All Questions
Tagged with electromagnetism capacitance
241
questions
2
votes
1
answer
29
views
Energy in Dielectric Medium as a Sea of Dipoles
Simply, why is the energy density in a dielectric medium = $\frac{1}{2}K\epsilon_oE^2$ where K is the dielectric constant of the dielectric medium?
Working proof
Consider a capacitor that is charged ...
- 35
0
votes
2
answers
49
views
Maxwell stress tensor on a capacitor late given a dielectric
Given a capacitor with large plates (area $A$) separated by a dielectric with relative permittivity $\varepsilon_r$ and thickness $g$, I believe the Maxwell stress tensor states that the force on each ...
0
votes
2
answers
57
views
Growth of Current in an Inductor Connected Across a Battery [duplicate]
When baterry is connected across inductor with no resistance in circuit, won't the current flowing through it be zero, as inductor back emf will counteract the effect of potential difference due to ...
0
votes
2
answers
73
views
Cannot catch a minus sign mistake when deriving the ODE for an LC circuit
It's probably a very basic question but I just cannot wrap my mind around it. I'll just try to derive the differential equation for an LC circuit:
According to the law of induction, in a solenoid we ...
- 4,887
0
votes
2
answers
34
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For a plane isolated capacitor with a dielectric slab partially introduced inside it, what's indpendent of the medium? $\mathbf E$ or $\mathbf D$?
Say we have an isolated capacitor with charge $Q$ and a dielectric slab, $\varepsilon_r$, partially introduced between the capacitor's plates, then, I'd say the displacement vector should be the same ...
- 296
2
votes
0
answers
23
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Capacitance coefficients of two enclosed spheres
I was reviewing G.B. Jeffery's work titled On a Form of the Solution of Laplace's Equation Suitable for Problems Relating to Two Spheres. To access the full text, you need to click "DOWNLOAD PDF&...
0
votes
2
answers
68
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Maximum Impedance for LCR series circuit
Impedance of an LCR series circuit can be maximum when either Inductive Reactance or Capacitive Reactance is maximum at an instant of time.
Max Capacitive Reactance is at minimum frequency, and max ...
0
votes
1
answer
53
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Why doesn't current flow stop in LC oscillation?
Why doesn't current flow stop in a LC circuit? It is supposed to stop when the magnitude of emf developed across capacitor is the same as the inductor, right?
2
votes
1
answer
45
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How does the presence of a static electric field affect the normal operation of an antenna (if at all)?
For arguments sake lets say I have a receiving antenna that has been positioned between two large conductive plates. Are there any differences in how that antenna operates (gain, efficiency etc.) when ...
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1
vote
2
answers
78
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Telegraphers' equations for lossy transmission line when $R$, $L$, $G$, and $C$ are frequency dependent
The telegraphers' equations are commonly written as
$$\frac{{\partial v(z,t)}}{{\partial z}} + R\space i(z,t) + L\frac{{\partial i(z,t)}}{{\partial t}} = 0$$
$$\frac{{\partial i(z,t)}}{{\partial z}} + ...
- 11
1
vote
2
answers
77
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How does capacitance of a capacitor not depend on the distance of the capacitor to a battery?
When you consider the positive terminal of battery, wire and capacitor plate, they form a single conductor, which is polarised by the field of the other capacitor plate, but it is not polarised as ...
- 155
2
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1
answer
67
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Does parasitic capacitance contribute to the magnetic flux?
When modeling a real inductor, typically the equivalent circuit is the following one:
Source
A series resistance and parallel capacitor is added.
This model works very well to explain the following ...
- 187
0
votes
0
answers
61
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Total capacitance from a capacitance matrix
I am currently doing a project where I need to find the capacitance of a multi-conductor system. Through my literature review, I read about the concept of a capacitance matrix. After more research, I ...
0
votes
1
answer
34
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How to find the capacitance between two metal sheets [closed]
Let’s say I have two metal sheets in a 2D plane. The first one has area A, the second one is infinitely long, and they are separated from each other by a distance d. How can I find the capacitance ...
0
votes
1
answer
83
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What is the electric potential difference for two spheres capacitors connected in series [closed]
Consider two spherical capacitors with Identical radii for the inner and outer spheres (let them be a and b). +q and -q charges are in the inner and outer sphere on Both the capacitors. Now I connect ...
- 123
2
votes
5
answers
109
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Magnetic field in a capacitor in a $LC$ circuit
Consider a simple $LC$ circuit in which the charge on the capacitor varies sinusoidally. Current flows in this circuit, changing the charge on the capacitor and hence changing the electric field ...
