Questions tagged [capacitance]

The ability of an object to store electric charge.

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21 votes
3 answers
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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 ...
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21 votes
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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}{\...
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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 ...
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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 ...
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13 votes
4 answers
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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 ...
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10 votes
4 answers
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Why are the two outer charge densities on a system of parallel charged plates identical?

One of the ways examiners torture students is by asking them to calculate charge distributions and potentials for systems of charged parallel plates like this: the ellipsis is meant to indicate any ...
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1 vote
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Capacitor in a dielectric fluid

In the above problem, it states that the dielectric fluid is "pulled up" by an electric force. However, I don't see why this should be true. The electric field (and therefore any electric force) ...
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0 answers
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What is the capacity of this circuit? [duplicate]

I simply can't figure out what the capacity of this circuit is. I can do the math myself, I just need a hint how to create an equivalent circuit where it is obvious what is parallel and what is in ...
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Why does a $1/2$ appear in this simple capacitor equation?

The maximum energy stored by a capacitor is when it’s (theoretically) fully charged, or just before it reaches dielectric breakdown, if you will. It seems, then, that the maximum energy ($E$) $=QV$, ...
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Capacitance of a single charged plate?

Lets say we have a single plate that has a charge of $+Q$ on it. A plate with charge $-Q$ is infinite distance away. Will the plate with $+Q$ have a capacitance associated with it? Why or why not? I ...
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21 votes
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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 ...
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What is the Capacitance of a spherical capacitor when either Inner/outer shell is grounded (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 ...
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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 ...
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7 votes
3 answers
10k views

Energy loss in Capacitors

We have this formula in our textbook for loss of energy when two capacitors are connected together. They mention that it is due to heat dissipation. However, we have not considered any such term in ...
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5 votes
1 answer
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Saving energy while charging capacitor

While charging a capacitor by a DC source, half energy is wasted as heat. Can't we save that energy? Here I am talking about $$(1/2)CV^2 $$ which is wasted while supplying $$CV^2$$
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2 votes
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Capacitor with multiple dielectrics [closed]

Let's say there is a capacitor as shown in the figure, $K_1$, $K_2$, $K_3$, $K_4$ being the dielectric constant of each quadrant. (the dimensions of all four parts are equal.) Let $C_1$, $C_2$, $C_3$,...
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1 vote
1 answer
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Magnetic field in a capacitor

If in a flat capacitor, formed by two circular armatures of radius $R$, placed at a distance $d$, where $R$ and $d$ are expressed in metres (m), a variable potential difference is applied to the ...
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6 votes
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Capacitance of two non parallel plates

What is the formula for capacitance of two non parallel plates at an angle with each other?If the plates were parallel then the value can be calculated as (PermittivityX area of one plate)/distance ...
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4 answers
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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 ...
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8 votes
5 answers
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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 ...
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7 votes
2 answers
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Energy conservation when capacitor charges in a zero resistance circuit

We know that when a capacitor charges from a source $E$, it stores energy of $E=\frac{1}{2}QV $. This is derived without taking into consideration any resistances present in the circuit. We also ...
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6 votes
4 answers
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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 ...
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5 votes
2 answers
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How is the "charge on a capacitor" defined when two plates are unequally charged? [duplicate]

In Concepts of Physics by Dr.. H.C.Verma, in the chapter on "Capacitors", in page 144, under the topic "Capacitor and Capacitance" the following statement is given: A combination of two conductors ...
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Capacitor connected to battery at one end

I was recently solving a problem that involved a circuit with a capacitor that was connected to a battery at one end(the second plate just had a wire which didnt lead anywhere).  In the solutions it ...
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Capacitor discharging through no load?

What exactly would happen in the ideal case where a capacitor were to discharge into a wire with zero resistance? Like in here: There is still charge on one plate of the capacitor, which will try to ...
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Proving electric field constant between two charged infinite parallel plates

It is known that the electric field intensity between two infinitely long charged parallel plates is constant. I had read that one explanation is that if a test charge is placed between the plates, ...
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3 answers
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I wonder why I cannot charge a capacitor with alternating current?

Why can't I charge the capacitor with AC? How do the plates block the flow of electrons with DC but not with AC. Somebody told me that the DC is blocked by the capacitor, so the capacitor gets charge,...
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6 votes
4 answers
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What is the reason for the edge effect in capacitors?

The electric field lines bend at the edges of the capacitors like this: What is the reason for this? Any quick explanation as to why they bend?
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14 votes
4 answers
13k 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 ? ...
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11 votes
2 answers
3k views

Why do the charges on a parallel plate capacitor lie only on the inner surface?

