The ability of an object to store electric charge.

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Can Van Allen Radiation belts act as a capacitor?

There are two Van Allen Belts at rougly 5000Km and 17000Km from the surface of the Earth each of them containing majority of one of the charges electrons and protons). Can these belts act as ...
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1answer
36 views

charged dieletric/conductor in capacitor

It is a standard problem to consider a dielectric or a conductor between the parallel plates of a capacitor. But what happens to capacity, voltage, charge, inserting between the plates of an ideal ...
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1answer
18 views

Capacitance per unit charge of three long rods, two of which are connected

Suppose we have three very long rods, each with diameter $a$, placed so that their "centers" form an equilateral triangle, with sided of length $d$. Two of the wires are connected with a thin wire ( ...
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15 views

Capacitance per unit length of a system containing uniformly charged rod and and infinite charged plate

Suppose we have an infinite plane, and a uniformly charged rod of radius $a$ and linear charge density $\alpha$, parallel to the plane and at distance $d$ from it. Net charge of the plane is $0$. Rod ...
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21 views

The force acting on a plate of a parallel capacitor

How can you prove (using high-school-knowledge) that the force which acts on one plate of a parallel capacitor is equal $F = \frac{E \cdot Q}{2}$ Where $E = \frac{U}{d}$ is the magnitude of the ...
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2answers
28 views

Physical Explanation of the steady state in RC circuits

Given a simple RC circuit with a resistance $R$ connected to a capacitor $C$, the differential equation that describes the voltage is $$C \dot V + V/R = 0$$ whose solution is $$V= V(t) = V_0 ...
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1answer
44 views

Force per unit length between two long parallel wires, one of which is insulated

Suppose we have two very long parallel wires of radius $a$ and distance between their axes $d$ ($d>>a$). The first wire if is carrying charge of linear charge density $\alpha$ and is insulated ...
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1answer
17 views

Charge density on a plate of a realistic capacitor

Suppose we have a realistic capacitor,connected to a constant voltage source, with plates at some distance $d$ and a varying charge density across plate due to edge effects. Is it correct to assume ...
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1answer
40 views

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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1answer
26 views

Dependence of capacitance on its build material

We know that the capacitance of a capacitor depends on the dielectric material in between the plates as $$C=\dfrac{K\epsilon_0 A}{d}\,.$$ But what if the electrode material is changed? For example ...
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22 views

capacitors - Square circuit of two capacitors charging in a changing magnetic field

I've been studying capacitors and DC circuits, and I received this question as part of my homework: The above circuit is a square circuit with side a. E and F are the midpoints of AB and CD ...
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25 views

The physics behind capacitive touch sensing

Can someone explain the mechanism of capacitive touch sensing? According to http://www.planetanalog.com/document.asp?doc_id=527410&site=planetanalog&print=yes, it mentions "fringing fields", ...
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32 views

Power in a circuit why is V constant

In deriving the power of a circuit we can do the following: $$\text{P}=\frac{dw}{dt}=\frac{dQ}{dt}\frac{dw}{dQ}=\text{IV}$$ But this assumes that $V$ (since $w=VQ$) is a constant. Why can we assume ...
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1answer
30 views

Finding energy stored in a capacitor

i am given a voltage time graph for a capacitor circuit. How to find energy stored in that capacitor at various time instants? Can we just calculate by using $\frac{1}{2}CV^2$ We are given capacitance ...
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1answer
30 views

What are the consequences of connecting a non-ideal conductor to a battery in open loop?

Suppose I have a battery with $\Delta V=5[\mathrm{V}]$. Now I connect a piece of a metal wire to the "+" side of the battery only. Let's assume that the ambient air is not conductive at all: Are ...
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3answers
245 views

Why capacitance is given as constant value in Farads, and not as max charge in Coulombs?

The equation for capacitance is $Q=CV$ or $V={1\over C}Q$. I don't understand what is the physical meaning of this "$C$": Does the charge in a system changes linearly with voltage under all ...
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1answer
46 views

Why does energy rise in a constant voltage capacitor after inserting a dielectric?

