Capacitors Definition and 79 Discussions

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. P

    Do charges always lie on the inner surface of capacitors?

    Parallel plates A, B are 5mm apart, with charges +1C and -1C respectively. Parallel plates C, D are 2mm apart, with charges +1C and -1C respectively. Capacitor CD is slid between capacitor AB. Find the potential difference between AB. The key idea to solving this problem is to suppose that +1C...
  2. P

    Electrical energy stored by charged concentric spherical shells

    I thought up of this problem myself, so I do not have solutions. I would appreciate if you could correct my approach to solving this problem. Firstly, the charge induced on the inner surface of shell B is -q, and so the charge on the outer surface of shell B is Q+q. The energy stored can be...
  3. P

    Principle of superposition for charges/induced charges

    The key observation to solve the above problem is that the charge Q can be dragged out into a flat capacitor plate parallel to the 2 existing plates. Apparently, while the charge distribution on the 2 existing plates changes, the total charge induced on each plate remains the same, due to the...
  4. P

    Meaning of F = -dU/dx for capacitors

    I have noticed that F = -dU/dx in gravitation gives the attractive force experienced by both bodies. For capacitors, does F = -dU/dx give the force experienced by each capacitor?
  5. P

    Force needed to hold together a capacitor

    I have 2 methods, which give 2 different solutions: Let sigma = charge per unit area Let plate 1 be the left plate, plate 2 = right plate. Method 1: Because they are insulating, consider the electric field at 3 regions; region 1 to the left of plate 1, region 2 between the plates, and region 3...
  6. B

    Redistribution of Pre-Charged Capacitors

    I first calculated the charge each capacitor has after its directly charged by the 36 V battery. ##Q_1 = C_1 * V = (2 \mu F) * 36 V = 72 \mu C## ##Q_2 = C_2 * V = (5 \mu F) * 36 V = 180 \mu C## ##Q_3 = C_3 * V = (7 \mu F) * 36 V = 252 \mu C## Then these capacitors connect in series, so I...
  7. G

    Questions on Capacitors again

    Well i don't you to solve the question for me but I want you to clarify the concepts pertaining to this question. My question is how do I write a equation for the circuit since the there is same charge on one of the capacitors. While writing the equation should i put the voltage across the...
  8. G

    A confusing question on capacitors and voltages across them

    In my opinion, the voltage across the C1 should be 9V as the potential on the side of the positive plate of the capacitor should be (15-6)V and on the other be 0V. Similarly the potential across C2 should be (7-0)V. Here I'm basically assuming that the voltage at the negative terminals of the...
  9. G

    Finding the potential across capacitors in capacitor-only circuits

    I tried to attempt it by applying KVL to both the loops. I tried to find a possible charge distribution for the capacitors. I guess this is right. On solving I get: from what I know potential difference between M and N is Q1/C2 but the solution is given as: Where am I wrong?
  10. H

    Electric field between two capacitor plates (proof)

    See attached image
  11. S

    Set up the differential equation showing the voltage V(t) for this RC circuit

    Hi all. I have another exam question that I am not so sure about. I've solved similar problems in textbooks but I have a feeling once again that the correct way to solve this problem is much simpler and eluding me. Especially because my answer to a) is already the solution to c) and d) (I did...
  12. S

    How to measure electric and magnetic fields in asymmetric capacitors?

    Summary: I need to build an asymmetric capacitor, but the mathematics of electromagnetics become too tough, do you have any info that can help? Appreciate it! Hello, I am an undergraduate student in engineering and I want to build an asymmetric capacitor, so I need electromagnetics which I...
  13. M

    2-Resistor, 2-Capacitor Circuit with Switch

    I already did part a and got the correct answer of 4.8 microC. However, I am unsure how to approach part b, where the switch is closed and apparently the two capacitors end up with different charges. For part b, since there is a path between the two capacitors, I don't know if I am able to...
  14. J

    Could someone help with this steady state capacitors questio

    Homework Statement Question: https://imgur.com/a/EmGDW87 Homework Equations Q =VC V = IR The Attempt at a Solution I don't understand how the capacitor C_2 is in parallel with R, which would dictate that they have the same p.d, but then again the circuit is in stead state and so no current...
  15. Muhammad Usman

    Capacitor Noise Removal?

    --> Hi, I am reading about the capacitors. I have read that the capacitors block DC and behaves as open circuit in DC while short circuit in AC. The reason behind this is the charging and discharging of the capacitor. What is my perception about it, I will mention about it. " During the DC...
  16. Lapse

    What is the Voltage on C1/C2 after the switch closes?

