Capacitor Definition and 196 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. Maximum charge on the plates of a capacitor

What I have done: The electromotive force due to Faraday's Law is: ##\mathcal{E}=-\frac{d\phi(\vec{B})}{dt}=\frac{d}{dt}(Ba^2)=a^2\frac{dB}{dt}=-10^{-4}V.## In the circuit, going around the loop in a clockwise fashion: ##\oint_{\Gamma}\vec{E}\cdot d\vec{l}=-\frac{d\phi(\vec{B})}{dt}\Rightarrow...
2. I Confused about charge on a dielectric-filled capacitor with the battery turned on

With a capacitor with a dielectric with the battery on, ##E_{total} = E_0 + E_i## ##\frac{Q_t}{dC_t} = \frac{Q_0}{dC_0} + \frac{Q_i}{dC_i}## thus, ##\frac{Q_t}{C_t} = \frac{Q_0}{C_0} + \frac{Q_i}{C_i}## since in a battery ##V_t = V_0, V_i = 0##, so either ##Q_i = 0## or ##C_i = infinite## but...
3. Minimizing the voltage drop across a capacitor (solution shown)

The following is the question and the solution to the question. I understand the solution to the part where you find the Ceq and derive Qeq from the equation Q = Ceq*V. However, I do not understand where V1 = V0-V2 come from. When calculating the minimum voltage, how do you come up with the...
4. A disconnected capacitor with two dielectrics in parallel

I considered the capacitor as two capacitors in parallel, so the total capacitance is ##C=C_1+C_2=\frac{\varepsilon_0\varepsilon_1 (A/2)}{d}+\frac{\varepsilon_0\varepsilon_2 (A/2)}{d}=\frac{\varepsilon_0 A}{2d}(\varepsilon_1+\varepsilon_2).## Since the parallel component of the electric field...
5. Maximum charge on a spherical capacitor

The electric field is the one generated by the charge ##+Q## on the inner sphere of the capacitor, which generates a radial electric field ##\vec{E}=\frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\hat{r}## which, due to the presence of the dielectric, become...
6. High Pass Filter Working Principle

A few days ago, I learnt to make an AC to DC converter. One question is troubling my mind. How does a High Pass Filter Capacitor work? Going through some websites I got a sketchy idea. As the current after passing through the bridge rectifier comes to capacitor, the capacitor starts charging...
7. Measuring Capacitance using a basic Multimeter

Is there any way to measure the capacitance of a capacitor indirectly using a multimeter that does not have the option to measure capacitance directly?
8. Comparing energy lost by the battery & energy gained by the capacitor.

Imagine the two terminal of a *parallel-plate capacitor* are connected to the two terminal of a battery with electric potential difference #V#. If the capacitance of the capacitor is #C#, and the area of each plate is $A$. In this process would the energy lost by the battery and the stored...
9. Engineering Capacitance for a capacitor with two dielectrics

The geometry of the capacitor can be either cylindrical or spherical.
10. Capacitance of a parallel-plate Capacitor with non uniform dielectric

Hey guys! I'm having trouble with the solution that I arrived at. Through boundary conditions I'm able to determine ##\vec{D}## as $$\vec{D}=-\frac{4Q}{R_0^2}\hat{e_z}$$ (In CGS units) Trough that I'm able to get the electric field as $$\vec{E}=-\frac{1}{\epsilon(r)}\frac{4Q}{R_0^2}\hat{e_z}$$...
11. Uncharged capacitors connected in series

I came across the following explanation from the famous book of Sears and Zemansky which I am unable to understand. I can get the initial part where a positive charge goes to the top plate of C1 since the point a is at a +ve potential causing free electrons to transfer from top plate of C1 to...
12. Some questions about capacitor discharging

why does the voltage of the capacitor eventually go to 0 when discharging the capacitor? I heard that's because "current starts flowing when discharging", but how exactly does that lead to V going down? I know that I = C * dV/dt, but that doesn't seem to help me understand why V goes down (which...