- 29
1
vote
1
answer
780
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Trying to understand how to apply Maxwell stress tensor to calculate forces
I'm struggling to understand how to use Maxwell's stress tensor to compute electromagnetic forces acting on surfaces. I'll take problem 8.7 from Griffths Introduction to Electrodynamics as an example.
...
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3
votes
1
answer
342
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Doubt regarding a possible mistake in Griffiths Electrodynamics
Griffiths, in section 7.3 Maxwell's Equations, says:
There’s another way to see that Ampère’s law is bound to fail for nonsteady
currents. Suppose we’re in the process of charging up a capacitor (Fig....
- 702
1
vote
1
answer
29
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Corona Discharge, Leyden Jar and Free Charge [closed]
Watching Walter Lewin's 8.02x- Lect 14, at the end of it, he explains the mechanism behind a Leyden jar. The (simplified) picture is the following:
Source: https://www.youtube.com/watch?v=By2ogrSwgVo
...
- 115
2
votes
0
answers
42
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What if two plates charged with different charges are placed face to face?
I'm asking this question because I can't solve it even if I ask a lot of smart people. (Please understand that I don't speak smoothly because I use a translator. I'm korean student..)
In the next ...
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0
votes
1
answer
77
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Capacitance matrix is diagonally dominant
I have heard of the following claim “the capacitance matrix of an isolated system of conductors is diagonally dominant” but I couldn’t find a way of proving it myself or any reference for that matter. ...
- 402
1
vote
2
answers
359
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Relation between resistance, capacitance and resistivity
I recently learnt that for a resistive medium, $RC = ρε$ where $R$ is the medium's resistance, $C$ is the capacitance, and $\rho$ is the resistivity. Now, I am able to prove that this is true in the ...
- 301
1
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1
answer
29
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Charging capacitor moving with respect to the dielectric
Suppose I am charging a capacitor, so that a magnetic field is generated. This capacitor is placed inside a dielectric material with permittivity $\epsilon$ and then it is moved parallel to it's ...
- 49
1
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0
answers
105
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Capacitance of a System of Two Spheres
To explain more clearly, I am looking for how one would derive an expression for the capacitance of a system which consists of two conducting spheres of radius $a$ separated by a distance $d>2a$. I ...
- 11
4
votes
5
answers
2k
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Can you charge a capacitor with only voltage (without current)? If No, then how does a capacitor correct power factor?
Let me explain you why I am asking this question.
The other day I was studying about power factor correction of a (step up or any) transformer. It said that on the output side of transformer's ...
- 129
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0
answers
83
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How do you determine the electromotive force when the negative plate of the capacitor is connected to the positive terminal?
$\newcommand{\d}{\mathrm{d}}$We have a square wave source with period $T$ and voltage $V_0$ connected to a circuit which is has a resistor of resistance $R$ in series with an uncharged capacitor of ...
- 221
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1
answer
51
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Enhanced capacitance by magnetism?
Consider the first picture.
This is much like a parallel plate capacitor charged by a battery. But instead of parallel plates we are using parallel rings. The rings are attracting each other because ...
1
vote
0
answers
40
views
Why do two infinite planes contribute to Electric Field but not two surfaces in co-axial cylinder
I understand why the electric field between two infinite planes is $\frac{\sigma}{\epsilon_0}{\vec{e_z}}$. Both planes contribute to the field - the upper positive charged plane "pushes" ...
- 149
1
vote
1
answer
151
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Does a spherical capacitor hold as much energy as a spherical shell?
Consider a spherical capacitor, that is, two concentrical spherical shells which serve as plates. Suppose the capacitor is charged. It is my understanding that the energy stored by a capacitor is ...
3
votes
0
answers
194
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Wave equation derivation in a transmission line
$$
V(x, t) = L\Delta x\frac{\partial I}{\partial t}(x, t) + \frac{1}{C \Delta x}\int{I_c(x, t) \; dt} \tag{1}
$$
$$
V(x + \Delta x, t) =\frac{1}{C \Delta x} \int {I_c(x, t) \; dt} \tag{2}
$$
$$
I(x) = ...
- 1,525
2
votes
1
answer
123
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Why is the product of the $L$ and $C$ matrices for coupled transmission lines diagonal?
Background - transmission line
$\newcommand{\ket}[1]{\left \lvert #1 \right \rangle}$
A transmission line can be modeled as an infinite sequence of inductors and capacitors:
...
- 24.6k
1
vote
2
answers
185
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How is the total energy conserved when an electron moves through a plate capacitor and gets accelerated?
We assume that the electron enters the plate capacitor with a constant velocity. If we also assume that the electric field is homogeneous and edge effects can be neglected, a force along the y-axis ...
- 47
0
votes
1
answer
73
views
Wave against a capacitor
Let's say you had an EM wave from a distant source hitting a parallel plate capacitor as in the above setup. Would the battery "hold" the voltage between the plates constant, thus holding ...