    Homework Statement Homework Equations Vf+(Vi-Vf)e-t/RC Tau = RC The Attempt at a Solution [/B] Normally, I would use the eq'n mentioned above to find Vr in an RC circuit. However, this second capacitor is throwing me off. Is there a new equation I should use? How do I think about this...
  17. R

    Engineering RC Circuit/Node voltage question

    Homework Statement Homework Equations The Attempt at a Solution So you need to find the voltage across node D while the capacitor is charging, so it is going to be related to the voltage of the capacitor, so a function of time, so you need to come up with the equation for Vc(t), which I...
  18. RedBaron

    Capacitors and regenerative braking

    So I'm interested in adding a bank of capacitors to help reduce the initial load on my batteries when accelerating from a stop but my current set up uses regenerative braking. My electrinics are this order, Motors, VESC, BMS, 10s5p Li-ion pack. If I add capacitors between the bms and vesc to...
  19. R

    Equivalent resistance with a capacitor

    Homework Statement So I need to find the equivalent resistance for the following RC circuit to calculate the time constant, but I got stuck in terms of finding out the equivalent resistance. Homework Equations V = IR The Attempt at a Solution So the thing I am not sure on is how do you...
  20. P

    Splitting Up Capacitors | Any Advantage?

    Can someone tell me what is the advantage of using the combination instead of a single capacitor? The same total power would be dissipated?
  21. A

    Finite dual disk capacitor: estimating charge distribution

    Homework Statement Working through Purcell (among others) as fun applied math/math modeling refresher. But, I have struggled all week in establishing from first principles that the potential/field/distribution for a configuration of two capacitive disks of radius 1 and separation s along the...
  22. Abhimessi10

    Charge entering into the field between capacitor plates

    Homework Statement A negative charge of mass m enters into the field between two deflecting parallel plates with length 'l' separated by distance 'd' with a velocity 'v'.Assuming electric field to be uniform,find the maximum charge so that it will not hit a plate Homework Equations Kinematic...
  23. U

    Electric potential of an electron in a capacitor

    Why is the electric potential of an electron in a capacitor measured from the negative plate and not the positive plate here? This is from Liboff Introductory quantum mechanics 1st(current is 4th) edition: I don't understand why the distance z is measured from the bottom plate if the...
  24. H

    Conductors in capacitor equal but opposite charge?

    Homework Statement If you put a capacitor in some circuit, why exactly do the conductors that form the capacitor have equal and opposite charge? This seems to be assumed in many cases. Homework Equations Is it possible for the conductors to have different magnitude of charge?
  25. Sunanda

    What is the potential of plate B with respect to infinity?

    I have two isolated plates A and B, kept parallel to each other. Now I give charge +Q to the plate A, it will redistribute itself as +Q/2 on the outer plate A and + Q/2 on the inner plate A. Right? Now this will induce charge -Q/2 on the inner plate B and +Q/2 charge on the outer plate B...
  26. C

    RC Circuit with parallel resistor

    Homework Statement A circuit contains a capacitor of capacitance C, a power supply of emf E, two resistors of resistances R1 and R2 , and a switch S2 . Resistor R1 is in series with the power supply and R2 is in parallel with the capacitor and the power supply. S2 switches the branch that...
  27. A

    Magnetic field in a capacitor

    When capacitor is leaking ie, when charge is leaking or when it is discharging then will there be any magnetic field between the parallel plates of capcitor?
  28. K

    Understanding surge capacitors

    I have a decent understanding of power factor caps, (far from perfect) but I’m not as clear on surge caps. Surge caps store transient voltages and discharge back into the system? Why are they tied to ground? And do they also provide capacitance in the motor circuit they are tied to providing...
  29. K

    Method for testing discharge resistors in 3 phase capacitors

    A couple of months ago I was troubleshooting a blown fuse on a power factor correction capacitor in a 4160v motor starter. While I was working through it I noticed that my DMM (fluke 289) would charge the capacitor to a certain degree while on the Ω function. The current from the meter would...
  30. C

    Voltage regulator question

    [mod note: thread moved from homework forum] I have this Mouser voltage regulator https://www.mouser.com/ProductDetail/STMicroelectronics/L7912CV/?qs=sGAEpiMZZMug9GoBKXZ75xbXFDWiKqpmvrbz5I0HaKg%3D Wired according to datasheet https://www.mouser.com/ds/2/389/l79-974159.pdf see insert photo...
  31. sam charlton

    Supercapacitors for dummies?

    Hello, what is the consensus on using super capacitors to power a 500w electric bike motor. cheap and innovative alternative to li-po batteries or explosion between thighs waiting to happen? the plan is to create something like this: i will use a 24V motor...
  32. A

    Cockcroft Walton Accelerator

    Hi everyone. I am doing some research about Cockcroft Walton Accelerators, because I need to be able to explain them to others as a part of a project. However, in order to understand the Cockcroft Walton Accelerator, I need to be able to understand the Cockcroft Walton Generator first. I have...
  33. J

    Inserting a conductor in a parallel-plate capacitor

    Consider a parallel plate capacitor formed by two plates of length ##L## and width ##d##, separated by a distance ##e##. There is a vacuum in between the plates. Let's note the capacitance of this arrangement ##C_0##. I insert a conducting plate of length ##l=L/2##, with ##D##, and thickness...
  34. M