For example from this link: http://te.fisica.edu.uy/fuerza_en_Capacitor.pdf we can deduce, that the work the electric field inside a constant-voltage capacitor does during the insertion of a ...
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3answers
106 views

RL vs RC circuits

I find it difficult to understand as to why current behaves differently in an RL and an RC circuit. At time $t=0$, in an RC circuit the current is maximum, whereas in an RL circuit its $0$. Why is it ...
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1answer
30 views

Discharging capacitor

When the capacitor is fully charged and the switch as it B, how can we apply KVL at that loop to reduce the equation to (-IR)? Why is it negative? At time = 0 is the current not zero? And it will ...
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1answer
22 views

Negative charge on capacitor, Kirchoff differential equation with step function

I'm solving a differential equation of the form $$200q'+\frac{200}{3}q=3U_o-6U_6+3U_{12}$$ Where $U$ is the Heaviside step function. $q(0)=0 [Coulombs] $ After solving it, I get that at time $t=18$ ...
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1answer
22 views

Variation of Peak Current and Peak Voltage with Capacitance in an AC circuit

The relation of Peak current, Peak voltage and Capacitive Reactance in Alternating Current is given by: $$i_m=\frac{v_m}{X_c}$$ and $$X_c=\frac{1}{C\omega }$$ So if we have a circuit with a ...
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1answer
43 views

Capacitors in Series and Parallel [closed]

I'm working on a physics project and we're disagreeing on how to analyze a circuit. I think the circuit should be $(1/c_1 + 1/c_2)^{-1} + c_3$ and other people in my group think it should be $((c_3 + ...
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1answer
53 views

Capacitance in a parallel plate cap with the real electric field

In many books of general physics, they prove the equations for capacitance of a cap of parallel plates as if the electric field where constant everywhere but what would happen if we take the real ...
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2answers
101 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 ...
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5answers
331 views

Capacitors and Resistors - Series or Parallel?

I recently came across a problem in which a 12v DC power supply is connected in parallel to (in one loop) a one microfarad capacitor and a one kilo-ohm resistor. In the other loop is a 1000 microfarad ...
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1answer
49 views

Why does an electric field oppose the flow of positive ion?

I'm solving a very interesting problem. Suppose a cell is divided into inside and outside. Inside we have positive ions and outside we also have positive ions in different concentration. Suppose ...
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1answer
45 views

Dependence of the Capacitance on the Material and Geometry of the Plates

I have two questions concerning capacitors: 1) We know that the capacitance of a capacitor also depend on the dielectric material inside as $C=$ $\dfrac{K \epsilon_0 A}{d}$ But what if the ...
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1answer
74 views

Effect of Motion on Capacitors

What will happen if a capacitor is accelerated or rotated very very fast? Will its capacitance decrease? Will one of the plates be able to hold the electrons which is providing the Potential ...
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1answer
32 views

How thin a practical capacitor can be?

According to the formula of a $parallel$ $plate$ capacitor... $C$ $=$ $\dfrac{\epsilon_0 A}{d}$ The thinner the capacitor, more the charge it will be able to store... And hence the graph should ...
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50 views

What stored charges do in a capacitor? [closed]

Why does a fan need capacitor? Capacitor is just providing charges to fan but it can be accomplished without it by directly connecting fan with electric supply. Which other appliances need capacitor?
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2answers
206 views

What is zero impedance in AC circuit?

If a capacitor is connected with an inductor, then because $$Z=\frac{1}{j\omega C}+j\omega L,$$ the Z may be zero. Does that mean when I apply a voltage, the current will be infinite large? What's ...
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82 views

Energy of moving capacitor

On the following link were a discussion about energy in capacitor moving parallel to its field: Where's the energy in a boosted capacitor? My question is what happen if capacitor is moving ...
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89 views

What is special about the ratio $Q/V$ that we give it the name Capacitance?