- 623
0
votes
3
answers
187
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How does a capacitor discharge? Shouldn't the electric field prevent the charges from leaving the plates?
If the charges on opposite plates of a charged capacitor are attracted to each other by the electric field between them, once we connect the capacitor to a device, how come the charges leave the ...
- 41
1
vote
1
answer
248
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Current through a capacitor in AC Circuits
I'm a little confused on the equation for the instantaneous current through a capacitor in AC circuits.
My textbook has it as:
$$i_C = \omega CV \ cos(\omega t + \pi/2) = -\omega CV \ sin(\omega t)$$
...
- 111
0
votes
3
answers
114
views
Current sign convention RC and LC circuits
When deriving the equations for RC and LC circuits we substitute the current in the circuit with the time derivative of the charge on the capacitor. However, when I think about it, the rate at which ...
user345007
0
votes
1
answer
40
views
What is the working mechanism for current increase with voltage increase (in case of a capacitor)
Image I have a loaded parallel-plate capacitor having capacity $C$ with a fixed charge $Q$ on the plates. It induces a voltage $U = Q / C$. If I connect a load with resistence $R$, then a current $I =...
- 737
1
vote
1
answer
181
views
What happens if a charged capacitor is connected to the ground on both sides through resistors? [closed]
Imagine we have the circuit below:
When the switch is connected to (a), we will be charging the capacitor. After the capacitor is fully charged, the left side of capacitor will be at 5V and right ...
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0
votes
0
answers
115
views
Why does changing the permittivity of the dielectric within a capacitor not have any effect on the electric field opposite the plate?
Utilizing FEMM 4.2, I built a pair of simple parallel plate capacitors, however I oriented them so that the anodes of these capacitors faced each other. As expected, the electric fields of the two ...
- 1
0
votes
1
answer
518
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Questions about Maxwell capacitance matrix and reference point selection
Let's consider a system of n conductors. If I'm not mistaken, the Maxwell capacitance matrix tells us that the following system of equations applies:
\begin{align}
Q_1 &= C_{11}V_1 + C_{12}V_{12}...
- 103
31
votes
10
answers
8k
views
How is energy "stored in an electric field"?
My physics teacher told me the statement "The energy of a capacitor is stored in its electric field".
Now this confuses me a bit. I understand the energy of a capacitor as a result of the ...
- 492
0
votes
2
answers
340
views
How much can we push the plates of capacitor?
I am currently a highschooler and came across this question in physics
A capacitor of plate area $A$ and separation $d$ is connected to battery of emf $V$ and is charged to steady state. Now the ...
- 3
1
vote
1
answer
118
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Electric Power of Displacement Current
Say we have displacement current $I_d$ inside a plate capacitor of alternating voltage $V(t)$.
We use the formula for electric power on the displacement current:
$P=I_dV$
What does $P$ account for? ...
- 297
1
vote
1
answer
91
views
Does the current inside a capacitor with dielectric becomes zero, if a DC (constant)voltage is applied?
Imagine a capacitor with dielectric with conductivity $\sigma$ and constant voltage source with voltage applied say $V$ is applied to it's ends, now using Ohm's Law we have $ \vec{J} = \sigma \vec{E}$....
0
votes
1
answer
99
views
Work done by battery [closed]
In charging a capacitor to a charge $Q$
Is there an equivalent to this in setting up current through an inductive circuit?
Energy stored in inductor $= 1/2Li^2$
What would be the work done in setting ...
- 3
-1
votes
1
answer
75
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What is electric permitivitty?
I have read 10 different explanations for this in 10 minutes. By definition, to permit means to allow something, but some sites say that it is actually how much a material resists the electric field?
...
- 183
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votes
2
answers
303
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What does equivalent capacitance in general mean?
If you have some capacitors in series then their EC (= equivalent capacitance) means the capacitance of the capacitor which will store the same charge as any individual capacitor in the series when ...
- 480
2
votes
2
answers
467
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Capacitor electromagnetic radiation
The plate capacitor is charged and discharged with sinusoidally changing electric current. Why does a capacitor emit electromagnetic radiation?
user331916
10
votes
5
answers
3k
views
Is there a magnetic field around a fully charged capacitor?
I know that a magnetic field exists when a capacitor is in the process of charging/discharging:
(a) But what if the capacitor is fully charged? Will the magnetic field still persist? Something like:
...
- 649
0
votes
1
answer
43
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Problem understanding the use of differentials in equation for energy stored in a capacitor
I was going through the Feynman Lecures on Physics Vol. II and arrived at section 8.1, where the equation for the energy stored in a capacitor is obtained.
We consider now the energy required to ...
- 1