The problem is shown below: (I am only asking about part b) ^Above is the problem. Below is the solution to part b. They have claimed that we can set potential at C = D = u(t), and A=B=0. Why is this claim true? What I realise: By applying Kirchoff's across ACDB, Voltage across C1 = Voltage...
14. Question on capacitors -- Can two charged conductors have a potential difference between them if they have equal charge?

Hi, I found the following question in a physics book, and so dusted off my 30yr old knowledge on capacitors and tried to answer it. The question is as follows :- "Suppose two nearby conductors carry the same negative charge. Can there be a potential difference between them? If so, can the...
15. Electrical How to use capacitors in a Solenoid circuit

I've been working on designing an experiment over the past few weeks as part of a school project, under the supervision of a teacher. I have designed a small low-power coil-gun. I have a coil of roughly 60m 24 AWG copper wire wrapped around a length of 2.5cm of clear PVC pipe. I tested the...
16. Capacitance of a spherical capacitor

When I try to do Gauss, the permeability is not always that of the free space, but it varies: up to a certain radius it is that of the void and then it is the relative one. How can I relate them? I'm trying to calculate the capacity of a spherical capacitor. The scheme looks like this: inside I...
17. Kirchoff's laws and capacitance

Equivalent capacitance before and after remains the same. Now the 10F capacitor (which was initially connected in parallel with 20F) would have 30 C charge. Hence an additional 20C must have been supplied to it. The only path which may supply the charge is through battery. However this leads...
18. Heat dissipated in a Resistor-Capacitor circuit

I tried to conserve the charge on the left plates of both the capacitors as intially the total charge on both is 48 and at t=t0 the total charge is 36(on c1) +4V(V is the potential across c2) so i got V=3 and then i conserved the energy Initial energy on both capacitor = final energy on both +...
19. Discharging this capacitor in an RC bridge circuit

The voltage source in the circuit below has been switched on for a long time when the switch S switches off. How long will it take before the current coming out of the capacitor has become less than 1 mA? My attempt: I am far from sure that my solution is correct. This is because i...
20. A circuit with switches, capacitators and resistors

To solve this question first I calculated the potential energy the capacitor A stored. It's equal a: Ca.V²/2. Ok, so when switch S1 is open and S2 is closed I calculated the equivalent capacitance as if they were in series --> 1/Ceq = 1/Ca + 1/Cb --> Ceq = (Ca.Cb)/(Ca+Cb). So I used the formula...
21. Determine the effective current value in the circuit with C and R

Given: U = 220 V f = 50 Hz r = 20 Ohm C = 100 μF Find: I Solution: 1) Xc = 1/(ω*C) = 1/(2*π*f*C) = 1/(2*π*50*10^-4) = 31.83 Ohm 2) R_eq - equivalent resistance R_eq = (r*Xc)/(r+Xc) = (20*31.83)/(20+31.83) = 12.28 Ohm 3) I = U/R_eq = 220/12.28 = 17.9 A True answer given in the textbook is 13 A...
22. Dielectric interface plate capacitor at angle alpha

Consider a plate capacitor with a dielectric interface (\epsilon = \epsilon_0*\epsilon_r, thickness=d) tilted at the angle \alpha . Outside the interface \epsilon = \epsilon_0. Without dielectric interface is the field \vec{E}=E_0*\vec{e_z}. Determine the E-field inside and outside the...
23. Calculating the capacitance of a capacitor

Summary: Two plates side by side, not parallel to each other. Hello everyone, Purpose of this capacitor is to detect changes in water level. It is constructed of a single copper plated pcb on which middle I have made a 1 mm of space separating now two copper plates on a single pcb. So, plates...
24. Gauss' law in Dielectrics

[PAGE 1] [PAGE 2] [PAGE 3] so in the 2nd page,when the dielectric material is introduced the gauss's law becomes $$\oint _ { S } \vec { E } \cdot \vec { d S } = \frac { ( q - q _ { i } ) } { \epsilon _ { 0 } }$$.But my question is why the ##{ \epsilon _ { 0 } }## is in the equation.Shouldn't...
25. Novice question about Relays, Capacitors, and Diodes