    Capacitors in Hybrid-pi Model and Frequency Response

    This is what I have and I was wondering where C2 might go if I make apply the hybrid pi model on this circuit. I know that all the DC Voltages should be shorted if I were to make use of an equivalent circuit, so that in the hybrid pi model R1 should be in the input and R3 in the output. But what...
  35. P

    Engineering Resonance and RCL circuits

    Homework Statement I'm just really lost on this topic and honestly don't have too much of an idea. (a) Using Kirchoff's loop rule, find the differential equation satisfied by the charge q(t). (b) Verify that q(t) = q_max e^-(a/t) cos(ωt) satisfies the differential equation for particular...
  36. C

    Capacitor discharge vs. Pole relaxation

    I'm trying to understand the difference between these two when it comes to measuring the amount of time they take. As of right now, my current understanding is that pole relaxation is based on the amount of time that it takes for polarization of the charges to go back to its relaxed state. For...
  37. J

    Why/how does distance affect capacitance?

    As distance between two capacitor plates decreases, capacitance increases - given that the dielectric and area of the capacitor plates remain the same. So, why does this occur?
  38. Mzzed

    Capacitor codes

    I have recently come across a datasheet for a buck converter regulator that uses the terms "c code" and "No." for certain capacitors but when going to the manufacturer datasheets for these capacitors they make no reference to any "c code" or one single number for a capacitor. If anyone could...
  39. Mzzed

    Basic capacitor concepts

    If a capacitor is charged to 10V where the negative side is connected to ground (0V), when the capacitor is disconnected from the power supply on both the positive and negative sides; 1) Will the negative side of the capacitor still be 0V relative to the ground it was just connected to? 2) Say...
  40. maxelcat

    Current in a capacitor question

    This is a question from an a level textbook. The R starts at 8k ohms (thought its not relevant) Switch S is closed - C fully charges up. Question:Switch S is opened. Explain why the current decreases and REVERSES direction in the ammeter. This is what I think. The decrease in current is...
  41. G

    Doubt regarding voltage balance on capacitors

    The text in the paper says that C1 has Vout/N-1 = Vout/6, C2 has 2*Vout/6 and so on... Can someone please explain how? Source : Liao, Zitao, Yutian Lei, and Robert CN Pilawa-Podgurski. "A GaN-based flying-capacitor multilevel boost converter for high step-up conversion." Energy Conversion...
  42. A

    Why do the capacitors in this circuit have the same voltage?

    Homework Statement Homework Equations C = Q/V The Attempt at a Solution I understand that capacitors in parallel have the same voltage, but wouldn't they be in series after the switch is flipped, since no current goes to the left most portion of the circuit.
  43. R

    Capacitor with uniform space charge between them

    Homework Statement 2 large plates are separated by a distance d and a space charge of uniform charge density p is placed between them and a potential difference V is applied accross the plates. Find the electric field stength at a distance x fromt the positive plate The answer is -V/d...
  44. CassieFordham

    Layering different dielectric materials to increase breakdown voltage

    This is a general question, not a homework question, and @berkeman said I should post it in the EE forum... I'm trying to get a better understanding of the relationship of dielectric constant (relative permitivity) to dielectric strength (breakdown voltage).I want to know what happens when you...
  45. Drako Amorim

    Unconventional capacitors connection

    Homework Statement Hello everyone. The problem is related to these two capacitor connections: In the enunciate it is said that these two are equivalent and it is requested to prove that: C1=(CxCy+CyCz+CzCx)/Cx C2=(CxCy+CyCz+CzCx)/Cy C1=(CxCy+CyCz+CzCx)/Cz Homework Equations C=Q/Vab...
  46. C

    Capacitance - does a general approach really exist?

    I have spent hours on the web and in libraries to obtain a general and careful approach to the notion of capacitance, without success. Please, read carefully the question: I already know the usual blabla about capacitance. Also, I appologize in advance for the length of this question. I...
  47. A

    Energy stored in Capacitor Network

    Homework Statement A potential difference Vab = 46.0 V is applied across the capacitor network of the following figure. If C1=C2=4.00μF and C4=8.00μF, what must the capacitance C3 be if the network is to store 2.80×10−3 J of electrical energy? Homework Equations U=(1/2)CV^2 (1/C) = (1/c1) +...
  48. Narayanan KR

    Will capacitors react to external electric field

    1. Passage of current via coil creates magnetic field (fig a), similarly external magnetic field linked to coil when changed created electric current in the circuit connected ( Faraday's em induction fig b). 2. Charging of capacitor using voltage source creates electric field in the capacitor...
  49. C

    Soldering High Voltage Capacitors

    Hi guys! I am fairly new to the forums so thank you for having me here :) I just wanted to ask some of you guys here who have a better knowledge about high voltage capacitors than me. I am creating a lightning impulse voltage generator as my final year project and have been provided with most...
  50. P

    Force between capacitor plates

    What will happen to the force between the capacitor plates (Will increase or decrease or remain constant) if we introduce a dielectric between them? As per the answer given the physics textbook , the force will remain same? But as per my understanding the force should decrease since electrical...
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