Why is the ratio Charge/Potential important? Also, usually when we add charge, the potential changes. Then why do we care how much charge we can put on a conductor for a given potential.
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1answer
65 views

Loading a capacitor in an RC circuit

I have an RC circuit to which I am going to apply voltages of 5V every 100ns. So every 100ns, the capacitor will change its "state" from charging up to uncharging. I have been trying to find the a ...
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5answers
1k views

Is voltage difference always proportional to its derivative?

We write, because of Ohm's law: $$V=RI(t),$$ but also we have $$C\frac{dV}{dt}=I(t).$$ From the first equation we deduce that $V\propto I$ and from the second $\dot V\propto I$. So we can conclude ...
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3answers
75 views

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 ...
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42 views

Capacitance changes by a factor of 3 or 9?

This is an example problem with solution in a book I am reading on electrostatics. "We have two square parallel plate capacitors with length $a$ and separation $d$ with $d<<a$. If we triple ...
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45 views

How can a paper battery behave like a capacitor?

I was trying to understand how paper batteries work and came across some explanations saying that the paper battery can also behave like a supercapacitor on account of its ability to discharge and to ...
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1answer
46 views

Capacitor-like-thing for controlling temperature of fluid?

I want to minimise the Gibbs' phenomenon like thing i.e. sudden peaks (temperature peaks here) in a container. Assume you have a cone where you want to block the transmittance of the temperature into ...
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2answers
122 views

Energy stored in a capacitor in an RC circuit

Suppose we have a capacitor of capacitance $C$ and a cell of emf $E$, why is the maximum energy stored in a capacitor equal to $\tfrac{1}{2}CE^2$? I feel confused because, the potential difference ...
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3answers
58 views

Energy stored in a capacitor

Suppose we have a capacitor connected in series with a resistor and this is connected to a cell. So would the voltage across the capacitor be less than the the emf of the cell? And hence the energy ...
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2answers
45 views

Can I calculate the current of any setup with capacitance by the same formulae?

There are known formulae relating a capacitor's voltage and current in its classic form (battery, wires, a capacitor circuit), but what if we had a charged plate (-ve), then we put on one of its ...
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1answer
66 views

Argument for symmetry of potential of a spherical capacitor filled with dielectric

A spherical capacitor with inner radius $r_1$ and outer radius $r_2$ is filled with dielectric material with permittivity $\epsilon=\epsilon_0+\epsilon_1\cos^2\theta $. $\theta$ is the polar angle. ...
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1answer
105 views

Why can rolled up capacitors be modelled as flat parallel plates?

If you have two parallel plate of opposite charge with a dielectric in between, then Capacitance, $ C = \frac{A \epsilon_{0}}{d} K$, where $ K $ is the dielectric constant. This is used to calculate ...
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1answer
516 views

How is capacitance of a cylindrical capacitor calculated?

I've just begun learning capacitance, and my lecture notes have a section on calculating capacitance for capacitors in vacuum of various shapes, e.g. two parallel plates and concentric spherical ...
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67 views

Why is capacitor used?

I don't quite get the practical use of a capacitor. The theory is fine, but in simple words please explain how, why and where capacitors are used? Is there any other equivalent circuit for capacitors? ...
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63 views

How to calculate the charge on a capacitor [closed]

I have a network in which $C_1$ ($8.73 \;\rm \mu F$) and $C_2$ ($2.46 \;\rm \mu F$) are connected in parallel and $C_3$ ($7.45 \;\rm \mu F$) and $C_4$ ($2.22 \;\rm \mu F$) are connected in parallel. ...
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1answer
313 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 $Q$ ...
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2answers
66 views

Two capacitor plates with equal positive charges $q$

I read in a book that if both the plates of a parallel plate capacitor are given equal positive charges $q$, then the charges on the facing surfaces will be zero and the charge on the outer surfaces ...
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2answers
99 views

Principle of superposition for capacitor

In a capacitor the parallel plates of opposite charge create equal electric fields in opposite directions. We know field outside the capacitor is zero but inside the capacitor it is non zero. My ...