Hello, In regards to this relay, TQ2SA-1.5V Panasonic 2 Form C AS Single side stable, 1.5VDC 2A DPDT NON-LATCHING SMD Relay Link to Relay PDF 1 of Relay PDF 2 of Relay (specifically pages 5-6) is attached at the bottom as a pdf file. I was previously told that I would need to wire a IN4001...
26. Determine the capacitor value so that this circuit is in resonance

L=100mh=0.1H ω=10^3 rad/s -> f = 159Hz XL= ωL= 2πfL= 2π*159*0.1= 99.90 Ω Z parallel = [(XL∠90º)*R2] / [(XL∠90º)-R2]= 37.13∠-21.8º XC= 1/ωC= 1/(2 π f C) I dont see how I am supposed to get to C
27. Capacitance of a circular capacitor with two parallel dielectrics

Hello, the problem is better illustrated at the picture below. The capacitor is isolated, with an initial charge Q0. I understand that Q0 does not distribute along the plates homogeneously. How could it be solved with the equivalent parallel circuit?
28. I RC charging - intuition behind equal dissipated energies?

Assume that a resistor R charges a capacitor C, whose other terminal is connected to the ground. The charge at time t = 0 is assumed to be null and the supply voltage is equal to V. We have, as is well known, ##i = \frac{V}{R} e^{-\frac{t}{RC}}##. Integrating ##\frac{i^2}{R}## between t = 0 and...
29. What is the voltage across this capacitor, inductor and resistor?

I can solve for the questions in completely series or parallel circuits however having the capacitor and inductor in parallel while the resistor stays in series is stumping me completely.
30. Capacitor and electrostatic field

Is there any electrostatic field around the leads of a charged capacitor? Let's take just the negative one. If I take a piece of tissue and put close to that terminal it will attract or repel the paper? And if not, why?
31. How does a Capacitor protect against voltage spikes?

Hi Everyone ! I have one confusion regarding the role of the capacitor in the circuit. How does the capacitor protects the circuit from the high voltage spikes. If the capacitor is connected from the circuit with battery. And sudden high spikes come then the capacitor is charged and the energy...
32. How to treat the "ideal" plate capacitor more rigorously?

Hi. The derivation of the capacity of an ideal parallel-plate capacitor is inconsistent: On the one hand, the plates are assumed to be infinitely large to exploit symmetries to compute an expression for the electric field, on the other the area is finite to get a finite expression for the...
33. Final potential difference of a 2 capacitor system

Homework Statement An isolated parallel-plate capacitor of area ##A_1## with an air gap of length ##s_1## is charged up to a potential difference ##\Delta V_1## A second parallel-plate capacitor, initially uncharged, has an area ##A_2## and a gap of length ##s_2## filled with plastic whose...
34. A change in the dielectric constant changes the current or voltage?

hi everyone! i am not into physics, but need it now,i am a programmer workin on an IOT project...so kinda need help. i want to use a capacitor to see what passed between two plates...knowing, it is the change in dielectric after certain intervals......how do i measure the change at the other...
35. Shell in a cylindrical capacitor

Homework Statement There is a capacitor (llenght ##L##) made of a conductor (cylinder of radius ##R_1##) and a cylindrical surface (radius ##R_2##). It is charged with a potential ##V_0##, then it is isolated. (There is vacuum between ##R_1## and ##R_2##) Now we insert a cylindrical...
36. Energy dissipated in the resistors in a 2 mesh RC circuit

Homework Statement Hi mates, I have problems solving the third part of this exercise, I've already done all the previous calculations. Given the following circuit, where the switch S is open, the power supply = 50 volts and: The initial charge in the C capacitor: QC = 0 coulombs The initial...
37. Confused by capacitors and potential energy

Hi I was reading about capacitors and potential energy. But the equation seems counter to how i thought. For potential energy you have: U = Q^2 / 2C or U = CV^2 / 2 But doesn't this suggest you lose potential energy the more capacitance you have? Since in the first equation as C increases U...
38. Engineering Finals Study Time! A Two Source Circuit w/Inductor, Cap, and Resistor

Homework Statement Homework Equations Z = √(XC2 + XL2) XL = 2πƒL XC = 1 / 2πƒC I = V/Z The Attempt at a Solution First off, thank you for all of the help this semester. I'm sure you'll be seeing questions from me in the spring also. Here is how I'm thinking about this problem: 1. Using the...
39. Can we extract energy out of a capacitor in the ocean?

