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Questions tagged [capacitance]

The ability of an object to store electric charge.

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6answers
12k views

Why do we use capacitors and not batteries in defibrillator?

Why do we use capacitors in defibrillators and not batteries? I know that capacitors are used to store electrical energy but isn't the function of a battery just the same? Moreover, I know that ...
28
votes
10answers
6k views

Is every open circuit a capacitor?

I think that even open-ended wires can let AC current flow through them, just with a low capacitance. I also think an antenna could be a capacitor and open ended. Am I thinking correctly?
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8answers
10k views

Why is capacitance defined as charge divided by voltage?

I understand that capacitance is the ability of a body to store an electrical charge and the formula is $C=\frac{Q}{V}$. What I don't understand, however, is why it is defined as coulomb per volt. Of ...
20
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3answers
5k views

Is there a deep reason why springs combine like capacitors?

I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
19
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4answers
2k views

Distorted colors of Google StreetView photographs near electric power lines

This is a followup to my question: Cyclist's electrical tingling under power lines Some users presented a convincing picture that the electric shocks under power lines are primarily from the ...
18
votes
2answers
734 views

Are there analogs to resistance, inductance, capacitance, and memristance connecting the weak force to electromagnetism?

A question was asked over at EE.SE recently which I tried to answer, but much of my answer was speculative. I'm hoping someone here can help my ignorance. In electronics design, there are four ...
16
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3answers
103k views

What is the electric field in a parallel plate capacitor?

When we find the electric field between the plates of a parallel plate capacitor we assume that the electric field from both plates is $${\bf E}=\frac{\sigma}{2\epsilon_0}\hat{n.}$$ The factor of two ...
15
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5answers
56k views

What is the difference between a battery and a charged capacitor?

What is the difference between a battery and a charged capacitor? I can see lot of similarities between capacitor and battery. In both these charges are separated and When not connected in a circuit ...
14
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11answers
75k views

Why does the voltage increase when capacitor plates are separated?

In lab, my TA charged a large circular parallel plate capacitor to some voltage. She then disconnected the power supply and used a electrometer to read the voltage (about 10V). She then pulled the ...
14
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3answers
54k views

Field between the plates of a parallel plate capacitor using Gauss's Law

Consider the following parallel plate capacitor made of two plates with equal area $A$ and equal surface charge density $\sigma$: The electric field due to the positive plate is $$\frac{\sigma}{\...
13
votes
5answers
1k views

Where's the energy in a boosted capacitor?

Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$. Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
12
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4answers
5k views

A problem of missing energy when charging a second capacitor

A capacitor is charged. It is then connected to an identical uncharged capacitor using superconducting wires. Each capacitor has 1/2 the charge as the original, so 1/4 the energy - so we only have 1/2 ...
12
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3answers
4k views

Why do charges enter the capacitor in the first place?

I have been studying Capacitors for the past year now and the one thing I don't understand is how a charge is stored on the capacitor. Essentially, a circuit with a capacitor is an incomplete circuit ...
12
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4answers
11k views

Will a charged capacitor discharge if one lead is connected to ground?

If I charge a capacitor and connect one lead to ground keeping the other lead floating, will the capacitor discharge ? ...
12
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4answers
2k views

Capacitance of two almost-touching hemispheres

This capacitor is composed of two half spherical shelled conductors both with radius $r$. There is a very small space between the two parts seeing to that no charge will exchange between them. So far,...
10
votes
5answers
7k views

Why does a ceiling fan start up slowly? [closed]

I think it probably has something to do with the capacitor inside but I don't get it why doesn't it just Start spinning instantly when we push the button why does it slowly start to spin and gradually ...
9
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4answers
2k views

Where does the extra energy come from in an LC circuit?

In an LC circuit, or an LC tank, the capacitor discharges in one direction through an inductor and then the inductor seems to carry energy in the form of a magnetic field , to charge the capacitor ...
9
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2answers
2k views

Are capacitors and inductors mirror images of each other?

