge from graph (capacitor)

    Apparently, we need to integrate the functions from 0 to the time when it is fully charged. However, I integrated in terms of t so the soultion (according to a graph programme) should be around 236 Vs but I don’t see how this could help me.
  38. A

    Engineering Short vs Open Circuit: Is That Correct?

    For the first circuit, Req = ZL + ZC = -j/(w*C) + j*w*L = 0 for short circuit, so w = 0? For the open circuit case, -j/(w*C) + j*w*L = infinity, so w = infinity? Is that correct?
  39. Idontknowhatimdoing

    Electric Field Inside Cylindrical Capacitor

    we know that flux is equal to the area integral of electric field dotted with dA and we can set this equal to charge enclosed divided by epsilon naught. Thus, in this case, the integral simplifies to E * A = (q_enclosed)/(ε_naught) when we choose a cylindrical gaussian surface with radius of r...
  40. A

    Energy conservation law question with capacitor

    I was wondering why energy of capacitor does not equal change in kinetic energy PLUS change in potential energy where potential energy is the change in the potential energy of the charges. I believe that should be so because energy conservation = change in kinetic energy plus change in potential...
  41. S

    Change in energy stored in a spherical Capacitor

    I have attached my solution. Unfortunately, after plugging in the values, my answer is 4 times more than the expected one. What am I missing?
  42. M

    Finding charge on a capacitor given potential difference across two points

    For this part(b) of this problem, The solution is However, I tried solving (b) like this: Since ##Q_{total} = 363 \times 10^{-6} C## then ##Q_1 = 181.5 \times 10^{-6} C ## since the equivalent upper capacitor is in series with the equivalent bottom capacitor so should store the same amount...
  43. Pushoam

    Charge upon a parallel plate capacitor

    a) if I take a Gaussian cylindrical surface whose circular area are present in the meat of the two plates of the capacitor, then the electric flux through this Gaussian surface is zero ( as the electric field inside the meatof the capacitor is zero and between the capacitors, electric field is...
  44. Pushoam

    Capacitance of a parallel plate capacitor with a metal plate kept inside

    Because of the plate P, the capacitor becomes a piece of conductor. It contains zero net charge and has 0 potential difference. Hence, the capacitance is ## \frac 0 0 # # that is undefined. The capacitance of a capacitor is defined as its capacity to store charge when a potential difference is...
  45. Pushoam

    Force between the plates of a capacitor with dielectric slab inserted

    The the electric field inside decreases due to the presence of a dielectric by a factor of dielectric constant K. Hence the force between the plates will decrease. Is this right?
  46. M

    Derivation for capacitance of cylindrical capacitor

    I don't understand how they got from the previous step to the next step of the derivation circled in red: Many thanks!
  47. sol47739

    Am vs Fm variable capacitor receiver, what is the difference?

    What is the difference between a variable capacitor in a AM receiver and a variable capacitor in a FM receiver? I understand that Am is amplitude modulation and that the signal is carried over a changing amplitude and that the frequency is constant. And the opposite in FM signals. And a variable...
  48. supak111

    Calculate current in a 120 VAC circuit with a series 10uF capacitor

    Hi can someone tell me please how much current is passed though below circuit: 120 AC 60Hz mains power going through a 10uF 500v capacitor in series
  49. G

    Capacitor Circuit Analysis: How to Find Voltage Across Resistors

    Figure: My attempt at a solution: Once the capacitors have been applied, we see directly that ##\boxed{v_c=v_0=0\, \textrm{V}}## Wouldn't this one be done like this? I know the following: in a circuit with capacitors and coils, if we have Capacitor: we change for an open circuit. Coil: we...
  50. Shreya

    Capacitor Network - Series or Parallel?

    My textbook solution states that 1 & 2 are in parallel and so is 3 & 4 and those 2 are in series. That is, (1 P 2) S (3 P 4). My thinking is such: points A & B are of same potential, say V, C & D are of same potential, say x and E & F are are of same potential, say 0. So I can say that 1 and 3...
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