Hello, sorry about my english it's not my mother tongue. I hope this is the right section to place this. 1. Homework Statement A cylindrical capacitor is placed in the sea so that when a wave comes (the water goes up), the water becomes the capacitors dielectric, when the wave has passed (the...
40. Question on the work done while charging up a capacitor.

So the work done when charging up a capacitor is ##dW=VdQ## However, when we add a charge ##dQ## to the capacitor, ##V## also changes accordingly, so I was wondering why the work done wasn't written as ##dW=VdQ+QdV## (one that also takes into account t he change in ##V##). Thanks in advance.
41. Combination of capacitors

Homework Statement Between the plates of parallel plate condenser having charge Q,a plate of thickness t1 and dielectric constant k1 is placed.In the rest of the space,there is another plate of thickness t2 and dielectric constant K2.The potential difference across the condenser will be...
42. Capacitor with Irregular size dielectric

Is it possible to make a capacitor with a dielectric not of the same size with its conducting plates?
43. Calculations for a capacitor charging from another capacitor

Hi all, I'd appreciate help in calculating the voltages in the circuit shown. I thought it should be fairly straight forward, but it has me stumped. This is a sample-and-hold circuit for an ADC -- the switch is closed to charge the hold capacitor with the sample voltage and then opened to...
44. Voltage vs charge?

If voltage is the difference in charge between 2 points, then why for a capacitor of a larger area or thinner dielectric cross section, do they say that it can store more charge given the same apppied voltage? Isnt voltage the difference in charge between 2 points? So if you can store more...
45. Electromagnetic coil and capacitors vs resistance

I'm not really sure if this is even scientific but while calculating how much energy is stored in electromagnetic coils and capacitors, pretty much the same formula is used: For electromagnetic coils it's U=0.5LI2 For capacitors it's U=0.5CV2 Why I think they're the same is that in a sense L to...
46. Find the transient current in this circuit

Homework Statement Homework Equations The Attempt at a Solution I often do circuit. But still there is always stuff i dont get.. I was asked to find 1) I1 I2 I3 immidiately after S closed 2) I1 I2 I3 after sufficient of time 3) I1 I2 I3 immidiately after S opened Just give me clue please...
47. Why is a capacitor a generator of lagging reactive power?

An ideal capacitor has power factor of zero degree leading as current leads the voltage in capacitor.But it also means that capacitor is generator of lagging reactive power.What does that mean.How does it do so.Capacitor is used for reactive power generation at inductive load sites.
48. Making Capacitance the subject of discharge equation

Homework Statement I have to rearrange the equation V=lnVo -t/RC to calculate C. The gradient from graph = -1/RC Homework Equations The Attempt at a Solution I got R/C=ln(Vo-V)/t but the answer I get is wrong
49. Find the Voltage in the parallel resistor-capacitor circuit

Homework Statement C1 = 4 micro F C2 = 8 mF R1 = 4 ohm R2 = 8 ohm V = 12 Questions : 1) Find current I when S not closed 2) find Vb, when Vc is 0 3) find Vd, when Vc is 0 4) total charge move from B to D Homework Equations Vf - Vi = F.d/q (not sure this below is useful equations) I = E/Z E...
50. Disconnected vs Connected parallel plate capacitor

I am having a hard time understanding the whole idea behind and the difference between disconnected vs connected parallel plate capacitor in terms of voltage and charge. How does this relate to the formula C(Q/V)=ke0A/d?