I'm reading through my E&M textbook (Physics for Scientists and Engineers 3rd edition, Knight) and watching as many lectures on YouTube (Shankar, Pomerantz, Lewin, etc) to prepare for next quarter....
9
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2answers
39k views

What happens to capacitor’s charge when the plates are moved further apart?

In my physics textbook there is an example of using capacitor switches in computer keyboard: Pressing the key pushes two capacitor plates closer together, increasing their capacitance. A larger ...
9
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2answers
5k views

Charge distribution on a plate of Capacitor with Dielectrics .

I had this Homework Problem with a capacitor (parallel plate) that has a group of 3 dielectrics between it like so : Now We were asked to find the equivalent capacitance and the distance of ...
8
votes
3answers
13k views

Electric field intensity in a dielectric inside a capacitor

In the question A parallel plate capacitor is charged from a cell and then isolated from it. A dielectric slab of dielectric constant $K$ is now introduced in the left half region between the ...
8
votes
1answer
775 views

Theoretically speaking what are the limits for super-capacitors in terms of their storage capability?

According to wikipedia, commercially available super-capacitors in 2010 can store 30Wh/kg, with 85Wh/kg achieved in a lab in 2011. Have any theoretical calculations been done to show what can be ...
7
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4answers
1k views

Why is there no current between two capacitors connected in series?

I want to ask a question about the figure below. Why the current does not flow from $-Q$ term to $+Q$ term or from a to b since there is a voltage?
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1answer
2k views

Lagrangian method for an $LC$-Circuit

In the paper http://physics.unipune.ernet.in/~phyed/26.2/File5.pdf, the author solves the LC-circuit using Euler-Lagrange equation. She assumes that the Lagrangian function for the circuit is $$L=T-...
7
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2answers
2k views

Capacitance of Non-concentric Spheres

We all know how to obtain the capacitance $C=\frac{ab}{b-a}$ (ignoring constants) for two concentric spheres of radii $a,b$. I was just thinking to myself, what would happen to the capacitance for ...
7
votes
2answers
917 views

What stops charges from jumping across a capacitor?

I've learnt in class that charges store up on capacitor plates (the electrons). But why don't the electrons just jump across capacitor, what stops them? At first I thought perhaps it might just be the ...
7
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5answers
10k views

Is a capacitor in an open circuit charged?

Say I have a circuit consisting of a battery, a wire, an open switch, and a capacitor. The circuit is open since the switch is open. My book says that the capacitor will only be charged when the ...
7
votes
1answer
2k views

$Q$ factor of parallel RLC circuit in series with a capacitor and resistor

I know that for parallel RLC circuits, the $Q$ factor is given by: $$ Q = R \sqrt {\frac{C}{L}} $$ But now suppose it is connected in series to a resistor $R_2$ and capacitor $C_2$. Would the $Q$ ...
7
votes
1answer
847 views

What does a relativistically moving capacitor “look” like? [duplicate]

Suppose I have a parallel plate capacitor with a vacuum between the plates, a voltage $V$ across them and a capacitance of $C$. What will this arrangement look like to an observer in a uniformly ...
7
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2answers
613 views

Express Laplace transform of voltage across a capacitor in terms of charge

In Charge Tunneling Rates in Ultrasmall Junctions section 2.1, the authors consider the problem of charge relaxation in a simple circuit shown in Figure A. The implicitly use an assumption made about ...
6
votes
5answers
13k views

If we charge a capacitor can we discharge it into a battery?

I have read that we can charge a capacitor using a battery, but can the vice versa happen? My project needs to show a battery being charged through a fully charged capacitor.
6
votes
4answers
3k views

What is the current of a capacitor when the derivative of voltage is undefined?