In most pictures I've seen of parallel plate capacitors, charges are drawn so that they're entirely on the inner surface of the plates. I accept that there can't be any net charge within the ...
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8 votes
1 answer
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$Q$ 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 $Q$ ...
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2 answers
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Direction of electric field at points on boundary between two dieletrics

I am ripping my hairs trying to understand this answer, it is written that, for the following set up of two capacitor plates separated by distance $d$ and area $A$ with three dielectrics insert with ...
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3 votes
2 answers
544 views

Battery charging a Capacitor

Ok I know there is a similar question already on here, but mine takes it just a step further. What I would really love to know is exactly how much energy is being transferred from a battery into a ...
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3 votes
2 answers
15k views

Energy changes due increasing plate separation of unconnected charged capacitor

If a capacitor is charged by a source with constant voltage, $V$, the energy, $W$, stored by a capacitor of capacitance $C$ is given by: $$ W = \frac{1}{2} QV = \frac{1}{2} V^{2}C = \frac{Q^{2}}{2C} $...
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2 votes
1 answer
295 views

Resultant capacitance of a capacitor having a combination of dielectrics between parallel plates

In few questions, I am having some confusion on how to take the resultant capacitance of a parallel plate capacitor with a combination of dielectric in between parallel plates. Here is a an example (...
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2 votes
4 answers
12k views

Capacitor Charging and Discharging when connected to the ground

When we charge a capacitor using a battery and then remove the battery, the plates of capacitor becomes charged. One holds positive charge and the other one gets equal negative charge. o. k. ? Now ...
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0 votes
3 answers
11k views

Charge On parallel plate capacitor?

Suppose if we supply two plates of parallel plate capacitor with charges $Q_1$ and $Q_2$.What would be the charge of the capacitor? As the field inside plate should be zero and hence using gauss law I ...
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8 votes
2 answers
3k 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 ...
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6 votes
2 answers
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What's the purpose of capacitors in parallel

In my school textbook it is written that the capacitor acts as a filter, that is, it decreases the fluctuations in the potential difference across the load. But since all the components are ...
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4 votes
4 answers
3k views

The moving capacitor

To what extent can a charged capacitor mounted on a moving platform (e.g. a rotating wheel) be considered an electric current generator? Electric current, after all, is nothing more than the transport ...
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2 votes
4 answers
139 views

How to convince myself that capacitance cannot be a function of voltage?

My textbook states that: ...The capacitance $C$ depends only on the geometrical configuration (shape, size, separation)of the system of two conductors. [As we shall see, later, it also depends on the ...
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0 votes
0 answers
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How to find the canonical equations of the RLC circuit

First, I consider the LC circuit with an ion in the capacitor, which is like this: The current $I$ can be derived easily by using Shockly-Ramo theorem \begin{equation} I = -\frac{qp}{dM} + C\dot{U} \...
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2 answers
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Electric Field from charged sphere within another charged sphere does not reinforce?

Lets say I have a positively charged conducting sphere of Q. The electric field outside the sphere is $$E = \frac Q{4\epsilon\pi r^2}.$$ Now suppose this sphere is enclosed inside another hollow ...
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8 votes
4 answers
2k 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 ...
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  • 185
5 votes
2 answers
3k 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 ...
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5 votes
2 answers
4k views

What physical properties of materials trigger the capacitive touch screen?

I have watched some youtube videos about capacitive touch screen http://www.youtube.com/watch?v=ZHzaVzYEZbw http://www.youtube.com/watch?v=GmCE18RMEeQ The videos show that a conductor(finger), in ...
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2 votes
5 answers
5k views

What happens in a circuit, when the wire and the battery are superconducting. And shorted

When a wire with no resistance is connected to the terminals of an ideal battery, will a current exist in the circuit? If a capacitor is added to the circuit, will it be charged by the battery or will ...
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1 vote
2 answers
3k views

Potential difference between two assymetrically charged plates

What is the potential diff between two plates of charges $q_1$ and $q_2$ ($q_1$ $>$ $q_2$)? I know the equation is $V = \frac{q_1-q_2}{2C}$ where $C$ is the capacitance of the system but how has ...
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1 vote
1 answer
5k views

What will happen if a conductor is introduced instead of a dielectric medium in parallel plate capacitor?

If a conductor like copper is placed between two plates of a parallel plate conductor,neither touching any of them, what will happen to the capacitance of the capacitor?
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1 vote
1 answer
188 views

Proof that elastance matrix is invertible

I was reading this lecture notes from MIT OCW on capacitance . It says $V_i =\sum_j P_{ij}Q_{j} $ where the constants $P_{ij}$ are determined by the geometry of the conductors. This matrix can ...
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