This is from the textbook I am reading: I know this equation for capacitors: $$i=C\cdot \frac { dv }{ dt }$$ Here is my question: how can diagram (a) be allowed if the derivative of the voltage ...
6
votes
2answers
39k views

Capacitor circuits with light bulb

Let's say we have a normal circuit with a light bulb, with wires and a battery. When one places a capacitor in this circuit, how is the light bulb able to light up, even when the capacitor prevents ...
6
votes
4answers
20k views

Electric field outside a capacitor

I know that the electric field outside of a capacitor is zero and I know it is easy to calculate using Gauss's law. We create cylindrical envelope that holds the same amount of charges (of opposite ...
6
votes
1answer
723 views

Energy to move a charge [closed]

If we want to move a charge q from the positive to the negative plate of a capacitor C (with a potential V), the energy that we need is E=1/2qV or E=qV ? Thanks for your help.
6
votes
4answers
8k views

Capacitor with different charges on each plate

I am confused as to whether/how capacitance changes when each plate has a different charge. For example, consider a coaxial cable and put $20Q$ on the outer cable, and $-Q$ on the inner. Or how about ...
6
votes
4answers
1k views

Energy Gain with capacitor?

I have a question about energy gain in capacitors. Assume the following system: As the electron gets accelerated inside the capacitor, it will have more kinetic energy coming out than going in. But ...
6
votes
3answers
555 views

Physics olympiad question about a charge capacitor

This is problem 1463 from the Russian magazine Quant. Unfortunately my Russian is not good enough, so I apologize if the translation of the problem contains errors. Problem. Inside a flat charge ...
6
votes
1answer
927 views

Grade 12 physics electric fields question

The question is: The initial electric field strength between oppositely charged parallel plates is $3.0\cdot 10^3 \frac{N}{C}$. What would the electric field strength become if half of the charge ...
6
votes
1answer
1k views

Grounding system of conducting plates

So, I always make mistakes on problems such as this (the grounding part), so I'm hoping someone could really explain to me how the process works. There are $n$ large parallel plate conductors ...
6
votes
1answer
324 views

Ring to infinite plate capacitance calculation

I don't know how to calculate the capacitance between a conductive ring and a conductive infinite plate. The ring has internal radius R1 and external radius R2 and is placed on a parallel plane with ...
6
votes
3answers
4k views

Charge Distribution on a Parallel Plate Capacitor

If a parallel plate capacitor is formed by placing two infinite grounded conducting sheets, one at potential $V_1$ and another at $V_2$, a distance $d$ away from each other, then the charge on either ...
5
votes
4answers
4k views

Attractive force between capacitor plates

A textbook question requires me to calculate the force of attraction between plates of a parallel-plate capacitor. The answer provided is $\frac{1}{2}QE$. I am not entirely sure how they arrived at ...
5
votes
1answer
28k views

What defines the maximum charge a capacitor can store?

The formula for a capacitor discharging is $Q=Q_0e^{-\frac{t}{RC}}$ Where $Q_0$ is the maximum charge. But what property defines the maximum charge a capacitor can store? If it depends on ...
5
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3answers
1k views

Physical interpretation of circuit with battery charging capacitor

In the picture below, we are charging up the capacitor, by connecting it (and the resistor to a battery of voltage $V_0$, at time $t = 0$). In terms of what is happening physicially, how would you ...
5
votes
2answers
300 views

Capacitor net charge on parallel and series arrangement

When the capacitors arrange as below, In the parallel arrangement, when we considering the net charge of the system,we take it as Q1+Q2. But in the series arrangement, The net charge is get as Q1. I ...
5
votes
2answers
500 views

Will the two capacitors be charged in this circuit?

Is such a circuit possible to exist with the two capacitors charging? Because if we considered the outer loop using Kirchhoff's law we get: ℰ = q1/C1 + q2/C2 But since the two capacitors are ...
5
votes
1answer
16k views

Capacitance of spherical capacitor when earthed [closed]

The capacitance of a spherical capacitor is given by $$C=\frac{4\pi \varepsilon_{0}}{\left ( \frac{1}{a}-\frac{1}{b} \right )}$$ How can I find out the capacitance of the capacitor when The inner ...
5
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2answers
2k views

Why does a capacitor discharge?

Suppose a charged capacitor (parallel plates), the negative and positive charges on two plates attract each other. Which force cause the negative charge carriers (electrons) move through the circuit ...
5
votes
3answers
622 views

Electric Potential Energy of a charged conductor

My physics textbook asks (I translate): A sphere of copper with a radius of 0.72 metres is charged with a Potential of 270,000 Volt. Find its charge and the electric energy it holds. I found the ...